in fact, like cords attached to levers, and operate according to strict mechanical principles.

The organs, by whose operation the digestion of food, the circulation of the blood, and the other important functions are performed, are contained in the three cavities of the cranium, the thorax, and the abdomen, which have been already cursorily described: We proceed to a consideration of these several functions, beginning with those of the brain and senses.

The brain, in man, is the grand centre of sensation and perception. It is the instrument through which the mind maintains its connection with the body; and this connection is extended from the brain to the other parts by means of nerves. The brain is a large organ, of a peculiar texture, occupying the whole of the cavity of the cranium, and consisting of several distinct parts. Several pairs of nerves proceed from it through different apertures in the skull, and are distributed to the parts about the head, to convey to them the powers of sensation and motion. But besides these, there is another large single nerve passing down into the canal formed by the vertebræ, already described, and supplying the greater part of the body and limbs. (Fig. 3.)

Through the nerves, impressions are transmitted from all parts of the body to the brain; and on the other hand, all the acts of the will produce an effect upon the different organs by their means. The nerves are necessary to the exercise of the senses (which in man are five: seeing, hearing, smelling, tasting, and feeling); for if the nerve going to the organ of either of these senses be injured, the mind no longer receives any impression from that sense, as happens in the disease of the eye called gutta serena, or amaurosis. And if the nerve going to any of the limbs be destroyed or obstructed, both sensation and power of motion in that limb are either destroyed or suspended. This happens when a limb, from long-continued pressure upon it, is said to be asleep; as, in sitting for some time in one particular position, the nerve going to one of the legs is pressed upon, and the connection with the brain being thus interrupted, the consequence is a loss of feeling and motion, which is sometimes so great as to cause the person affected to fall down, on attempting to walk.

The senses, taken altogether, are more perfect in man than

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; 7, 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