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.