The pallor produced by fright and by extreme anxiety, is purely the result of a local modification of the circulation, brought about by an over-stimulation of the nerves which supply the small arteries, causing them to contract, and to thus cut off more or less completely the supply of blood.
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THE ORGANS OF RESPIRATION.
THE ORGANS OF RESPIRATION are the Trachea, or windpipe, the Bronchia, formed by the subdivision of the trachea, and the Lungs, with their air-cells. The Trachea is a vertical tube situated between the lungs below, and a short quadrangular cavity above, called the larynx, which is part of the windpipe, and used for the purpose of modulating the voice in speaking or singing. In the adult, the trachea, in its unextended state, is from four and one-half to five inches in length, about one inch in diameter, and, like the larynx, is more fully developed in the male than in the female. It is a fibro-cartilaginous structure, and is composed of flattened rings, or segments of circles. It permits the free passage of air to and from the lungs.
The Bronchia are two tubes, or branches, one proceeding from the windpipe to each lung. Upon entering the lungs, they divide and subdivide until, finally, they terminate in small cells, called the bronchial or air-cells, which are of a membranous character.
The Lungs are irregular conical organs rounded at the apex, situated within the chest, and filling the greater part of it, since the heart is the only other organ which occupies much space in the thoracic cavity. The lungs are convex externally, and conform to the cavity of the chest, while the internal surface is concave for the accommodation of the heart. The size of the lungs depends upon the capacity of the chest. Their color varies, being of a pinkish hue in childhood but of a gray, mottled appearance in the adult. They are termed the right and left lung. Each lung resembles a cone with its base resting upon the diaphragm, and its apex behind the collar-bone. The right lung is larger though shorter, than the left, not extending so low, and has three lobes, formed by deep fissures, or longitudinal divisions, while the left has but two lobes. Each lobe is also made up of numerous lobules, or small lobes, connected by cellular tissue, and these contain great numbers of cells. The lungs are abundantly supplied with blood-vessels, lymphatics, and nerves. The density of a lung depends upon the amount of air which it contains. Thus, experiment has shown that in a foetus which has never breathed, the lungs are compact and will sink in water; but as soon as they become inflated with air, they spread over a larger surface, and are therefore more buoyant. Each lung is invested, as far as its root, with a membrane, called the pleura, which is then continuously extended to the cavity of the chest, thus performing the double office of lining it, and constituting a partition between the lungs. The part of the membrane which forms this partition is termed the mediastinum. Inflammation of this membrane is called pleurisy. The lungs are held in position by the root, which is formed by the pulmonary arteries, veins, nerves, and the bronchial tubes. Respiration is the function by which the venous blood, conveyed to the lungs by the pulmonary artery, is converted into arterial blood. This is effected by the elimination of carbonic acid, which is expired or exhaled from the lungs, and by the absorption of oxygen from the air which is taken into the lungs, by the act of inspiration or inhalation. The act of expiration is performed chiefly by the elevation of the diaphragm and the descent of the ribs, and inspiration is principally effected by the descent of the diaphragm and the elevation of the ribs.
When the muscles of some portions of the air-passages are relaxed, a peculiar vibration follows, known as snoring. Coughing and sneezing are sudden and spasmodic expiratory efforts, and generally involuntary. Sighing is a prolonged deep inspiration, followed by a rapid, and generally audible expiration. It is remarkable that laughing and sobbing, although indicating opposite states of the mind, are produced in very nearly the same manner. In hiccough, the contraction is more sudden and spasmodic than in laughing or sobbing. The quantity of oxygen consumed during sleep is estimated to be considerably less than that consumed during wakefulness.
It is difficult to estimate the amount of air taken into the lungs at each inspiration, as the quantity varies according to the condition, size, and expansibility of the chest, but in ordinary breathing it is supposed to be from twenty to thirty cubic inches. The consumption of oxygen is greater when the temperature is low, and during digestion. All the respiratory movements, so far as they are independent of the will of the individual, are controlled by that part of the brain called the medulla oblongata. The respiratory, or breathing process, is not instituted for the benefit of man alone, for we find it both in the lower order of animals and in plant life. Nature is very economical in the arrangement of her plans, since the carbonic acid, which is useless to man, is indispensable to the existence of plants, and the oxygen, rejected by them, is appropriated to his use. In the lower order of animals, the respiratory act is similar to that of the higher types, though not so complex; for there are no organs of respiration, as the lungs and gills are called. Thus, the higher the animal type, the more complex its organism. The effect of air upon the color of the blood is very noticeable. If a quantity be drawn from the body, thus being brought into contact with the air, its color gradually changes to a brighter hue. There is a marked difference between the properties of the venous and the arterial blood.
The venous blood is carried, as we have previously described, to the right side of the heart and to the lungs, where it is converted into arterial blood. It is now of uniform quality, ready to be distributed throughout the body, and capable of sustaining life and nourishing the tissues. Man breathes by means of lungs; but who can understand their wonderful mechanism, so perfect in all its parts? Though every organ is subservient to another, yet each has its own office to perform. The minute air-cells are for the aeration of the blood; the larger bronchial tubes ramify the lungs, and suffuse them with air; the trachea serves as a passage for the air to and from the lungs, while at its upper extremity is the larynx, which has been fitly called the organ of the human voice. At its extremity we find a sort of shield, called the epiglottis, the office of which is supposed to be to prevent the intrusion of foreign bodies.
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Through digestion and respiration, the blood is continually supplied with material for its renewal; and, while the nutritive constituents of the food are retained to promote the growth of the body, those which are useless or injurious are in various ways expelled. There are, perhaps, few parts of the body more actively concerned in this removal than the skin.
The skin is a membranous envelope covering the entire body. It consists of two layers, termed the Cutis Vera, or true skin, and the Epidermis, or cuticle. The Cutis Vera is composed of fibers similar to those of the cellular tissue. It consists of white and yellow fibers, which are more densely woven near the surface than deeper in the structure; the white give strength, the yellow strength and elasticity combined. The true skin may be divided into two layers, differing in their characteristics, and termed respectively the superficial or papillary layer, and the deep or fibrous layer. Upon the external surface, are little conical prominences, known as papillae. The papillae are irregularly distributed over the body, in some parts being smaller and more numerous than in others, as on the finger-ends, where their summits are so intimately connected as to form a tolerably smooth surface. It is owing to their perfect development, that the finger-tips are adapted to receive the most delicate impressions of touch. Although every part of the skin is sensitive, yet the papillae are extremely so, for they are the principal means through which the impressions of objects are communicated. Each papilla not only contains a minute vein and artery, but it also incloses a loop of sensitive nerves. When the body is exposed to cold, these papillae can be more distinctly seen in the form of prominences, commonly known as "goose-pimples."
The internal, or fibrous layer of the skin, contains numerous depressions, each of which furnishes a receptacle for fat. While the skin is supplied with a complete net-work of arteries, veins, and nerves, which make it sensitive to the slightest touch, it also contains numerous lymphatic vessels, so minute that they are invisible to the naked eye.
Among the agents adapted for expelling the excretions from the system, few surpass the Sudoriferous Glands. These are minute organs which wind in and out over the whole extent of the true skin, and secrete the perspiration. Though much of it passes off as insensible transpiration, yet it often accumulates in drops of sweat, during long-continued exercise or exposure to a high temperature. The office of the perspiration is two-fold. It removes noxious matter from the system, and diminishes animal heat, and thereby equalizes the temperature of the body. It also renders the skin soft and pliable, thus better adapting it to the movements of the muscles. The Sebaceous Glands, which are placed in the true skin, are less abundant where the sudoriferous glands are most numerous, and vice versa. Here, as elsewhere, nature acts with systematic and intelligent design. The perspiratory glands are distributed where they are most needed,—in the eyelids, serving as lubricators; in the ear passages, to produce the cerumen, or wax, which prevents the intrusion of small insects; and in the scalp, to supply the hair with its natural pomatum.
