Fig. 5

a, the salivary gland in the cheek; b, the duct leading to the mouth; c, the gland under the edge of the under jaw. The stomach next receives the food which passes down the meat-pipe (fig. 6, gg.) The whole coating of the stomach discharges a liquid slightly acid, called gastric juice, which is the solvent. The muscles of the stomach cause it to make a churning action, by contracting alternately in the length and circumference. Thus, by mechanical and chemical operations, the food is converted into chyme, in which state it passes the pylorus (k, fig. 6), a valve which allows the passage downwards, but not upwards, except in unnatural excitement. After passing the pylorus, which is shown at d, fig. 7, with the internal coating of the stomach, the gall-bladder, k, discharges upon the chyme in the duodenum, g, the liquid called bile (the produce of the liver), by the action of which, and also of the pancreatic juice through the duct

pared for reception into the blood-vessels, and is then called chyle. In this state it travels through the small guts, n n, and is taken up by a number of small vessels called lacteals. Figs. 8 and 9 will show how the lacteals absorb the chyle and carry it to the thoracic duct, and thence how it flows to the veins and the heart.

The alimentary canal is about twelve times the length of a man's trunk. The rejected portions of the food, together with the waste portions of the discharge from the liver and pancreas, are received into the large gut called the colon for evacuation, while the kidneys and bladder receive the liquid impurities for the same purpose. The food has been traced to its conversion into chyle, which is a white transparent fluid, four-fifths of its weight being water. The chyle passes on to the heart, and thence flows through the lungs, to be operated upon by the air, and afterwards supplies every portion of the human frame. The air-passage to the lungs (f, fig. 6), is called the windpipe, and is separated from the meat-pipe by a valve e, called the epiglottis, which is so situated as to shut in the act of swallowing, but is otherwise always open to admit of respiration. It is from a peculiar and complicated arrangement of the cartilageous muscles on the top of the windpipe,

called the larynx, that the sound and modulation of the voice is obtained, and under the control of the individual. The complex arrangement of these parts adapting them to the varied expression of sound, it is impossible to give an idea of, being in itself a complete study for the anatomist. The respiratory organs, the lungs, and the heart are above the stomach, and divided from it by a strong muscular partition, on which they rest (m m, fig. 6); they are also protected by the ribs.

known by Harvey. The heart is of a conical form; the blood all passes through it, and from thence is distributed to the whole body. The pure blood goes out from the heart in the arteries; the impure blood returns to it in the veins; so that there is a double set of what are ordinarily termed veins over the whole system. These veins and arteries are composed of three coats, one over the other, and internally are furnished with proper valves to keep the flow of blood in the right direction.

The quantity of blood in a healthy person is estimated at 28 lbs., which is propelled by the heart, so as to complete the circulation of the body in about 2 minutes; and it is calculated one hogshead must pass through the heart every hour. From this fact the importance of pure blood may be felt, as also the injury that must be produced by the introduction of poisons. It has been hitherto reckoned that some force was exerted by the heart to cause this circulation; but the most rational and enlightened mode of accounting for the circulation of the blood is that electricity is the agency, the arterial vessels being furnished with nerves, which are the conductors of the positive electric fluid, while the veins are without the nervous conductors, and convey the blood in a negative state to the heart, the beating of which is caused by a sort of electric shock. The blood goes back to the heart in the veins in an impure state, and is joined by the chyle from the thoracic duct, (see figs. 8 and 9).. From the heart the blood is forced or conveyed to the lungs (see fig. 11), and spreading through the delicate arteries of this porous body, it there, meets with the air which is sent in at every inspiration through the windpipe and its branches (C e e, fig. 11.) The purification of the blood is here

effected by the affinity of the oxygen of the air for the carbon contained in the blood, thus forming carbonic acid gas, which flies off at every expiration. The blood is thus both oxygenized and electrified, and is now converted from a dark thick fluid to a clear bright red. It then returns by the veins K, to the left ventricle of the heart, and by the aorta H, it spreads through the arteries over the whole system, to nourish all parts of the Fig. 9.

body. The lungs A B, which purify the blood, and into which we impel the air at every breath, are a wonderful arrangement of tubes and air vessels, nerves, and glands; and so numerous are the membraneous linings, that if spread out, they would occupy a surface equal to thirty times that of the body. The quantity of air inhaled daily is supposed to be about fifty-seven hogsheads; from this great quantity the necessity of pure air and ventilation for the enjoyment of health may easily be imagined.

Fig. 10.

m

Shows the spinal column, with dd, the thoracic duct, ascending in front of it, and entering the subclavian vein at s.

a, the left ventricle; b, the right ventricle; ce f, the aorta, the great artery that goes off from the left ventricle; g h i, the arteries that are sent from the arch of the aorta; k, the pul monary artery, that goes from the right ventricle to the lungs 11, branches of the pulmonary artery, going to the two sides of the lungs; m m, the pulmonary veins, which bring the blood back from the lungs to the left side of the heart; n, the right auricle; o, the ascending vena cava; q, the lescending: these two meet, and by their union form the right auricle; p, the veins from the liver, spleen, and bowels; s, the left coronary artery, one of the arteries which nourish the heart.