In the first chapter reference was made to organs producing substances which pass directly into the circulation; these substances act by control of the activities of other parts, stimulating or depressing or altering their function.

Two of these glands, the thymus, lying in front, where the neck joins the body and which attains its greatest size at puberty, and the pituitary body, placed beneath the brain but forming no part of it, have been shown by recent investigations to have a very definite relation to growth, especially the growth of the skeleton.
The growth energy chiefly resides in the skeleton, and if the growing animal has a diet sufficient only to maintain the body weight, the skeleton will continue to grow at the expense of the other tissues, literally living upon the rest of the body.

Disease of the glands mentioned leading to an increase or diminution or alteration of their function may not only inhibit or unduly increase the growth of the skeleton, but may also interfere with the sexual development which accompanies the skeleton growth.
The difficulties which arise in an endeavor to comprehend normal growth are greater when the growth of tumors is considered.

A tumor is a mass of newly formed tissue which in structure, in growth, and the relations which it forms with adjoining tissues departs to a greater or less degree from the type of the tissue to which it is related in structure or from which it originates.

It is an independent structure which, like a parasite, grows at the expense of the body, contributing nothing to it, and its capacity for growth is unlimited.