incretions, biologically active substances formed by the endocrine glands and secreted directly into the blood.

The term “hormone” was introduced by the English physiologists W. Bayliss and E. Starling in 1902. Hormones are transported by the blood and influence the activity of organs, modifying physiological and biochemical reactions by activating or inhibiting enzyme processes. There are more than 30 known hormones secreted by the endocrine glands of mammals and man (see Table 1). Hormones do not affect the organs in which they are formed. They are relatively easily destroyed; constant manufacture and secretion are therefore necessary in order to maintain sufficient amounts in the blood. Their principal function is the hormonal regulation of the activities of various organs and systems and of the organism as a whole.

Hormones may be classified according to their chemical nature as steroid hormones, such as those of the adrenal cortex, testes, and ovaries; proteo hormones, such as insulin, growth (somatotropic) hormone, and the gonadotropic hormones (follicle-stimulating, luteinizing, and luteotropic); polypeptide hormones, such as adrenocorticotropic hormone, melanocyte-stimulating hormone, oxytocin, vasopressin, glucagon, and thyrocalcitonin; and amino acid derivatives, such as thyroxine, triiodothyronine, epinephrine, and norepinephrine. Hormones are also found in invertebrates (for example, the hormones of molting and pupation in insects). Plants also have biologically active compounds (for example, auxin, gibberellins, and quinines) that are sometimes called hormones, or phytohormones. Many active compounds (for example, histamine and serotonin) are formed in the tissues of vertebrates that are not in fact hormones, but rather parahormones, or histohormones; these are often grouped with the so-called hormonoids.