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.

Experiments.

The action produced by the tendency of fluids to mix, or become equally diffused in contact with each other, is known as osmosis, a form of molecular attraction allied to that of adhesion.  The various physical processes by which the products of digestion are transferred from the digestive canal to the blood may be illustrated in a general way by the following simple experiments.

The student must, however, understand that the necessarily crude experiments of the classroom may not conform in certain essentials to these great processes conducted in the living body, which they are intended to illustrate and explain.