The Epidermis, or Cuticle, so called because it is placed upon the skin, is the outer layer of the skin. Since it is entirely destitute of nerves and blood-vessels, it is not sensitive. Like the cutis vera, it has two surfaces composed of layers. The internal, or Rete Mucosum, which is made up chiefly of pigment cells, is adapted to the irregularities of the cutis vera, and sends prolongations into all its glandular follicles. The external surface, or epidermis proper, is elastic, destitute of coloring matter, and consists of mere horny scales. As soon as dry, they are removed in the form of scurf, and replaced by new ones from the cutis vera. These scales may be removed by a wet-sheet pack, or by friction. The cuticle is constantly undergoing renewal. This layer serves to cover and protect the nervous tissue of the true skin beneath. We may here observe that the cuticle contains the pigment for coloring the skin. In dark races, as the negro, the cuticle is very thick and filled with black pigment. The radiation of animal heat is dependent upon the thickness and color of this cuticle. Thus, in the dark races, the pigment cells are most numerous, and in proportion as the skin is dark or fair do we find these cells in greater or lesser abundance. The skin of the Albino is of pearly whiteness, devoid even of the pink or brown tint which that of the European always possesses. This peculiarity must be attributed to the absence of pigment cells which, when present, always present a more or less dark color. The theory that climate alone is capable of producing all these diversities is simply absurd. The Esquimaux, who live in Greenland and the arctic regions of America, are remarkable for the darkness of their complexion. Humboldt remarks that the American tribes of the tropical regions have no darker skin than the mountaineers of the temperate zone. Climate may modify the complexion, but it cannot make it.
Hairs are horny appendages of the skin, and, with the exception of the hands, the soles of the feet, the backs of the fingers and toes, between the last joint and the nail, and the upper eyelids, are distributed more or less abundantly over every part of the surface of the body. Over the greater part of the surface the hairs are very minute, and in some places are not actually apparent above the level of the skin; but the hair of the head, when permitted to reach its full growth, attains a length of from twenty inches to a yard, and, in rare instances, even six feet. A hair may be divided into a middle portion, or shaft, and two extremities; a peripheral extremity, called the point; and a central extremity, inclosed within the hair sac, or follicle, termed the root. The root is somewhat greater in diameter than the shaft, and cylindrical in form, while its lower part expands into an oval mass, called the bulb. The shaft of the hair is not often perfectly cylindrical, but is more or less flattened, which circumstance gives rise to waving and curling hair; and, when the flattening is spiral in direction, the curling will be very great. A hair is composed of three different layers of cell-tissues: a loose, cellulated substance, which occupies its center, and constitutes the medulla, or pith; the fibrous tissue, which incloses the medulla, and forms the chief bulk of the hair; and a thin layer, which envelops this fibrous structure, and forms the smooth surface of the hair. The medulla is absent in the downy hairs, but in the coarser class it is always present, especially in white hair. The color of hair is due partly to the granules and partly to an inter-granular substance, which occupies the interstices of the granules and the fibers. The quantity of hair varies according to the proximity and condition of the follicles. The average number of hairs of the head may be stated at 1,000 in a superficial square inch; and, as the surface of the scalp has an area of about one hundred and twenty superficial square inches, the average number of hairs on the entire head is 120,000. The hair possesses great durability, as is evinced by its endurance of chemical processes, and by its discovery, in the tombs of mummies more than two thousand years old. The hair is remarkable for its elasticity and strength. Hair is found to differ materially from horn in its chemical composition. According to Vauquelin, its constituents are animal matter, a greenish-black oil, a white, concrete oil, phosphate of lime, a trace of carbonate of lime, oxide of manganese, iron, sulphur, and silex. Red hair contains a reddish oil, a large proportion of sulphur, and a small quantity of iron. White hair contains a white oil, and phosphate of magnesia. It has been supposed that hair grows after death, but this theory was probably due to the lengthening of the hair by the absorption of moisture from the body or atmosphere.
The nails constitute another class of appendages of the skin. They consist of thin plates of horny tissue, having a root, a body, and a free extremity. The root, as well as the lateral portion, is implanted in the skin, and has a thin margin which is received into a groove of the true skin. The under surface is furrowed, while the upper is comparatively smooth. The nails grow in the same manner as the cuticle.
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The term Secretion, in its broadest sense, is applied to that process by which substances are separated from the blood, either for the reparation of the tissues or for excretion. In the animal kingdom this process is less complicated than in vegetables. In the former it is really a separation of nutritive material from the blood. The process, when effected for the removal of effete matter, is, in a measure, chemical, and accordingly the change is greater.
Three elementary constituents are observed in secretory organs: the cells, a basement membrane, and the blood-vessels. Obviously, the most essential part is the cell.
The physical condition necessary for the healthy action of the secretory organs is a copious supply of blood, in which the nutritive materials are abundant. The nervous system also influences the process of secretion to a great extent. Intense emotion will produce tears, and the sight of some favorite fruit will generally increase the flow of saliva.
The process of secretion depends upon the anatomical and chemical constitution of the cell-tissues. The principal secretions are (1), Perspiration; (2), Tears; (3), Sebaceous matter; (4), Mucus; (5), Saliva; (6), Gastric juice; (7), Intestinal juice; (8), Pancreatic juice; (9), Bile; (10), Milk.
PERSPIRATION is a watery fluid secreted in minute glands, which are situated in every part of the skin, but are more numerous on the anterior surfaces of the body. Long thread-like tubes, only 1/100th of an inch in diameter, lined with epithelium, penetrate the skin, and terminate in rounded coils, enveloped by a net-work of capillaries, which supply the secretory glands with blood. It is estimated by Krause that the entire number of perspiratory glands is two million three hundred and eighty-one thousand two hundred and forty-eight, and the length of each glandular coil being 1/16 of an inch, we may estimate the length of tubing to be not less than two miles and a third. This secretion has a specific gravity of 1003.5, and, according to Dr. Dalton, is composed of
Water, 995.50 Chloride of Sodium, 2.23 Chloride of Potassium, 0.24 Sulphate of Soda and Potassa, 0.01 Salts of organic acids, with Soda and Potassa, 2.02 ———- 1000.00
Traces of organic matter, mingled with a free volatile acid, are also found in the perspiration. It is the acid which imparts to this secretion its peculiar odor, and acid reaction. The process of its secretion is continuous, but, like all bodily functions, it is subject to influences which augment or retard its activity. If, as is usually the case when the body is in a state of repose, evaporation prevents its appearance in the liquid form, it is called invisible or insensible perspiration. When there is unusual muscular activity, it collects upon the skin, and is known as sensible perspiration. This secretion performs an important office in the animal economy, by maintaining the internal temperature at about 100 deg. Fahr. Even in the Arctic regions, where the explorer has to adapt himself to a temperature of 40 deg. to 80 deg. below zero, the generation of heat in the body prevents the internal temperature from falling below this standard. On the contrary, if the circulation is quickened by muscular exertion, the warmer blood flowing from the internal organs into the capillaries, raises the temperature of the skin, secretion is augmented, the moisture exudes from the pores, and perceptible evaporation begins. A large portion of the animal heat is thrown off in this process, and the temperature of the skin is reduced. A very warm, dry atmosphere can be borne with impunity but if moisture is introduced, evaporation ceases, and the life of the animal is endangered. Persons have been known to remain in a temperature of about 300 deg. Fahr. for some minutes without unpleasant effects. Three conditions may be assigned as effective causes in retarding or augmenting this cutaneous secretion, variations in the temperature of the atmosphere, muscular activity, and influences which affect the nerves. The emotions exert a remarkable influence upon the action of the perspiratory glands. Intense fear causes great drops of perspiration to accumulate on the skin, while the salivary glands remain inactive.
TEARS. The lachrymal glands are small lobular organs, situated at the outer and upper orbit of the eye, and have from six to eight ducts, which open upon the conjunctiva, between the eyelid and its inner fold. This secretion is an alkaline, watery fluid. According to Dr. Dalton, its composition is as follows:
Water, 882.0 Albuminous matter, 5.0 Chloride of Sodium, 13.0 Mineral Salts, a trace, ——— 1000.0
The function of this secretion is to preserve the brilliancy of the eye. The tears are spread over this organ by the reflex movement of the eyelid, called winking, and then collected in the puncta lachrymalia and discharged into the nasal passage. This process is constant during life. The effect of its repression is seen in the dim appearance of the eye after death. Grief or excessive laughter usually excite these glands until there is an overflow.
