Involuntary Regulation - Glands of Internal Secretion
There are two general devices, nervous and chemical, for effecting coordination within the animal body. The former is somewhat like a telegraph system, whereby messages are conveyed from one point to another without material transfer, while the latter is more like the parcel post, in that substances produced by certain organs are distributed through the blood system to other parts of the body, where they bring about results affecting the entire organism.
The chemical regulators of the body have their origin in glands. Some glands, for example those of the digestive tract, are provided with ducts and furnish chemical substances called enzymes, that are indispensable agents in the utilization of food materials. Others have no ducts and can only deliver their products by the agency of blood vessels with which they are in intimate contact. These are the endocrine glands, or “glands of internal secretion.”
The chemical substances that they elaborate from the blood are termed hormones, which like enzymes may act in very small quantities, suggesting that they are catalytic in action, or in other words that they hasten chemical action without themselves undergoing any permanent change. They not only bring about functional results by maintaining chemical equilibrium within the body, but they also play an important part in developmental processes. The term hormone means “exciter,” which indicates in general the character of this kind of intramural messengers. Some ductless glands produce substances, however, that act as restraining bridles rather than as stimulating spurs, and such glandular products are called chalones in distinction to autocoids, or hormones proper.
The total mass of human endocrine glands all rolled together would equal, according to an eminent English biologist, a parcel small enough to “go in a waistcoat pocket,” yet their importance is so great that, with the advance of knowledge gained within the present century largely through pathological studies and animal experimentation, the new biological science of Endocrinology has arisen, absorbing the attention of a small army of specialists and possessing a rapidly increasing literature all its own.
The glandular system involves considerably more than the ductless glands, since there are glands having ducts, for example the pancreas and the gonads, that in addition deliver hormones directly into the blood, as well as substances through ducts. Moreover, certain substances not produced by glands act also through the medium of the blood in a manner similar to that of the hormones. Carbon dioxide, for instance, resulting from the activity of muscle tissues, which is transported by the blood stream to the respiratory nervous center in the medulla where movements essential to breathing are regulated, belongs in this category.
The endocrine glands are subject to great variation and, although largely interdependent, their hormones, carried everywhere that blood flows, are effective only upon certain specific organs. They are like keys to fit particular locks but not master keys to fit all locks.
Upsets in the balance of endocrine activities result in various pathological manifestations, while the loss of certain of these glands of internal secretion results fatally. The interlocking endocrine system among other things is responsible, in part at least, for (1) the time, order, and rate of organic development; (2) metamorphosis in amphibians; (3) adaptive coloration in certain larval salamanders; (4) the development of secondary sexual characters; (5) a considerable part of visceral control; and (6) many phases of behavior, particularly the “emotions” that contribute so much to the psychic life and to the determination of “personality.”
The possibility of controlling and regulating the functions of the ductless glands has opened a new broad field for experimentation and medical practice, with the inevitable and often deplorable accompaniment of much unfounded speculation and quackery.
No ductless mass of cells which is not muscular, nervous, or skeletal can escape the suspicion of being involved in the endocrine system. For example, the spleen has been suspected of endocrine affiliations although the only functions definitely shown for it have to do with the circulatory system. Substances similar to vertebrate hormones, associated with such phenomena as color changes in crustaceans and metamorphosis of insects, have already been discovered in invertebrates despite limited investigations along this line. Plants have also been found to produce certain substances that act after the long-arm method of hormones upon structures distant from the point of their elaboration.
For convenience in description, the principle endocrine structures of the vertebrate body (Fig. 412) may be grouped into four categories according to their general location: (1) pharyngeal (thyroid, parathyroid, and associated glands); (2) cranial (pineal, anterior and posterior pituitary); (3) sexual (testes, ovaries); and (4) abdominal (pancreas, intestinal, adrenals).