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.

Although each gland has its particular and distinctive function, they collaborate with the nervous system to maintain the internal environment and the appropriate general and specific responses to external and internal stimuli.

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 one another and with the nervous system is entitled endocrinology. Because of its clinical implications, the endocrinology is one of the most important and most active branches of biology. This chapter is mainly concerned with the gross anatomy of the glands.

The endocrine organs are collectively termed an endocrine system. However, it must be understood that the individual organs are scattered, achieve no physical continuity, and have very diverse embryologic origins, targets, and functions. They are united only by their general subservience to the central nervous system (hypothalamus), the similar patterns of their target organ effects, and some common structural features; these last 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 (Fig. 6.1).

Three types of endocrine organs may be recognized pragmatically. The first group includes the few discrete primary endocrine organs: 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 group are the unobtrusive endocrine components of organs with quite different primary functions; the brain, kidneys, liver, thymus, heart, and gastrointestinal tract are the best examples. There are notable species differences.