SEBACEOUS MATTER. Three varieties of this secretion are found in the body. A product of the sebaceous glands of the skin is found in those parts of the body which are covered with hairs; also, on the face and the external surface of the organs of generation. The sebaceous glands consist of a group of flask-shaped cavities, opening into a common excretory duct. Their secretion serves to lubricate the hair and soften the skin. The ceruminous glands of the external auditory meatus, or outer opening of the ear, are long tubes terminating in a glandular coil, within which is secreted the glutinous matter of the ear. This secretion serves the double purpose of moistening the outer surface of the membrana tympani, or ear-drum, and, by its strong odor, of preventing the intrusion of insects. The Meibomian glands are arranged in the form of clusters along the excretory duct, which opens just behind the roots of the eyelashes. The oily nature of this secretion prevents the tears, when not stimulated by emotion, from overflowing the lachrymal canal.
MUCUS. The mucous membranes are provided with minute glands which secrete a viscid, gelatinous matter, called mucus. The peculiar animal matter which it contains is termed mucosin. These glands are most numerous in the Pharynx, Esophagus, Trachea, Bronchia, Vagina and Urethra. They consist of a group of secreting sacs, terminating at one extremity in a closed tube, while the other opens into a common duct. The mucus varies in composition in different parts of the body; but in all, it contains a small portion of insoluble animal matter. Its functions are threefold. It lubricates the membranes, prevents their injury, and facilitates the passage of food through the alimentary canal.
SALIVA. This term is given to the first of the digestive fluids, which is secreted in the glands of the mouth. It is a viscid, alkaline liquid, with a specific gravity of about 1005. If allowed to stand, a whitish precipitate is formed. Examinations with the microscope show it to be composed of minute, granular cells and oil globules, mingled with numerous scales of epithelium. According to Bidder and Schmidt, the composition of saliva is as follows:
Water, 995.16 Organic matter, 1.34 Sulpho-cyanide of Potassium, 0.06 Phosphates of Sodium, Calcium and Magnesium, .98 Chlorides of Sodium and Potassium, .84 Mixture of Epithelium, 1.62 ———- 1000.00
Two kinds of organic matter are present in the saliva; one, termed ptyalin, imparts to the saliva its viscidity, and it obtained from the secretions of the parotid, submaxillary and sublingual glands; another, which is not glutinous, is distinguished by the property of coagulating when subjected to heat. The saliva is composed of four elementary secretions, derived respectively, from the mucous follicles of the mouth, and the parotid, the submaxillary, and the sublingual glands. The process of its secretion is constant, but is greatly augmented by the contact of food with the lining membrane. The saliva serves to moisten the triturated food, facilitate its passage, and has the property of converting starch into sugar; but the latter quality is counteracted by the action of the gastric juice of the stomach.
GASTRIC JUICE. The minute tubes, or follicles, situated in the mucous membrane of the stomach, secrete a colorless, acid liquid, termed the gastric juice. This fluid appears to consist of little more than water, containing a few saline matters in solution, and a small quantity of free hydrochloric acid, which gives it an acid reaction. In addition to these, however, it contains a small quantity of a peculiar organic substance, termed pepsin, which in chemical composition, is very similar to ptyalin, although it is very different in its effects. When food is introduced into the stomach, the peristaltic contractions of that organ roll it about, and mingle it with the gastric juice, which disintegrates the connective tissue, and converts the albuminous portions into the substance called chyme, which is about the consistency of pea-soup, and which is readily absorbed through the animal membranes into the blood of the delicate and numerous vessels of the stomach, whence it is conveyed to the portal vein and to the liver. The secretion of the gastric juice is influenced by nervous conditions. Excess of joy or grief effectually retard or even arrest its flow.
INTESTINAL JUICE. In the small intestine, a secretion is found which is termed the intestinal juice. It is the product of two classes of glands situated in the mucous membrane, and termed respectively, the follicles of Lieberkuhn and the glands of Brunner. The former consist of numerous small tubes, lined with epithelium, which secrete by far the greater portion of this fluid. The latter are clusters of round follicles opening into a common excretory duct. These sacs are composed of delicate, membranous tissue, having numerous nuclei on their walls. The difficulty of obtaining this juice for experiment is obvious, and therefore its chemical composition and physical properties are not known. The intestinal juice resembles the secretion of the mucous follicles of the mouth, being colorless, vitreous in appearance, and having an alkaline reaction.
PANCREATIC JUICE. This is a colorless fluid, secreted in a lobular gland which is situated behind the stomach, and runs transversely from the spleen across the vertebral column to the duodenum. The most important constituent of the pancreatic juice is an organic substance, termed pancreatin.
THE BILE. The blood which is collected by the veins of the stomach, pancreas, spleen, and intestines, is discharged into a large trunk called the portal vein, which enters the liver. This organ also receives arterial blood from a vessel called the hepatic artery, which is given off from the aorta below the diaphragm. If the branches of the portal vein and hepatic artery be traced into the substance of the liver, they will be found to accompany one another, and to subdivide, becoming smaller and smaller. Finally, the portal vein and hepatic artery will be found to terminate in capillaries which permeate the smallest perceptible subdivisions of the liver substance, which are polygonal masses of not more than one-tenth of an inch in diameter, called the lobules. Every lobule rests upon one of the ramifications of a great vessel termed the hepatic vein, which empties into the inferior vena cava. There is also a vessel termed the hepatic duct leading from the liver, the minute subdivisions of which penetrate every portion of the substance of that organ. Connected with the hepatic duct, is the duct of a large oval sac, called the gall-bladder.
Each lobule of the liver is composed of minute cellular bodies known as the hepatic cells. It is supposed that in these cells the blood is deprived of certain materials which are converted into bile. This secretion is a glutinous fluid, varying in color from a dark golden brown to a bright yellow, has a specific gravity ranging from 1018 to 1036, and a slightly alkaline reaction. When agitated, it has a frothy appearance. Physiologists have experienced much difficulty in studying the character of this secretion from the instability of its constituents when subjected to chemical examination.
Biliverdin is an organic substance peculiar to the bile, which imparts to that secretion its color. When this constituent is re-absorbed by the blood and circulates through the tissues, the skin assumes a bright yellow hue, causing what is known as the jaundice. Cholesterin is an inflammable crystallizable substance soluble in alcohol or ether. It is found in the spleen and all the nervous tissues. It is highly probable that it exists in the blood, in some state or combination, and assumes a crystalline form only when acted upon by other substances or elements. Two other constituents, more important than either of the above, are collectively termed biliary salts. These elements were discovered in 1848, by Strecker, who termed them glycocholate and taurocholate of soda. Both are crystalline, resinous substances, and, although resembling each other in many respects, the chemist may distinguish them by their reaction, for both yield a precipitate if treated with subacetate of lead, but only the glycocholate will give a precipitate with acetate of lead. In testing for biliary substances, the most satisfactory method is the one proposed by Pettenkoffer. A solution of cane-sugar, one part of sugar to four parts of water, is mixed with the suspected substance. Dilute sulphuric acid is then added until a white precipitate falls, which is re-dissolved in an excess of the acid. On the addition of more sulphuric acid, it becomes opalescent, and passes through the successive hues of scarlet, lake, and a rich purple. Careful experiments have proved that it is a constant secretion; but its flow is mere abundant during digestion. During the passage through the intestines it disappears. It is not eliminated, and Pettenkoffer's test has failed to detect its existence in the portal vein. These facts lead physiologists to the conclusion, that it undergoes some transformation in the intestines and is re-absorbed.
After digestion has been going on in the stomach for some time, the semi-digested food, in the form of chyme, begins to pass through the pyloric orifice of the stomach into the duodenum, or upper portion of the small intestine. Here it encounters the intestinal juice, pancreatic juice, and the bile, the secretion of all of which is stimulated by the presence of food in the alimentary tract. These fluids, mingling with the chyme, give it an alkaline reaction, and convert it into chyle. The transformation of starch into sugar, which is almost, if not entirely, suspended while the food remains in the stomach, owing to the acidity of the chyme, is resumed in the duodenum, the acid of the chyme, being neutralized by the alkaline secretions there encountered.
Late researches have demonstrated that the pancreatic juice exerts a powerful effect on albuminous matters, not unlike that of the gastric juice.
Thus, it seems that while in the mouth only starchy, and while in the stomach only albuminous substances are digested, in the small intestine all kinds of food materials, starchy, albuminoid, fatty and mineral, are either completely dissolved, or minutely subdivided, and so prepared that they may be readily absorbed through the animal membranes into the vessels.
