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.

[Illustration: Fig. 62.]

Experiment 62. _Simple Apparatus for Illustrating Endosmotic
Action._ "Remove carefully a circular portion, about an inch in
diameter, of the shell from one end of an egg, which may be done without
injuring the membranes, by cracking the shell in small pieces, which are
picked off with forceps. A small glass tube is then introduced through
an opening in the shell and membranes of the other end of the egg, and
is secured in a vertical position by wax or plaster of Paris, the tube
penetrating the yelk. The egg is then placed in a wine-glass partly
filled with water. In the course of a few minutes, the water will have
penetrated the exposed membrane, and the yelk will rise in the
tube."