Although much is known about the functions of the endocrine glands, their study still remains the most fertile field for advancement in medicine. The actions of these glands are so greatly influenced by environment, vitamins, chemicals and emotional states that the results of animal experimentation are not conclusive. The action of one gland is so intimately bound up with the actions of all the other glands that it is almost impossible to isolate hyperfunction or hypofunction of any single gland. In an effort to make up for the deficiency of a hypofunctioning gland, other glands may develop hyperfunction of their primary secretions or of their component glands. Then the vitamins, which are to plant life what the hormones are to animal life, are essential to the proper functioning of the endocrine system. Finally, biochemical reactions and the nervous system regulate the endocrine function, and the endocrine function controls