MILK. The milk is a white, opaque fluid, secreted in the lacteal glands of the female, in the mammalia. These glands consist of numerous follicles, grouped around an excretory duct, which unites with similar ducts coming from other lobules. By successive unions, they form large branches, termed the lactiferous ducts, which open by ten to fourteen minute orifices on the extremity of the nipple. The most important constituent of milk is casein; it also contains oily and saccharine substances. This secretion, more than any other, as influenced by nervous conditions. A mother's bosom will fill with milk at the thought of her infant child. Milk is sometimes poisoned by a fit of ill-temper, and the infant made sick and occasionally thrown into convulsions, which in some instances prove fatal. Sir Astley Cooper mentions two cases in which terror instantaneously and permanently arrested this secretion. It is also affected by the food and drink. Malt liquors and other mild alcoholic beverages temporarily increase the amount of the secretion, and may, in rare instances, have a beneficial effect upon the mother. They sometimes affect the child, however, and their use is not to be recommended unless the mother is extremely debilitated, and there is a deficiency of milk.
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The products resulting from the waste of the tissues are constantly being poured into the blood, and, as we have seen, the blood being everywhere full of corpuscles, which, like all living things, die and decay, the products of their decomposition accumulate in every part of the circulatory system. Hence, if the blood is to be kept pure, the waste materials incessantly poured into this fluid, or generated in it, must be as continually removed, or excreted. The principal sets of organs concerned in effecting the separation of excrementitious substances from the blood are the lungs, the skin, and the kidneys.
The elimination of carbonic acid through the lungs has already been described on page 66, and the excretory function of the skin on page 70.
The kidneys are two bean-shaped organs, placed at the back of the abdominal cavity, in the region of the loins, one on each side of the spine. The convex side of each kidney is directed outwards, and the concave side is turned inwards towards the spine. From the middle of the concave side, which is termed the hilus, a long tube of small caliber, called the ureter, proceeds to the bladder. The latter organ is an oval bag, situated in the pelvic cavity. It is composed principally of elastic muscular fibers, and is lined internally with mucous membrane, and coated externally with a layer of the peritoneum, the serous membrane which lines the abdominal and pelvic cavities. The ureters enter the bladder through its posterior and lower wall, at some little distance from each other. The openings through which the ureters enter the bladder are oblique, hence it is much easier for the secretion of the kidneys to pass from the ureters into the bladder than for it to get the other way. Leading from the bladder to the exterior of the body is a tube, called the urethra, through which the urine is voided.
The excretion of the kidneys, termed the urine, is an amber-colored or straw-colored fluid, naturally having a slightly acid reaction, and a specific gravity ranging from 1,015 to 1,025. Its principal constituents are urea and uric acid, together with various other animal matters of less importance, and saline substances, held in solution in a proportionately large amount of water. The composition of the urine and the quantity excreted vary considerably, being influenced by the moisture and temperature of the atmosphere, by the character of the food consumed, and by the empty or replete condition of the alimentary tract. On an average a healthy man secretes about fifty ounces of urine in the twenty-four hours. This quantity usually holds in solution about one ounce of urea, and ten or twelve grains of uric acid. In the amount of other animal matters, and saline substances, there is great variation, the quantity of these ranging from a quarter of an ounce to an ounce. The principal saline substances are common salt, the sulphates and phosphates of potassium, sodium, calcium, and magnesium. In addition to the animal and the saline matters, the urine also contains a small quantity of carbonic acid, oxygen and nitrogen.
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THE NERVOUS SYSTEM.
Hitherto, we have only considered the anatomy and functions of the organs employed in Digestion, Absorption, Circulation, Respiration, Secretion and Excretion. We have found the vital process of nutrition to be, in all its essential features, a result of physical and chemical forces; in each instance we have presupposed the existence and activity of the nerves. There is not an inch of bodily tissue into which their delicate filaments do not penetrate, and form a multitude of conductors, over which are sent the impulses of motion and sensation.
Two elements, nerve-fibers and ganglionic corpuscles, enter into the composition of nervous tissue. Ordinary nerve-fibers in the living subject, or when fresh, are cylindrical-shaped filaments of a clear, but somewhat oily appearance. But soon after death the matter contained in the fiber coagulates, and then the fiber is seen to consist of an extremely delicate, structureless, outer membrane, which forms a tube through the center of which runs the axis-cylinder. Interposed between the axis-cylinder and this tube, there is a fluid, containing a considerable quantity of fatty matter, from which is deposited a highly refracting substance which lines the tube. There are two sets of nerve-fibers, those which transmit sensory impulses, called afferent or sensory nerves, and those which transmit motor impulses, called efferent or motor nerves. The fibers when collected in bundles are termed nerve trunks. All the larger nerve-fibers lie side by side in the nerve-trunks, and are bound together by delicate connective tissue, enclosed in a sheath of the same material, termed the neurilemma. The nerve-fibers in the trunks of the nerves remain perfectly distinct and disconnected from one another, and seldom, or never, divide throughout their entire length. However, where the nerves enter the nerve-centers, and near their outer terminations, the nerve-fibres often divide into branches, or at least gradually diminish in size, until, finally, the axis-cylinder, and the sheath with its fluid contents, are no longer distinguishable. The investing membrane is continuous from the origin to the termination of the nerve-trunk.
In the brain and spinal cord the nerve-fibers often terminate in minute masses of a gray or ash-colored granular substance, termed ganglia, or ganglionic corpuscles.
The ganglia are cellular corpuscles of irregular form, and possess fibrous appendages, which serve to connect them with one another. These ganglia form the cortical covering of the brain, and are also found in the interior of the spinal cord. According to Koelliker, the larger of these nerve-cells measure only 1/200 of an inch in diameter. The brain is chiefly composed of nervous ganglia.
Nerves are classified with reference to their origin, as cerebral—those originating in the brain, and spinal—those originating in the spinal cord.
There are two sets of nerves and nerve-centers, which are intimately connected, but which can be more conveniently studied apart. These are the cerebro-spinal system, consisting of the cerebro-spinal axis, and the cerebral and spinal nerves; and the sympathetic system, consisting of the chain of sympathetic ganglia, the nerves which they give off, and the nervous trunks which connect them with one another and with the cerebro-spinal nerves.
THE CEREBRO-SPINAL SYSTEM.
THE CEREBRO-SPINAL AXIS consists of the brain and spinal cord. It lies in the cavities of the cranium and the spinal column. These cavities are lined with a very tough fibrous membrane, termed the dura mater, which serves as the periosteum of the bones which enter into the formation of these parts. The surface of the brain and spinal cord is closely invested with an extremely vascular, areolar tissue, called the pia mater. The numerous blood-vessels which supply these organs traverse the pia mater for some distance, and, where they pass into the substance of the brain or spinal cord, the fibrous tissue of this membrane accompanies them to a greater or less depth. The inner surface of the dura mater and the outer surface of the pia mater are covered with an extremely thin, serous membrane, which is termed the arachnoid membrane. Thus, one layer of the arachnoid envelopes the brain and spinal cord, and the other lines the dura mater. As the layers become continuous with each other at different points, the arachnoid, like the pericardium, forms a shut sac, and, like other serous membranes, it secretes a fluid, known as the arachnoid fluid. The space between the internal and the external layers of the arachnoid membrane of the brain is much smaller than that enclosed by the corresponding layers of the arachnoid membrane of the spinal column.
THE SPINAL CORD is a column of soft, grayish-white substance, extending from the top of the spinal canal, where it is continuous with the brain, to about an inch below the small of the back, where it tapers off into a filament. From this nerve are distributed fibers and filaments to the muscles and integument of at least nine-tenths of the body.
The spinal cord is divided in front through the middle nearly as far as its center, by a deep fissure, called the anterior fissure, and behind, in a similar manner, by the posterior fissure. Each of these fissures is lined with the pia mater, which also supports the blood-vessels which supply the spinal cord with blood. Consequently, the substance of the two halves of the cord is only connected by a narrow isthmus, or bridge, perforated by a minute tube, which is termed the central canal of the spinal cord.
Each half of the spinal cord is divided lengthwise into three nearly equal parts, which are termed the anterior, lateral, and posterior columns, by the lines which join together two parallel series of bundles of nervous filaments, which compose the roots of the spinal nerves. The roots of those nerves, which are found along that line nearest the posterior surface of the cord, are termed the posterior roots; those which spring from the other line are known as the anterior roots.
Several of these anterior and posterior roots, situated at about the same height on opposite sides of the spinal cord, converge and combine into what are called the anterior and posterior bundles; then two bundles, anterior and posterior, unite and form the trunk of a spinal nerve.
The nerve trunks make their way out of the spinal canal through apertures between the vertebra, called the inter-vertebral foramina and then divide into numerous branches, their ramifications extending principally to the muscles and the skin. There are thirty-one pairs of spinal nerves, eight of which are termed cervical, twelve dorsal, five lumbar, and six sacral, with reference to that part of the cord from which they originate.
When the cord is divided into transverse sections, it is found that each half is composed of two kinds of matter, a white substance on the outside, and a grayish substance in the interior. The gray matter, as it is termed, lies in the form of an irregular crescent, with one end considerably larger than the other, and having the concave side turned outwards. The ends of the crescent are termed the horns, or cornua, the one pointing forward being called the anterior cornu, the other one the posterior cornu. The convex sides of these cornua approach each other and are united by the bridge, which contains the central canal.
There is a marked difference in the structure of the gray and the white matter. The white matter is composed entirely of nerve fibers, held together by a framework of connective tissue. The gray matter contains a great number of ganglionic corpuscles, or nerve-cells, in addition to the nerve-fibers.
When the nerve-trunks are irritated in any manner, whether by pinching, burning, or the application of electricity, all the muscles which are supplied with branches from this nerve-trunk immediately contract, and pain is experienced, the severity of which depends upon the degree of the irritation; and the pain is attributed to that portion of the body to which the filaments of the nerve-trunk are distributed. Thus, persons who have lost limbs often complain in cold weather of an uneasiness or pain, which they locate in the fingers or toes of the limb which has been amputated, and which is caused by the cold producing an irritation of the nerve-trunk, the filaments, or fibers of which, supplied the fingers or toes of the lost member.
On the other hand, if the anterior bundle of nerve-fibers given off from the spinal cord is irritated in precisely the same way, only half of these effects is produced. All the muscles which are supplied with fibers from that trunk contract, but no pain is experienced. Conversely, if the posterior bundle of nerve-fibers is irritated, none of the muscles to which the filaments of the nerve are distributed contract, but pain is felt throughout the entire region to which these filaments are extended. It is evident, from these facts, that the fibers composing the posterior bundles of nerve-roots only transmit sensory impulses, and the filaments composing the anterior nerve-roots only transmit motor impulses; accordingly, they are termed respectively the sensory and the motor nerve-roots. This is illustrated by the fact that when the posterior root of a spinal nerve is divided, all sensation in the parts to which the filaments of that nerve are distributed is lost, but the power of voluntary movement of the muscles remains. On the other hand, if the anterior roots are severed, the power of voluntary motion of the muscles is lost, but sensation remains.
It appears from these experiments, that, when a nerve is irritated, a change in the arrangement of its molecules takes place, which is transmitted along the nerve-fibers. But, if the nerve-trunks are divided, or compressed tightly at any point between the portion irritated, and the muscle or nerve-centre, the effect ceases immediately, in a manner similar to that in which a message is stopped by the cutting of a telegraph wire. When the nerves distributed to a limb are subjected to a pressure sufficient to destroy the molecular continuity of their filaments, it "goes to sleep," as we term it. The power of transmitting sensory and motor impulses is lost, and only returns gradually, as the molecular continuity is restored.
From what has been said, it is plain that a sensory nerve is one which conveys a sensory impulse from the peripheral or outer part of a nerve to the spinal cord or brain, and which is, therefore, termed afferent; and that a motor nerve is one which transmits an impulse from the nerve centre, or is efferent. So difference in structure, or in chemical or physical composition, can be discerned between the afferent and the efferent nerves. A certain period of time is required for the transmission of all impulses. The speed with which an impulse travels has been found to be comparatively slow, being even less than that of sound, which is 1,120 feet per second.
The experiments heretofore related have been confined solely to the nerves. We may now proceed to the consideration of what takes place when the spinal cord is operated upon in a similar way. If the cord be divided with a knife or other instrument, all parts of the body supplied with nerves given off below the division will become paralyzed and insensible, while all parts of the body supplied with nerves from the spinal cord above the division will retain their sensibility and power of motion. If, however, only the posterior half of the spinal cord is divided, or destroyed, there is loss of sensation alone; and, if the anterior portion is cut in two, and the continuity of the posterior part is left undisturbed, there is loss of voluntary motion of the lower limbs, but sensation remains.
REFLEX ACTION OF THE SPINAL CORD. In relation to the brain, the spinal cord is a great mixed motor and sensory nerve, but, in addition to this, it is also a distinct nervous centre, in which originate and terminate all those involuntary impulses which exert so potent an influence in the preservation and economy of the body. That peculiar power of the cord by which it is enabled to convert sensory into motor impulse is that which distinguishes it, as a central organ, from a nerve, and is called reflex action.
The gray matter, and not the white, is the part of the cord which possesses this power. This reflex action is a special function of the spinal cord, and serves as a monitor to, and regulator of the organs of nutrition and circulation, by placing them, ordinarily, beyond the control of conscious volition.
If the foot of a decapitated frog is irritated, there is an instant contraction of the corresponding limb; if the irritation is intense the other limb also contracts. These motions indicate the existence, in some part of the spinal cord, of a distinct nerve-centre, capable of converting and reflecting impulses. It has been found by experiment, that the same movements will take place if the irritation be applied to any portion of the body to which the spinal nerves are distributed, thus giving undoubted evidence that the spinal cord in its entirety is capable of causing these reflections. Fig. 57 represents the course of the nervous impulses. The sensory impulse passes upward along the posterior root, a, until it reaches the imbedded gray matter, b, of the cord, by which it is reflected, as a motor impulse, downward along the anterior root, c, to the muscles whence the sensation was received. This is the reflex action of the spinal cord. There is no consciousness or sensation connected with this action, and the removal of the brain and the sympathetic system does not diminish its activity. Even after death it continues for some time, longer in cold-blooded than in warm-blooded animals, on account of the difference in temperature, thus showing this property of the spinal cord. By disease, or the use of certain poisons, this activity may be greatly augmented, as is frequently observed in the human subject. A sudden contact with a different atmosphere may induce these movements. The contraction of the muscles, or cramp, often experienced by all persons, in stepping into a cold bath, or emerging from the cozy sitting-room into a chilly December temperature, are familiar illustrations of reflex movements. It has been demonstrated that the irritability of the nerves may be impaired or destroyed, while that of the muscles to which they are distributed remains unchanged; and that the motor and sensory classes of filaments may be paralyzed independently of each other.
The reflex actions of the spinal cord have been admirably summed up by Dr. Dalton, as exerting a general, protective influence over the body, presiding over the involuntary action of the limbs and trunk, regulating the action of the sphincters, rectum, and bladder, and, at the same time, exercising an indirect influence upon the nutritive changes in all parts of the body to which the spinal filaments are distributed.
THE BRAIN. The brain is a complex organ, which is divided into the medulla oblongata, the cerebellum, and the cerebrum.
The medulla oblongata is situated just above the spinal cord, and is continuous with it below, and the brain above. It has distinct functions which are employed in the preservation and continuance of life. It has been termed the "vital knot," owing to the fact that the brain may be removed and the cord injured and still the heart and lungs will continue to perform their functions, until the medulla oblongata is destroyed.
The arrangement of the white and gray matter of the medulla oblongata is similar to that of the spinal cord; that is to say, the white matter is external and the gray internal; whereas in the cerebellum and cerebrum this order is reversed. The fibres of the spinal cord, before entering this portion of the brain, decussate, those from the right side crossing to the left, and those from the left crossing to the right side. By some authors this crossing of the sensory and motor filaments has been supposed to take place near the medulla oblongata. Dr. Brown-Sequard shows, however, that it takes place at every part of the spinal cord. The medulla oblongata is traversed by a longitudinal fissure, continuous with that of the spinal cord. Each of the lateral columns thus formed are subdivided into sections, termed respectively the Corpora Pyramidalia, the Corpora Olivaria, the Corpora Restiformia and the Posterior Pyramids.
The Corpora Pyramidalia (see 1, 1, Fig. 58) are two small medullary eminences or cords, situated at the posterior surface of the medulla oblongata; approaching the Pons Varolii these become larger and rounded.
The Corpora Olivaria (3, 3, Fig. 58) are two elliptical prominences, placed exterior to the corpora pyramidalia. By some physiologists these bodies are considered as the nuclei, or vital points, of the medulla oblongata. Being closely connected with the nerves of special sensation, Dr. Solly supposed that they presided over the movements of the larynx.
