Absorption.

154. Absorption. While food remains within the alimentary canal it is as much outside of the body, so far as nutrition is concerned, as if it had never been taken inside. To be of any service the food must enter the blood; it must be absorbed. The efficient agents in absorption are the blood-vessels, the lacteals, and the lymphatics. The process through which the nutritious material is fitted to enter the blood, is called absorption. It is a process not confined, as we shall see, simply to the alimentary canal, but one that is going on in every tissue.

The vessels by which the process of absorption is carried on are called absorbents. The story, briefly told, is this: certain food materials that have been prepared to enter the blood, filter through the mucous membrane of the intestinal canal, and also the thin walls of minute blood-vessels and lymphatics, and are carried by these to larger vessels, and at last reach the heart, thence to be distributed to the tissues.

155. Absorption from the Mouth and Stomach. The lining of the mouth and œsophagus is not well adapted for absorption. That this does occur is shown by the fact that certain poisonous chemicals, like cyanide of potash, if kept in the mouth for a few moments will cause death. While we are chewing and swallowing our food, no doubt a certain amount of water and common salt, together with sugar which has been changed from starch by the action of the saliva, gains entrance to the blood.

In the stomach, however, absorption takes place with great activity. The semi-liquid food is separated from the enormous supply of blood-vessels in the mucous membrane only by a thin porous partition. There is, therefore, nothing to prevent the exchange taking place between the blood and the food. Water, along with any substances in the food that have become dissolved, will pass through the partition and enter the blood-current. Thus it is that a certain amount of starch that has been changed into sugar, of salts in solution, of proteids converted into peptones, is taken up directly by the blood-vessels of the stomach.

156. Absorption by the Intestines. Absorption by the intestines is a most active and complicated process. The stomach is really an organ more for the digestion than the absorption of food, while the small intestines are especially constructed for absorption. In fact, the greatest part of absorption is accomplished by the small intestines. They have not only a very large area of absorbing surface, but also structures especially adapted to do this work.

157. The Lacteals. We have learned in Section 144 that the mucous lining of the small intestines is crowded with millions of little appendages called villi, meaning “tufts of hair.” These are only about 1/30 of an inch long, and a dime will cover more than five hundred of them. Each villus contains a loop of blood-vessels, and another vessel, the lacteal, so called from the Latin word lac, milk, because of the milky appearance of the fluid it contains. The villi are adapted especially for the absorption of fat. They dip like the tiniest fingers into the chyle, and the minute particles of fat pass through their cellular covering and gain entrance to the lacteals. The milky material sucked up by the lacteals is not in a proper condition to be poured at once into the blood current. It is, as it were, in too crude a state, and needs some special preparation.

The intestines are suspended to the posterior wall of the abdomen by a double fold of peritoneum called the mesentery. In this membrane are some 150 glands about the size of an almond, called mesenteric glands. Now the lacteals join these glands and pour in their fluid contents to undergo some important changes. It is not unlikely that the mesenteric glands may intercept, like a filter, material which, if allowed to enter the blood, would disturb the whole body. Thus, while the glands might suffer, the rest of the body might escape. This may account for the fact that these glands and the lymphatics may be easily irritated and inflamed, thus becoming enlarged and sensitive, as often occurs in the axilla.

Having been acted upon by the mesenteric glands, and passed through them, the chyle flows onward until it is poured into a dilated reservoir for the chyle, known as the receptaculum chyli. This is a sac-like expansion of the lower end of the thoracic duct. Into this receptacle, situated at the level of the upper lumbar vertebræ, in front of the spinal column, are poured, not only the contents of the lacteals, but also of the lymphatic vessels of the lower limbs.

158. The Thoracic Duct. This duct is a tube from fifteen to eighteen inches long, which passes upwards in front of the spine to reach the base of the neck, where it opens at the junction of the great veins of the left side of the head with those of the left arm. Thus the thoracic duct acts as a kind of feeding pipe to carry along the nutritive material obtained from the food and to pour it into the blood current. It is to be remembered that the lacteals are in reality lymphatics—the lymphatics of the intestines.

Fig. 61.—Section of a Lymphatic Gland.

159. The Lymphatics. In nearly every tissue and organ of the body there is a marvelous network of vessels, precisely like the lacteals, called the lymphatics. These are busily at work taking up and making over anew waste fluids or surplus materials derived from the blood and tissues generally. It is estimated that the quantity of fluid picked up from the tissues by the lymphatics and restored daily to the circulation is equal to the bulk of the blood in the body. The lymphatics seem to start out from the part in which they are found, like the rootlets of a plant in the soil. They carry a turbid, slightly yellowish fluid, called lymph, very much like blood without the red corpuscles.

Now, just as the chyle was not fit to be immediately taken up by the blood, but was passed through the mesenteric glands to be properly worked over, so the lymph is carried to the lymphatic glands, where it undergoes certain changes to fit it for being poured into the blood. Nature, like a careful housekeeper, allows nothing to be wasted that can be of any further service in the animal economy (Figs. [63] and [64]).

The lymphatics unite to form larger and larger vessels, and at last join the thoracic duct, except the lymphatics of the right side of the head and chest and right arm. These open by the right lymphatic duct into the venous system on the right side of the neck.

The whole lymphatic system may be regarded as a necessary appendage to the vascular system ([Chapter VII].). It is convenient, however, to treat it under the general topic of absorption, in order to complete the history of food digestion.

160. The Spleen and Other Ductless Glands. With the lymphatics may be classified, for convenience, a number of organs called ductless or blood glands. Although they apparently prepare materials for use in the body, they have no ducts or canals along which may be carried the result of their work. Again, they are called blood glands because it is supposed they serve some purpose in preparing material for the blood.

The spleen is the largest of these glands. It lies beneath the diaphragm, and upon the left side of the stomach. It is of a deep red color, full of blood, and is about the size and shape of the palm of the hand.

The spleen has a fibrous capsule from which partitions pass inwards, dividing it into spaces by a framework of elastic tissue, with plain muscular fibers. These spaces are filled with what is called the spleen pulp, through which the blood filters from its artery, just as a fluid would pass through a sponge. The functions of the spleen are not known. It appears to take some part in the formation of blood corpuscles. In certain diseases, like malarial fever, it may become remarkably enlarged. It may be wholly removed from an animal without apparent injury. During digestion it seems to act as a muscular pump, drawing the blood onwards with increased vigor along its large vein to the liver.

The thyroid is another ductless gland. It is situated beneath the muscles of the neck on the sides of “Adam’s apple” and below it. It undergoes great enlargement in the disease called goitre.

The thymus is also a blood gland. It is situated around the windpipe, behind the upper part of the breastbone. Until about the end of the second year it increases in size, and then it begins gradually to shrivel away. Like the spleen, the thyroid and thymus glands are supposed to work some change in the blood, but what is not clearly known.

The suprarenal capsules are two little bodies, one perched on the top of each kidney, in shape not unlike that of a conical hat. Of their functions nothing definite is known.