both in health and in disease, lactic acid may be found in the stomach. In addition to free hydrochloric acid, the gastric juice contains pepsin, a proteolytic enzyme acting only in an acid medium, and rennin, an enzyme that curdles milk.

Appetite is a great factor in the secretion of the gastric juice, as Pawlow and others have shown. Food introduced without the knowledge of the individual is not digested as rapidly, nor is as much gastric juice secreted as when the appetite has been excited. After digestion has begun there are certain substances which cause a second and more uniform flow of gastric juice. These have been called hormones. The bodies which most easily cause this flow of gastric juice are the products of protein digestion, so that where the flow has been started by appetite it goes on more satisfactorily. Meat juices also contain such substances, and meat soups or broths are, therefore, very useful in stimulating the flow of gastric juice.

Pepsin changes proteins into peptones. The process is a complicated one, and is effected gradually. Kühne's investigations have done much to enlighten us on this point. His methods and terminology have been adopted extensively by recent writers. The process is described as follows: The protein material is changed first into syntonin or acid-albumin. If the solution is rendered alkaline, acid-albumin will be precipitated. The next step is the taking up of water by syntonin, which splits up into several soluble proteins called collectively proteoses-albumose from albumin ; globulose from globulin, etc. These in turn take up more water, and split up into another series of soluble proteins known as the secondary proteoses or deuteroproteoses. These again undergo the same process and form peptones. Gelatinoids are acted upon by pepsin in much the same way as are proteins. The intermediate products have been termed gelatoses or glutoses. The end-product is known as gelatin-peptone.

Rennin curdles milk very rapidly at the body-temperature. The casein is converted from a soluble protein into a more or less solid clot, which gradually becomes firmer and expresses all the whey that was contained in the mass. The casein of cow's milk precipitates in large firm clots; that of human milk, into very fine flocculent particles, which explains the great difference in the digestibility of the two milks.

Action of the Gastric Juice on other Food Elements.— Beyond the mechanical alterations that take place from the presence of fluid and from the churning movements of the

stomach, the starches are not acted upon by the gastric juice. Lusk believes that sugar is inverted in the stomach, whereas in reality it undergoes inversion much more completely in the small intestine. In the stomach fats are, for the most part, dissolved by the body-heat and become thoroughly mixed with the other food elements by the movements of the stomach. Beyond this they remain unchanged.

Intestinal Digestion.-When the food has been passed into the small intestine, it is acted on simultaneously by three secretions-the pancreatic juice, the intestinal juice, and the bile. Although these secretions, as stated, act together, for the sake of simplicity each will be considered separately.

Pancreatic Juice. Our knowledge of the functions of the pancreatic juice is obtained largely from experiments made on the lower animals. In man it enters the intestine together with or closely following or preceding the bile, being mixed with the latter secretion and the food-material at the same time. It is alkaline in reaction, and contains at least three, and probably more, enzymes-viz., trypsin, amylopsin, steapsin, and, it is said, a milk-curdling ferment similar to rennin.

According to Pawlow, the amounts of the various ferments in the pancreatic juice varies with the nature of the food taken, starchy food causing an increase in the amylopsin, and so forth. These statements have not been fully confirmed as yet by other observers. Pawlow has also shown that the presence of bile doubles the activity of the digestive juices.

Trypsin.-Trypsin is a more active ferment than is pepsin, and acts in alkaline, neutral, or even in slightly acid media. It is most active, however, in alkaline solutions. The process by which peptones are formed from proteins is similar to that of peptic digestion, but differs somewhat in detail. Trypsin, however, is capable of carrying on the digestion of peptones further than is pepsin. The steps of the process consist in separating the peptone into an antipeptone, a peptone that can not be acted on further by the ferment, and hemipeptone, which is split up into various simpler substances, such as amido-acids and nitrogenous bases. Among these simpler substances are leucin and tyrosin. Just what rôle these end-products play in the animal economy has not been definitely determined. They do not replace tissuewaste, and are less useful sources of energy than is peptone itself, which is absorbed as such and utilized by the body.

Howell gives the following scheme, modified from Neumeister, to explain graphically tryptic digestion :

The digestion of gelatinoids is similar to that of the proteins. Trypsin produces gelatin-peptone, whereas pepsin, as previously stated, ceases to act with the formation of gelatose.

Amylopsin converts starch into sugar in the same way that ptyalin does. Inasmuch as ptyalin digestion ceases with the entrance of the food into the stomach, it is important that the starches should be completely digested in the small intestine, especially as a large part of the heat and energy consumed by the body is derived from some form of starchy food.