The Corpora Restiformia (5, 5, Fig. 59) are lateral and posterior rounded projections of whitish medulla, which pass upward to the cerebellum and form the crura cerebelli, so called because they resemble a leg. The filaments of the pneumogastric nerve originate in the ganglia of these parts.
The Posterior Pyramids are much smaller than the other columns of the medulla oblongata. They are situated (4, 4, Fig. 59) upon the margin of the posterior fissures in contact with each other.
The functions of the medulla oblongata, which begin with the earliest manifestations of life, are of an instinctive character. If the cerebellum and cerebrum of a dove be removed, the bird will make no effort to procure food, but if a crumb of bread be placed in its bill, it is swallowed naturally and without any special effort. So also in respiration the lungs continue to act after the intercostal muscles are paralyzed; if the diaphragm loses its power, suffocation is the result, but there is still a convulsive movement of the lungs for sometime, indicating the continued action of the medulla oblongata.
The Cerebellum, or little brain, is situated in the posterior chamber of the skull, beneath the tentorium, a tent-like process of the dura mater which separates it from the cerebrum. It is convex, with a transverse diameter of between three and one-half and four inches, and is little more than two inches in thickness. It is divided on its upper and lower surfaces into two lateral hemispheres, by the superior and inferior vermiform processes, and behind by deep notches. The cerebellum is composed of gray and white matter, the former being darker than that of the cerebrum. From the beautiful arrangement of tissue, this organ has been termed the arbor vitae.
The peduncles of the cerebellum, the means by which it communicates with the other portions of the brain, are divided into three pairs, designated as the superior, middle and inferior. The first pass upward and forward until they are blended with the tubercles of the corpora quadrigemina. The second are the crura cerebelli, which unite in two large fasciculi, or pyramids, and are finally lost in the pons varolii. The inferior peduncles are the corpora restiformia, previously described, and consist of both sensory and motor filaments. Some physiologists suppose that the cerebellum is the source of that harmony or associative power which co-ordinates all voluntary movements, and effects that delicate adjustment of cause to effect, displayed in muscular action. This fact may be proved by removing the cerebellum of a bird and observing the results, which are an uncertainty in all its movements, and difficulty in standing, walking, or flying, the bird being unable to direct its course. In the animal kingdom we find an apparent correspondence between the size of the cerebellum and the variety and extent of the movements of the animal. Instances are cited, however, in which no such proportion exists, and so the matter is open to controversy. The general function of the cerebellum, therefore, cannot be explained, but the latest experiments in physiological and anatomical science seem to favor the theory that it is in some way connected with the harmony of the movements. This co-ordination, by which the adjustment of voluntary motion is supposed to be effected, is not in reality a faculty having its seat in the brain substance, but is the harmonious action of many forces through the cerebellum.
The Cerebrum occupies five times the space of all the other portions of the brain together. It is of an ovoid form, and becomes larger as it approaches the posterior region of the skull. A longitudinal fissure covered by the dura mater separates the cerebrum into two hemispheres, which are connected at the base of the fissure, by a broad medullary band, termed the corpus callosum. Each hemisphere is subdivided into three lobes. The anterior gives form to the forehead, the middle rests in the cavity at the base of the skull, and the posterior lobe is supported by the tentorium, by which it is separated from the cerebellum beneath. One of the most prominent characteristics of the cerebrum is its many and varied convolutions These do not correspond in all brains, nor even on the opposite sides of the same brain, yet there are certain features of similarity in all; accordingly, anatomists enumerate four orders of convolutions. The first order begins at the substantia perforata and passes upward and around the corpus callosum toward the posterior margin of that body, thence descends to the base of the brain, and terminates near its origin. The second order originates from the first, and subdivides into two convolutions, one of which composes the exterior margin and superior part of the corresponding hemisphere, while the other forms the circumference of the fissure of Sylvius. The third order, from six to eight in number, is found in the interior portion of the brain, and inosculates between the first and second orders. The fourth is found on the outer surface of the hemisphere, in the space between the sub-orders of the second clasp. A peculiar fact relating to these convolutions is observed by all anatomists: mental development is always accompanied by an increasing dissimilarity between their proportional size.
The cerebral hemispheres may be injured or lacerated without any pain to the patient. The effect seems to be one of stupefaction without sensation or volition. A well-developed brain is a very good indication of intelligence and mental activity. That the cerebrum is the seat of the reasoning powers, and all the higher intellectual functions, is proved by three facts. (1.) If this portion of the brain is removed, it is followed by the loss of intelligence. (2.) If the human cerebrum is injured, there is an impairment of the intellectual powers. (3.) In the animal kingdom, as a rule, intelligence corresponds to the size of the cerebrum. This general law of development is modified by differences in the cerebral texture. Men possessing comparatively small brains may have a vast range of thought and acute reasoning powers. Anatomists have found these peculiarities to depend upon the quantity of gray matter which enters into the composition of the brain.
In the cerebro-spinal system there are three different kinds of reflex actions. (1.) Those of the spinal cord and medulla oblongata are performed without any consciousness or sensation on the part of the subject. (2.) The second class embraces those of the tuber annulare, where the perception gives rise to motion without the interference of the intellectual faculties. These are denominated purely instinctive reflex actions, and include all those operations of animals which seem to display intelligent forethought; thus, the beaver builds his habitation over the water, but not a single apartment is different from the beaver homestead of a thousand years ago; there is no improvement, no retrogression. Trains of thought have been termed a third class of reflex actions. It is evident that the power of reasoning is, in a degree, possessed by some of the lower-animals: for instance, a tribe of monkeys on a foraging expedition will station guards at different parts of the field, to warn the plunderers of the approach of danger. A cry from the sentinel, and general confusion is followed by retreat. Reason only attains its highest development in man, in whom it passes the bounds of ordinary existence, and, with the magic wand of love, reaches outward into the vast unknown, lifting him above corporeal being, into an atmosphere of spiritual and divine Truth.
THE CRANIAL NERVES. From the brain, nerves are given off in pairs, which succeed one another from in front backwards to the number of twelve. The first pair, the olfactory nerves, are the nerves of the sense of smell. The second pair are the optic, or the nerves of the sense of sight. The third pair are called the motores oculi, the movers of the eye, from the fact that they are distributed to all the muscles of the eye with the exception of two. The fourth pair and the sixth pair each supply one of the muscles of the eye, on each side, the fourth extending to the superior oblique muscle, and the sixth to the external rectus muscle. The nerves of the fifth pair are very large; they are each composed of two bundles of filaments, one motor and the other sensory, and have, besides, an additional resemblance to a spinal nerve by having a ganglion on each of their sensory roots, and, from the fact that they have three chief divisions, are often called the trigeminal, or trifacial, nerves. They are nerves of special sense, of sensation, and of motion. They are the sensitive nerves which supply the cranium and face, the motor nerves of the muscles of mastication, the buccinator and the masseter, and their third branches, often called the gustatory, are distributed to the front portion of the tongue, and are two of the nerves of the special sense of taste. The seventh pair, called also the facial nerves, are the motor nerves of the muscles of the face, and are also distributed to a few other muscles; the eighth pair, termed the auditory nerves, are the nerves of the special sense of hearing. As the seventh and eighth pairs of nerves emerge from the cavity of the skull together, they are frequently classed by anatomists as one, divided into the facial, or portio dura, as it is sometimes called, and the auditory, or portio mollis. The ninth pair, called the glosso-pharyngeal, are mixed nerves, supplying motor filaments to the pharyngeal muscles and filaments of the special sense of taste to the back portion of the tongue. The tenth pair, called the pneumogastric, or par vagum, are very important nerves, and are distributed to the larynx, the lungs, the heart, the stomach, and the liver, as shown in Fig. 60. This pair and the next are the only cerebral nerves which are distributed to parts of the body distant from the head. The eleventh pair, also called spinal accessory, arise from the sides of the spinal marrow, between the anterior and posterior roots of the dorsal nerves, and run up to the medulla oblongata, and leave the cranium by the same aperture as the pneumogastric and glosso-pharyngeal nerves. They supply certain muscles of the neck, and are purely motor. As the glosso-pharyngeal, pneumogastric, and spinal accessory nerves leave the cranium together, they are by some anatomists counted as the eighth pair. The twelfth pair, known as the hypoglossal, are distributed to the tongue, and are the motor nerves of that organ.
