Hormones are biochemical messengers that regulate physiological events in living organisms. More than 100 hormones have been identified in humans. Hormones are secreted by endocrine (ductless) glands such as the hypothalamus, the pituitary gland, the pineal gland, the thyroid, the parathyroid, the thymus, the adrenals, the pancreas, the ovaries, and the testes. Hormones are secreted directly into the blood stream from where they travel to target tissues and modulate digestion, growth, maturation, reproduction, and homeostasis. The word hormone comes from the Greek word, hormon, to stir up, and indeed excitation is characteristic of the adrenaline and the sex hormones. Most hormones produce an effect on specific target tissues that are sited at some distance from the gland secreting the hormone. Although small plasma concentrations of most hormones are always present, surges in secretion trigger specific responses at one or more targets. Hormones do...