and is especially distinguished from other such solutions by its diffusibility—i.e., the ease with which it passes through a membrane. Some of these peptones, with

the sugars of the food, whether original or the product of the action of the saliva, are absorbed from the stomach. The food, while in the stomach, is kept in continual motion, and, after a time, is discharged in gushes into the intestine. The name chyme is given to the pulpy mass of food in the stomach.103 In the intestine the

chyme meets three fluids - bile, pancreatic juice, and intestinal juice. All of these are alkaline, and at once give the acid chyme an alkaline reaction. This change permits the action of the saliva to recommence, which is aided by the pancreatic and intestinal juices. The pancreatic juice has much more important functions. It changes albuminoid food into peptones, and probably breaks up the fats into very small particles, which are suspended in the fluid chyle. This forms an emulsion, like milk, and causes the chyle to appear whitish. The bile has important functions, but little understood. It emulsifies and saponifies part of the fats, so that they are dissolved, and perhaps aids in preventing the food from decomposing during the process of digestion and absorption. The chyle is slowly driven through the small intestine by the creeping, peristaltic motion of its walls, 104 the nutritious portion being taken up by the absorbents, as described in the next chapter, while the undigested part remaining is discharged from the large intestine. 105

CHAPTER XIV

THE ABSORBENT SYSTEM

THE nutritive matter (chyle), prepared by the digestive process, is still outside of the organism. How shall it enter the living tissue?

In animals, like the Infusoria and polyps, whose digestive department is not separated from the body cavity, the food, as soon as dissolved, mingles freely with the parts it has to nourish. In the higher invertebrates having an alimentary canal, the chyle passes, by simple transudation, through the walls of the canal directly into the soft tissues, as in insects, or is absorbed from the canal by veins in contact with it, as in sea urchins, mollusks. f worms, and crusta- FIG. 256. ceans, and then distributed through the body.

Section of Injected Small Intestine of Cat: a, b, mucosa; g, villi; i, their absorbent vessels; h, simple follicles; c, muscularis mucosa; d, submucosa; e, e', circular and longitudinal layers of muscle; f, fibrous coat. All the dark lines represent blood vessels filled with an injection mass.

In vertebrates only do we find a special absorbent system. Three sets of vessels are concerned in the general process by which fresh material is taken up and added to the blood: Capillaries, Lacteals, and Lymphatics. Only the two former draw material from the alimentary canal.

The food probably is absorbed almost as fast as it is dissolved, and, therefore, there is a constant loss in the passage down the canal. In the mouth and esophagus, the absorption is slight; but much of that which has yielded to the gastric juice, with most of the water, is greedily absorbed by the capillaries of the stomach, and made to join the current of blood which is rushing to the liver. Absorption by the capillaries also takes place from the skin and lungs. Medicinal or poisonous gases and liquids are readily introduced into the system by these channels.

We have seen that the oily part of the food passes unchanged from the stomach into the small intestine,

where, acted upon by the

pancreatic juice, it is cut up into extremely minute d particles, and that the undigested albuminoids and starches are digested in the intestine. Two kinds of absorbents are present in the intestine, lacteals and blood capillaries. Both the lymphatic and blood systems send vessels into the velvety villi 106 with which the intestine is

FIG. 257. Lacteal System of Mammal: a,

descending aorta, or principal artery; b,

thoracic duct; c, origin of lacteal vessels,

g, in the walls of the intestine, d; e,

mesentery, or membrane attaching the

intestine to walls of the body; f, lacteal,

or mesenteric, glands.

lined. The blood capillaries lie toward the outside of the villus and the lacteal in the center. The albumi

noids and sugars are chiefly absorbed by the blood vessels and go to the liver. The fats pass on into the lacteals, which receive their name from the milky appearance of the chyle. These lacteals unite into larger trunks, which lie in the mesentery (or membrane which suspends the intestine from the back wall of the abdomen), and these pour their contents into one large vessel, the thoracic duct, lying along the backbone, and joining the jugular vein in the neck.

The lacteals are only a special part of the great lymphatic system, which absorbs and carries to the thoracic duct matter from all parts of the body.10 107 The lymph is a transparent fluid having many white blood corpuscles. It is, in fact, blood, minus the red corpuscles, while chyle is the same fluid rendered milky by numerous

a

fat globules. During the FIG. 258. - Principal Lymphatics of the Hu

intervals of digestion, the lacteals carry ordinary lymph. This fluid is the

man Body: a, union of left jugular and subclavian veins; b, thoracic duct; c, receptaculum chyli. The oval bodies are glands.

overflow of the blood - the which escape from the blood capillaries, and carry nutriment to, and waste from, those parts of the various tissues which are not in contact with the blood capillaries.

plasma and white corpuscles

This surplus overflow is returned to the blood by the lymphatics. The current is kept up by the movements of the body, and in many vertebrates, as frogs and fishes, by lymph hearts.

Like the roots of plants, the absorbent vessels do not commence with open mouths; but the fluid which enters them must traverse the membrane which covers their minute extremities. This membrane is, however, porous, and the fluids pass through it by the processes of filtration and diffusion, or dialysis. How the fat gets into the lacteals is not yet well understood, but the lacteals are themselves rhythmically contractile, and force the absorbed chyle toward the heart. The valves of the lymphatics prevent its return.