THE GREAT SYMPATHETIC.
A double chain of nervous ganglia extends from the superior to the inferior parts of the body, at the sides and in front of the spinal column, and is termed, collectively, the system of the great sympathetic. These ganglia are intimately connected by nervous filaments, and communicate with the cerebro-spinal system by means of the motor and sensory filaments which penetrate the sympathetic. The nerves of this system are distributed to those organs over which conscious volition has no direct control.
Four of the sympathetic centers, situated in the front and lower portions of the head, are designated as the ophthalmic, spheno-palatine, submaxillary and otic ganglia. The first of these, as its name indicates, is distributed to the eye, penetrates the sclerotic membrane (the white, opaque portion of the eyeball, with its transparent covering), and influences the contraction and dilation of the iris. The second division is situated in the angle formed by the sphenoid and maxillary bone, or just below the ear. It sends motor and sensory filaments to the palate, and velum palati. Its filaments penetrate the carotid plexus, are joined by others from the motor roots of the facial nerve and the sensory fibres of the superior maxillary. The third division is located on the submaxillary gland. Its filaments are distributed to the sides of the tongue, the sublingual, and submaxillary glands. The otic ganglion is placed below the base of the skull, and also connects with the carotid plexus. Its filaments of distribution supply the internal muscles of the malleus, the largest bones of the tympanum, the membranous linings of the tympanum and the eustachian tube. Three ganglia, usually designated as the superior, middle, and inferior, connect with the cervical and spinal nerves. Their interlacing filaments are distributed to the muscular walls of the larynx, pharynx, trachea, and esophagus, and also penetrate the thyroid gland. The use of this gland is not accurately known. It is composed of a soft, brown tissue, and consists of lobules contained in lobes of larger size. It forms a spongy covering for the greater portion of the larynx, and the first section of the trachea. That it is an important organ, is evident from the fact that it receives four large arteries, and filaments from two pairs of nerves.
The sympathetic ganglia of the chest correspond in number with the terminations of the ribs, over which they are situated. Each ganglion receives two filaments from the intercostal nerve, situated above it, thus forming a double connection. The thoracic ganglia supply with motor fibres that portion of the aorta which is above the diaphragm, the esophagus, and the lungs.
In the abdomen the sympathetic centers are situated upon the coeliac artery, and are termed, collectively, the semilunar coeliac ganglion. Numerous inosculating branches radiate from this center and are called, from the method of their distribution, the solar plexus. From this, also, originate other plexi which are distributed to the stomach, liver, kidneys, intestines, spleen, pancreas, supra-renal glands, and to the organs of generation. Four other pairs of abdominal ganglia connected with, the lumbar branches are united by filaments to form the semilunar ganglion.
The sympathetic ganglia of the pelvis consist of five pairs, which are situated upon the surface of the sacrum. At the extremity of the spinal column this system terminates in a single knot, designated as the ganglion impar.
Owing to the position of the sympathetic ganglia, deeply imbedded in the tissues of the chest and abdomen, it is exceedingly difficult to subject them to any satisfactory experiments. A few isolated facts form the basis of all our knowledge concerning their functions. They give off both motor and sensory filaments. The contraction of the iris is one of the most familiar examples of the action of the sympathetic system.
In the reflex actions of the nerves of special sense, the sensation is transmitted through the cerebro-spinal system, and the motor impulse is sent to the deep-seated muscles by the sympathetic system. Physiologists enumerate three kinds of reflex actions, which are either purely sympathetic, or partially influenced by the cerebro-spinal system. Dr. Dalton describes them as follows:
First.—"Reflex actions taking place from the internal organs, through the sympathetic and cerebro-spinal systems, to the voluntary muscles and sensitive surfaces.—The convulsions of young children are often owing to the irritation of undigested food in the intestinal canal. Attacks of indigestion are also known to produce temporary amaurosis [blindness], double vision, strabismus, and even hemiplegia. Nausea, and a diminished or capricious appetite, are often prominent symptoms of early pregnancy, induced by the peculiar condition of the uterine mucous membrane."
Second.—"Reflex actions taking place from the sensitive surfaces, through the cerebro-spinal and sympathetic systems to the involuntary muscles and secreting organs.—Imprudent exposure of the integument to cold and wet, will often bring on a diarrhea. Mental and moral impressions, conveyed through the special senses, will affect the motions of the heart, and disturb the processes of digestion and secretion. Terror, or an absorbing interest of any kind, will produce a dilatation of the pupil, and communicate in this way a peculiarly wild and unusual expression to the eye. Disagreeable sights or odors, or even unpleasant occurrences, are capable of hastening or arresting the menstrual discharge, or of inducing premature delivery."
Third.—"Reflex actions taking place through the sympathetic system from one part of the body to another.—The contact of food with the mucous membrane of the small intestine excites a peristaltic movement in the muscular coat. The mutual action of the digestive, urinary, and internal generative organs upon each other takes place entirely through the medium of the sympathetic ganglia and their nerves. The variation of the capillary circulation in different abdominal viscera, corresponding with the state of activity or repose of their associated organs, are to be referred to a similar nervous influence. These phenomena are not accompanied by any consciousness on the part of the individual, nor by any apparent intervention of the cerebro-spinal system."
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THE SPECIAL SENSES.
The eye is the organ through which we perceive, by the agency of light, all the varied dimensions relations, positions, and visible qualities of external objects.
The number, position, and perfection of the eyes, vary remarkably in different orders, in many instances corresponding to the mode of life, habitation, and food of the animal. A skillful anatomist may ascertain by the peculiar formation of the eye, without reference to the general physical structure, in what element the animal lives. Sight is one of the most perfect of the senses, and reveals to man the beauties of creation. The aesthetic sentiment is acknowledged to be the most refining element of civilized life. Painting, sculpture, architecture, and all the scenes of nature, from a tiny way-side flower to a Niagara, are subjects in which the poet's eye sees rare beauties to mirror forth in the rhythm of immortal verse.
In the vertebrates, the organs of vision are supplied with filaments from the second pair of cranial nerves. In mammalia, the eyes are limited to two in number, which in man are placed in circular cavities of the skull, beneath the anterior lobes of the cerebrum. Three membranes form the lining of this inner sphere of the eye, called respectively the Sclerotic, Choroid, and Retina.
The Sclerotic, or outer covering, is the white, firm membrane, which forms the larger visible portion of the eyeball. It is covered in front by a colorless, transparent segment, termed the cornea, which gives the eye its lustrous appearance. Within the sclerotic, and lining it throughout, is a thin, dark membrane termed the Choroid. Behind the cornea it forms a curtain, called the iris, which gives to the eye its color. The muscles of the iris contract or relax according to the amount of light received, thus enlarging or diminishing the size of the circular opening called the pupil. The Retina is formed by the optic nerve, which penetrates the sclerotic and choroid and spreads out into a delicate, grayish, semi-transparent membrane. The retina is one of the most essential organs of vision, and consists of two layers. A spheroidal, transparent body, termed the crystalline lens, is situated directly behind the pupil. It varies in density, increasing from without inward, and forms a perfect refractor of the light received. The space in front of the crystalline lens is separated by the iris into two compartments called respectively the anterior and posterior chambers. The fluid contained within them, termed the aqueous humor, is secreted by the cornea, iris, and ciliary processes. The space behind the crystalline lens is occupied by a fluid, called the vitreous humor. This humor is denser than the other fluids and has the consistency of jelly, being perfectly transparent. "The function of the crystalline lens is to produce distinct perception of form and outline." The transparent humors of the eye also contribute to the same effect, but only act as auxiliaries to the lens.
The figure on the next page represents the course of the rays of light proceeding from an object a b, refracted by the lens, and forming the inverted image x y on the screen. All rays of light proceeding from b are concentrated at y, and those proceeding from a converge at x. Rays of light emanating from the center of the object a b pursue a parallel course, and form the center of the image. Rays of light passing through a double convex lens converge at a point called the focus. In the organ of vision, if perfect, the focus is on the retina, which serves as a screen to receive the image or impression. We have a distinct perception of the outline of a distant hill, and also of a book lying before us. The rays of light we receive from these objects cannot have the same focus. How, then, can we account for the evident accommodation of the eye to the varying distances? Various theories have been advanced to explain this adjustment; such as changes in the curvature of the cornea and lens; a movement of the lens, or a general change in the form of the eyeball, by which the axis may be lengthened or shortened.
