The endocrine or ductless glands are those that deliver their secretory products (hormones) into the blood, lymph, or tissue fluid, which transports them to the target organs susceptible to the instructions these products represent.

Each gland has its particular and distinctive function; collectively, they collaborate with the nervous system in maintaining the internal environ­ment and securing the appropriate general and specific responses to stimuli from both external and internal sources.

The study of the anatomy of the glands, the produc­tion and the chemistry of the hormones, the responses of the target organs, and the complicated interplay of the various endocrine tissues with each other and with the nervous system is entitled endocrinology. Endocri­nology is one of the most important and currently most active branches of biology, and because derangements are common in clinical medicine, its significance is not to be measured by the brevity of this chapter, which is essentially concerned with the gross anatomy of the glands.

Some writers regard these organs as together consti­tuting an “endocrine system.” Although there is no serious objection to this practice, it must be appreciated that the components, unlike those of other body systems, are scattered, achieve no physical continuity, and have very diverse embryological origins, targets, and func­tions. They are united only by their general subservience to the central nervous system (hypothalamus), by the similar patterns of their government of other organs, and by some common features of structure; these com­prise the epithelioid character of the secretory cells, the absence of drainage ducts, the sparse supporting frame­works, the generous vascularity, and the intimate asso­ciation with blood vascular or other transport media (Figure 6-1).

Three types of endocrine organ may be recognized pragmatically. The first comprises the few discrete organs of a primary endocrine nature: the hypophysis (pituitary gland), the epiphysis (pineal gland), and the thyroid, parathyroid, and adrenal glands. The second comprises those organs that combine major endocrine functions with other important related functions: the pancreas, testes, ovaries, and placenta. The last com­prises the unobstrusive endocrine component of organs with a quite different primary function; the brain, kidneys, liver, thymus, heart, and the gastrointestinal tract are the best examples.

The existing knowledge of endocrine functions has been obtained in part from observation of human and animal patients with derangements of these glands and in part from experimental studies. Although much remains to be discovered, it is already clear that notable species differences exist.