All vertebrates (animals with backbones) have a well-developed and highly organized endocrine system. The system consists of the following glands: the pituitary, the pineal, the thyroid, the thymus, the pancreas, a pair of adrenals (each adrenal actually acts as two glands—the adrenal cortex produces unique hormones and functions independently of the adrenal medulla), a pair of parathyroids, and a pair of ovaries or testes. Endocrine tissue in the gastrointestinal tract readies the system for the digestive process. During a pregnancy, the placental tissue assumes an endocrine function. Although the kidneys do not produce a hormone directly, they release an enzyme which converts a blood protein into a hormone that stimulates red blood cell production.

All vertebrates have a pituitary. The pituitary is a small, round organ found at the base of the brain. This major endocrine gland interacts with the hypothalamus of the nervous system. Together, they control behavior. The hypothalamus receives information about physiological events in the body by monitoring the composition of the blood. In turn, the hypothalamus signals the pituitary by either a nerve impulse or a chemical messenger. The pituitary responds by releasing or ceasing to release hormones that will have a direct effect on physiology or will stimulate other endocrines to release their hormones in order to alter the physiological event and influence behavior. The endocrine system exerts its effects on a biochemical level.

The human endocrine system is typical of vertebrate endocrine systems and their effect on behavior, although certain hormones may have a more pronounced and obvious effect in other vertebrates. For example, melanocyte-stimulating hormone, which is generated by the anterior lobe of the pituitary, greatly increases skin pigmentation in amphibians. This creates a protective coloration. In humans, the darkening effect is not achieved unless excessive hormone is administered. The protective function is not apparent. There are enough similarities among human and animal endocrine functions and

Glands of the Endocrine System

(Hans & Cassidy, Inc.)

effects, however, to warrant the use of data from both ethology and human behavioral studies in determining the biological bases for behavior.

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