Two facts comprise all the positive knowledge which we possess on this subject. Every person is conscious of a muscular effort in directing the eye to a near object" as a book, and of fatigue, if the attention is prolonged. If, now, the eyes be directed to a distant object, there will result a sense of rest, or passiveness. By various experiments it has been proved that the accommodation or adjustment of the eye for near objects requires a muscular effort, but for distant objects the muscles are in an essentially passive condition. An increase in the convexity of the crystalline lens is now admitted to be necessary for a distinct perception of near objects. We may give two simple illustrations, cited by Dr. Dalton in his recent edition of Human Physiology. If a candle be held near the front of an eye which is directed to a distant object, three reflected images of the flame will be seen in the eye, one on each of the anterior surfaces of the cornea and lens, and a third on the posterior surface of the latter. If the eye is directed to a near object, the reflection on the cornea remains unchanged, while that on the anterior surface of the lens gradually diminishes and approximates in size the reflection on the cornea, thus giving conclusive evidence that, in viewing a near object, the anterior surface of the crystalline lens become more convex, and at the same time approaches the cornea. Five or six inches is the minimum limit of the muscular adjustment of the eye. From that point to all the boundless regions of space, to every star and nebulae which send their rays to our planet, human vision can reach. It is the sense by which we receive knowledge of the myriads of worlds and suns which circle with unfailing precision through infinite space.
Hearing depends upon the sonorous vibrations of the atmosphere. The waves of sound strike the sensitive portions of the ear, and their impressions upon the auditory nerves are termed the sensations of hearing. The ear is divided into three parts, called respectively the External, Middle, and Internal ear.
The external organs of hearing are two in number, and placed on opposite sides of the head. In most of the higher order of vertebrates, they are so situated as to give expression and proportion to the facial organs, and, at the same time, to suit the requirements of actual life.
The External ear is connected with the interior part by a prolongation of its orifice, termed the external auditory meatus. In man, this gristly portion of the auditory apparatus is about one inch in length, lined by a continuation of the integument of the ear, and has numerous hairs on its surface, to prevent the intrusion of foreign substances. Between the external MEATUS and the cavity of the middle ear is the membrana tympani, which is stretched across the opening like the head of a drum. The tympanum, or ear-drum, communicates with the pharynx by the eustachian tube, which is a narrow passage lined with delicate, ciliated epithelium. On the posterior portion it is connected with the mastoid cells. Three small bones are stretched across the cavity of the tympanum, and called, from their form, the malleus, incus and stapes, or the hammer, anvil, and stirrup. Agassiz mentions a fourth, which he terms the os orbiculare. Each wave of sound falling upon the membrana tympani, throws its molecules into vibrations which are communicated to the chain of bones, which, in turn, transmits them to the membrane of the foramen ovale. The three muscles which regulate the tension of these membranes are termed the tensor tympani, laxator tympani, and stapedium tympani.
The Labyrinth, or Internal ear, is a complicated cavity, consisting of three portions termed the vestibule, cochlea, and semi-circular canals. The vestibule is the central portion and communicates with the other divisions. The labyrinth is filled with a transparent fluid, termed perilymph, in which are suspended, in the vestibules and canals, small membranous sacs, containing a fluid substance, termed endolymph (sometimes called vitrine auditive from its resemblance to the vitreous humor of the eye). The filaments of the auditory nerve penetrate the membranous tissues of these sacs, and also of those suspended at the commencement of the semi-circular canals. These little sacs are supposed to be the seat of hearing, and to determine, in some mysterious way, the quality, intensity and pitch of sounds.
The determination of the direction of sound is a problem of acoustics. Some have contended that the arrangement of the semi-circular canals is in some way connected with this sensation. But this supposition, together with the theory of the transmission of sound through the various portions of the cranial bones, has been exploded.
From the foregoing description, it will be seen that the labyrinth and tympanum are the most essential parts of the organs of hearing. In delicacy and refinement this sense ranks next to sight. The emotions of beauty and sublimity, excited by the warbling of birds and the roll of thunder, are scarcely distinguishable from the intense emotions arising from sight. It is a remarkable fact, that the refinement or cultivation of these senses is always found associated. Those nations which furnish the best artists, or have the highest appreciation of painting and sculpture, produce the most skillful musicians, those who reduce music to a science.
Next in order of delicacy, and more closely allied with the physical functions, is the sense of smell. Delicate perfumes, or the fragrance of a flower, impart an exhilarating sensation of delight, while numerous odors excite a feeling of disgust. The organ of smell is far less complicated in its structure than the eye or the ear. It consists of two cavities having cartilaginous walls, and lined with a thick mucous coat, termed the pituitary membrane, over which are reflected the olfactory nerves. Particles of matter, too minute to be visible even through the microscope, are detached from the odorous body and come in contact with the nerves of smell, which transmit the impressions or impulses thus received to the brain. Fig. 65 shows the distribution of the olfactory nerves in the nasal passages. The nose is supplied with two kinds of filaments which are termed respectively nerves of special and nerves of general sensation. Compared with the lower animals, especially with those belonging to the carnivorous species, the sense of smell in man is feeble. The sensation of smell is especially connected with the pleasures and necessities of animal life.
The sense of taste is directly connected with the preservation and nutrition of the body. A delicious flavor produces a desire to eat a savory substance. Some writers on hygiene have given this sense an instinctive character, by assuming that all articles having an agreeable taste are suitable for diet. The nerves of taste are distributed over the surface of the tongue and palate, and their minute extremities terminate in well developed papillae. These papillae are divided into three classes, termed, from their microscopic appearance, filiform, fungiform and circumvallate. The organ of taste is the mucous membrane which covers the back part of the tongue and the palate. The papillae of the tongue are large and distinct, and covered with separate coats of epithelium. The filiform papillae are generally long and pointed and are found over the entire surface of the tongue. The fungiform are longer, small at the base and broad at the end. The circumvallate are shaped like an inverted V and are found only near the root of the tongue; the largest of this class of papillae have other very small papillae upon their surfaces. It is now pretty satisfactorily established that the circumvallate, or fungiform papillae are the only ones concerned in the special sense of taste.
The conditions necessary to taste are, that the substance be in solution either by artificial means, or by the action of the saliva; and that it be brought in contact with the sensitive filaments imbedded in the mucous membrane. The nerves of taste are both general and special in their functions. If the general sensibility of the nerves of taste is unduly excited, the function of sensibility is lost for some time. If a peppermint lozenge is taken into the mouth, it strongly excites the general sensibilities of taste, and the power of distinguishing between special flavors is lost for a few moments. A nauseous drug may then be swallowed without experiencing any disagreeable taste.
Paralysis of the facial nerve often produces a marked effect in the sensibility of the tongue. Where this influence lies has not been fully explained; probably it is indirect, being produced by some alteration in the vascularity of the parts or a diminution of the salivary secretions.
By the sense of touch, we mean the general sensibility of the skin. Sensations of heat and cold are familiar illustrations of this faculty. By the sense of touch, we obtain a knowledge of certain qualities of a body, such as form consistency, roughness, or smoothness of surface, etc. The tip of the tongue possesses the most acute sensibility of any portion of the body, and next in order are the tips of the fingers. The hands are the principal organs of tactile sensation. The nerves of general sensibility are distributed to every part of the cutaneous tissue. The contact of a foreign body with the back, will produce a similar tactile sensation, as with the tips of the fingers. The sensation, however, will differ in degree because the back is supplied with a much smaller number of sensitive filaments; in quality it is the same.
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By means of the nervous system, an intimate relation is maintained between mind and body, for nervous energy superintends the functions of both. The fibres of nervous matter are universally present in the organization, uniting the physical and spiritual elements of man's being. Even the minutest nerve-rootlets convey impressions to the dome of thought and influence the intellectual faculties. We recognize muscular force, the strength of the body, molecular force, molecules in motion, as heat, light, chemical force, electricity, and nervous force, a certain influence which reacts between the animal functions and the cerebrum, thus connecting the conditions of the body with those of the mind. We cannot speak of the effects of mind or body separately, but we must consider their action and reaction upon each other, for they are always associated. There are many difficulties in understanding this relationship, some of which may be obviated by a study of the development of nervous matter, and its functions in the lower orders of organization.