The substances of which our food is composed then, may be divided into two classes; the nitrogenized, capable of being converted into blood, and thus forming all the organized tissues, which might be called the elements of nutrition, and the non-nitrogenized, which are strictly elements of respiration. Let us now look at the process of digestion, and the change necessary for the food to undergo before it can be taken up by the absorbents. All nitrogenized articles of food are undoubtedly converted into albumen before they can contribute to nutrition.

The food, in passing into the stomach, becomes mingled with the saliva, which possesses to a certain extent, the power like the gastric juice, of acting on azotized compounds. With the saliva there is mingled a large quantity of atmospheric air; this entering the stomach, the oxygen aids digestion, the nitrogen passes off in respiration, perspiration, etc. The inner coat of the stomach, in its natural or healthy state is of a light or pale pink color, varying in its hues according to its full or empty state. It is of a soft or velvet-like appearance, and is constantly covered with a thin, transparent, viscid mucus, lining the whole interior of the organ. By applying irritants to the stomach and observing the effect through a magnifying glass, innumerable lucid points and very fine nervous or vascular papillae can be seen arising from the villous membrane, and protruding through the mucous coat, from which distils a pure, limpid, colorless, slightly viscid fluid. The fluid, thus excited, is invariably acid, and is called the gastric juice. The gastric juice never appears to accumulate in the stomach when fasting, and is seldom if ever discharged from its vessels, except when excited by the natural stimulus of aliment, mechanical irritation of the tubes, or other excitants.

The quantity of the gastric juice secreted from the walls of the stomach depends on the general requirements of the system rather than the amount of food. The juice refuses to dissolve only a certain amount of food, and when that is accomplished, if an excess has been taken, the residue either remains in the stomach or passes into the bowels in its crude form, becoming a source of irritation, pain, and disease. This abuse of the stomach by introducing into it more food than it can readily digest, is often considered a trouble of but few hours' duration, when in reality the worst effects are not felt at first, but the foundations of a disease are laid, oftentimes exceedingly troublesome and painful, and not unfrequently terminating in death.

The gastric juice is decidedly acid, containing among other ingredients, a considerable amount of hydro-chloric or muriatic acid. The process of digestion, at one time, looked upon as the result of vital force, is now generally conceded to be carried on by chemical action. The food in passing into the stomach is subjected to a peculiar peristaltic movement, not only producing a thorough intermixture of the gastric fluid with the alimentary mass, but causing the contents to revolve about the interior from point to point.

As the process of digestion goes on, its result in the form of chyme, varying in color from that of cream to gruel, gradually passes through the pyloric orifice into the duodenum. Here, before it is taken up by the lac-teals, it is mingled with the bile and pancreatic juice, the soda in these fluids neutralizing the acidity of the chyme which it has obtained from the gastric juice. The effect of this admixture is to separate the chyme into three distinct parts, - a reddish brown sediment at the bottom, - a whey-colored fluid in the centre, - and a creamy pellicle at the top. The central portion with the creamy pellicle seems to constitute the chyle absorbed by the lacteals; the creamy matter being chiefly composed of oily particles, and the wheyey fluid having proteine-compounds, saccharine and saline matters in solution, the sediment, partly consisting of the insoluble portion of the food, and partly of the biliary matter itself, is evidently excrementitious.

Let us now glance briefly at the circulation of the blood. The popular explanation given to the circulation of the blood in man, makes the heart the prime-mover of the mechanism. This organ, they say, is devoted to a double purpose, that of a force-pump, in driving the arterial blood through the arteries to every part of the body, and of a suction-pump, to enable the venous blood to return to the heart. In order to accomplish this twofold purpose, it is furnished with valvular and tubular arrangements, and, at specific periods, contracts and dilates for the purpose of ejecting or sucking up the circulating liquid.

Dr. Draper, professor of chemistry in the New-York University, has advanced another theory, which, to say the least, is plausible and worthy of careful investigation. To keep a continuous flow of a liquid through a capillary tube it is necessary, either by evaporation, or some chemical action, to remove the superficial portions of the elevated liquid when they stand at the extremity of the tube. Fill a lamp with oil, and unless you light the wick, no oil would be removed. In an alcohol lamp, if the wick should be uncovered evaporation of the alcohol in the upper portion of the capillary tubes of the wick would take place, and thus a continuous flow of the alcohol would be kept up until the contents of the lamp were exhausted. Take for instance a bladder, which is of course full of capillary tubes, fill it with alcohol, making the mouth of the bladder tight, so that none of the fluid can escape. On placing the bladder in a vessel of water, you find the alcohol gradually coming out of the bladder into the water, and the water flowing more readily, passing much more rapidly into the bladder, so in a short time you have an accumulation in the bladder which distends it, and finally causes it to burst. Thus a constant current is kept up through the walls of the bladder, that having the most affinity for the tubes flowing more rapidly. Watching these phenomena, Dr. Draper lays down the following principle: