NERVOUS SYSTEM IN MAN. - In this figure is represented the general distribution of the brain and nerves throughout the body.

a, the principal portions of the brain or cerebrum, called the hemispheres; b, a smaller distinct portion, the cerebellum; c, the spinal nerve which passes down the back through a canal formed by the vertebræ; d, nerves of the face; e, a network or collection of nerves to supply the arm; f, g, h, i, nerves of the arm; j, of the ribs; k, of the loins; 1, of the hip and thigh or the sciatic; m, n, o, of the leg.

in any other animal. Yet in each of them, individually, he is probably excelled by some particular species. Thus, in sight, he is exceeded by the vulture and eagle; in hearing, by the greater number of rapacious quadrupeds; in smell, by the dog; in taste, by a great many animals; and in nicety and delicacy of touch and feeling, by most insects.

In performing the function of digestion, the food is in the first place taken into the mouth, mixed with the saliva, and ground into a kind of paste, by the action of the jaws and teeth. It is then swallowed through a long muscular canal, the œsophagus or gullet, which passes through the thorax behind the heart and lungs, near the backbone, and is conveyed into the stomach, through its upper or cardiac orifice. This is an irregularly shaped muscular bag or sack, situated in the upper part of the abdomen, at the spot usually called the pit of the stomach. It is capable of great distention or contraction, according to the quantity which is put into it. In the stomach, the food is acted on by a peculiar fluid, called the gastric juice. It has no remarkable sensible qualities, and is nearly tasteless and destitute of odor; but its operation upon the substances exposed to its influence is very decided and powerful. They are gradually reduced, of whatever kind they may be, to one homogeneous mass, called chyme, of a grayish color, and of a consistence like that of thick cream. This operation being completed, the chyme passes out of the stomach, by its lower or pyloric orifice, situated towards the right side, into the intestines, which form a long canal, and, taken together, are many times longer than the body.

In the intestines, the chyme is subjected to the action of the bile and pancreatic juice. The bile, or gall, is a brown-colored, viscid, and very bitter fluid, prepared by the liver, a large organ on the right side, just beneath the ribs, and collected into the gall-bladder, where a part of it is reserved for use. The pancreatic juice resembles very nearly the saliva in color and appearance, and is prepared by the pancreas, an organ situated just below the stomach. These two fluids, the whole of whose office is only imperfectly understood, are mixed with the chyme, which is then separated into two parts. One of these is a thin, milky fluid, called chyle; the remainder consists of those portions

of the food which are not fit for the nourishment of the system, but are rejected and thrown out of it, as useless. The chyle is gradually absorbed by capillary vessels, called the lacteals, opening into the intestines through nearly their whole course, and conveyed into a vessel called the thoracic duct. This duct ascends from the abdomen along the back into the thorax, and there empties its contents into the vein coming from the left arm, or left subclavian, where the chyle is immediately mingled with the mass of blood, and enters with it into the circulation. The chyle is essentially of the same composition from whatever substance it is prepared, when the digestion is perfect. Some kinds of food, however, are capable of furnishing a larger proportion of it than others: this is the case with animal food, of which it takes a smaller quantity to supply the system with nourishment, than of vegetable. Animal substances are not required to undergo so great a change in their conversion into chyle, as vegetable; and hence it is observed, that in animals subsisting on vegetables, the digestive organs are more various, extensive, and complicated, than in those which are carnivorous.

The chyle, being mixed with the blood, becomes a part of it, although it is not known where or how its change from the white

Contents of the thorax or chest in man: the windpipe or trachea, lungs, heart, and

blood vessels.

to the red color is effected. It is then circulated throughout the body, by the heart, the arteries, and the veins. The heart is a hollow muscular organ, the main-spring of the circulation; the arteries are long cylindrical canals or pipes, carrying the blood from the heart to the different parts of the body; the veins are vessels of a similar form and structure, bringing the blood back to the heart; after it has gone the round of circulation.

The relation of the several parts of the heart to each other is illustrated in the following diagram. It supposes the organ to be cut open, and its cavities with their connecting valves to be displayed. The relative size and position of the parts are varied from nature in order to present a more intelligible view of their mechanism.

The heart in man is a double organ; that is, it consists of two complete and distinct organs, united together into one mass, but performing their functions without interference or connection. These two parts are called the right and left sides of the heart; and each has two distinct cavities, called auricles and ventricles. The right side of the heart receives the blood from the body at large, and sends it to the lungs; the left receives it from the lungs, and sends it to the body. The heart is of a conoidal shape, situated in the thorax, just within the sternum, a little inclining to the left side. It is, however, placed with the apex, or point of the cone, extending downwards and to the left, so that it touches the ribs at the spot where the beating is felt, and hence

is often supposed to lie entirely on the left side. The main body of the heart is composed of the two ventricles, which are strong muscular cavities, the left far more so than the right; the auricles are situated around the base of the organ, seeming rather to be loose appendages than constituent parts of it.

We shall begin with the course of the blood at the point where it receives its new supply from the chyle. The subclavian vein, after uniting with the vein from the other arm and the veins coming down from the head and neck, conveys its blood immediately to the right auricle, where it meets with that brought from the lower parts of the body. The two trunks which bring the venous blood in this way to the heart, are called the descending and ascending vena cava. They pour their blood into the right auricle, which contracts, and expels it, through an opening for that purpose, into the right ventricle. This opening is guarded by the tricuspid valves, which prevent the flowing back of the blood, by completely closing the passage. When the ventricle has become distended, it contracts in its turn, and the blood, being prevented from returning to the auricle, is thrown forward into the pulmonary artery, which carries it to the lungs. This passage is also guarded by valves.

At the time of its passage through the right side of the heart, the blood is of a dark bluish red or purple color, approaching almost to black. It is generally called black blood, and is neither fit for circulation in the vessels, nor for the nourishment of the different parts. In this state it is sent to the lungs. These fill up all that part of the cavity of the chest not occupied by the heart, which they nearly surround upon all sides. They consist principally of a collection of blood and air vessels, and are constantly supplied with air, which is drawn in through the windpipe, and distributed to every part of them. The blood is circulated throughout their substance, by the branches of the pulmonary artery, and is, in its course, exposed to the influence of the air. By this means, its color is changed to a bright crimson or vermilion, and it becomes fit for the purposes of life.

It is now brought back to the left side of the heart, by the pulmonary veins, and passes through the left auricle and ventricle, in a manner similar to that which has been already de