The internal secreting or endocrine glands secretions
About one hundred years have passed since a scientist who removed the sex glands of a rooster found that the animal could be restored to its masculine vigor by transplanting into its tissue the glands of another rooster. Later a British doctor named Addison described a disease due to insufficiency of action of the adrenal glands which is now known as "Addison's disease." In 1889 a French scientist named Brown-Sequard attracted world attention by the claim that the injection into his own body of male sex hormone had produced a rejuvenating effect. It didn't work with him, and it still doesn't work to produce rejuvenation or restore lost sex-power, notwithstanding the imaginative and emotional claims of a few too-credulous scientists. Nevertheless, the new knowledge of the glands, most of which has developed since 1900, is one of the greatest contributions ever made to human health and happiness.
From the point of view of chemical study four types of hormones are known, including those, like insulin, which have large protein molecules, those, like ACTH, which are called polypeptides, those, like thyroxin and adrenalin, which are aromatic derivatives, and the steroid hormones which include Cortisone and the sex hormones.
Much has been learned about hormones from the study of animals in which the glands can be removed and in which other glands can be transplanted to determine the effects. Similarly glandular materials may be given to human beings and the effects then studied. Sometimes a tumor will cause overgrowth and overactivity of a gland. The results are reflected by various changes in the body and in its functions.