Steapsin, known also as lipase, splits up the neutral fats into glycerin and free fatty acids. This emulsification is of paramount importance in fat-digestion and absorption. The process now becomes again one of hydrolysis. The fat takes up water and splits up into other products. The following formula explains the process:

CH(CH2n+1COO), +3H2O=C2H5(OH), +3(CH2 + 1COOH)
Free fatty acid.

Fat.

Glycerin.

There are two views concerning the absorption of fat. The older view is that the fat splits or is saponified only to a small extent, the larger part of it being emulsified by the fatty acids formed during the splitting-up process. This emulsified fat is then directly absorbed as neutral fat. The view more recently adopted is that all the fat is split up into glycerin and fatty acids, whether or not emulsification has previously occurred. The fatty acids are saponified by the action of the alkaline salts in the intestine, the products being then absorbed, and brought into combination again to form a neutral fat. This recombination may occur in the epithelial cells of the intestine. As the action of lipase is reversible, that is, may split up the fats or it may cause synthesis of the split products. Lipase is found in many tissues of the body, as liver, muscle, and mammary glands.

It is possible that fat is split and re-formed many times in the processes of nutrition.

Emulsification takes place more rapidly in the presence of bile and pancreatic fluid than in the presence of pancreatic fluid alone. Although bile itself causes no emulsification, it aids very materially in the process.

Intestinal Secretion. This is the secretion of the intestinal glands, the crypts of Lieberkühn. It is strongly alkaline from the presence of sodium carbonate, and this may aid in the emulsification of fat. Otherwise the intestinal secretion probably has no action on the proteins or fats. The secretion and the walls of the small intestines contain three ferments which act upon carbohydrates. These are invertase, which acts upon canesugar; maltase, which acts upon maltose and dextrin, and lactase, which acts upon lactose. The walls of the intestines contain also erepsin and enterokinase. Erepsin probably continues or supplements the changes begun by trypsin. It is supposed to act upon the deutero-albumoses and peptones. Erepsin has been found by Vernon in all the tissues of the body. It is present in the kidneys in greater quantities than in the intestinal mucosa. Enterokinase acts upon the pancreatic juice. Apart from the small intestine the pancreatic juice has no digestive action on proteins. The explanation of this is that the juice contains a substance, trypsinogen, capable of being converted into trypsin by the action of enterokinase. As soon as the pancreatic juice comes in contact with the intestinal wall the previously inert trypsinogen is changed into the very active ferment, trypsin.

Secretin. This is not an enzyme, but a definite chemical compound. It is secreted by the wall of the small intestine when acids are brought in contact with it. Secretin is supposed to be absorbed by the blood, and being thus carried to the pancreas, excites the secretion of the pancreatic juice. As we descend the intestinal tract the quantity of enzymes contained in the intestinal secretion becomes smaller. The large intestine secretes mucus but no enzymes.

Bacterial Changes.-The changes produced by bacteria are an extremely important factor in digestion, especially from the pathologic standpoint. The subject can not, however, be entered upon fully here, and for a complete knowledge the student should consult the special text-books on bacteriology. For our present purpose it is sufficient to say that, in the small intestine,

bacterial changes are probably limited to the carbohydrates. Under abnormal conditions, or when excessive quantities of protein food are taken, putrefaction of the proteins may occur. In the large intestine, however, the extreme alkalinity overcomes this acidity, and allows putrefaction of the feces to take place. The products of bacterial action are many, and consist of leucin, tyrosin, phenol, skatol, and various acids and gases. Some of these, after having undergone certain changes, are absorbed and excreted again in the urine. It is not definitely known just what part they play in the nutrition of the body. Judging from the experiments of Nuttall, it is reasonably certain, however, that bacterial action is not essential to nutrition.

ABSORPTION.

In order properly to understand digestion and assimilation it is necessary to know something of absorption. This occurs in two ways: either by the material absorbed entering directly into the blood and passing thence to the liver, or by its entering the lacteals and passing thence through the thoracic duct to enter the blood-current of the left jugular and subclavian veins.

Absorption was formerly believed to take place to a very marked extent in the stomach. This view is now held to be erroneous, probably little or no absorption taking place in this organ. Water, as well as most other liquids, may be absorbed slightly from the stomach. Alcohol may be absorbed in it, and solutions of various salts may be absorbed slowly. Condiments, by stimulating the mucous membrane of the stomach, and increasing the secretion of gastric juice, aid in stomach absorption. Fats are not absorbed by the stomach. Proteins and sugars, if taken in sufficiently concentrated solutions, may be absorbed, the congestion brought about by the use of alcohol or condiments aiding the absorption. On the whole, however, absorption from the stomach is of trifling importance.

Absorption in the Intestine.-Absorption takes place principally in the small intestine. Food passes from the small intestine in from five to twenty hours. On entering the large intestine the food is still in a very fluid condition, notwithstanding the large amount of absorption of water and salts that takes place during its passage through the small intestine.