The organs of response and adjustment.Effectors are the mechanisms by which organisms react to their environment. Effector cells were the first cells to be differentiated in the course of evolution, and in primitive species such as the sponge they combine the two functions of irritability and con- tractability—that is, they both receive and respond to stimuli. In time a further differentiation occurred, and certain cells or groups of cells became receptor mechanisms—the eye, for example—while others became response mechanisms or effectors. In man and other animals effectors consist of highly developed muscles and glands.There are three kinds of muscles in the human body. The striated or striped muscles, made up of bands called fibril- lae, enable the body to move or hold itself rigid. The smooth muscles, which are more primitive and spindle-shaped, control the movements involved in digestion as well as the expansion and contraction of blood vessels. The cardiac muscle is a special network of striated muscle cells found only in the heart. It performs its cycle of contraction and relaxation about eighty times a minute throughout life. This muscle and the smooth muscles are termed involuntary, while the striated muscles are termed voluntary, even though they frequently operate by automatic, reflex activity.The glands are classed as effectors because most but not all of them are innervated by effector neurons of the nervous system. The duct or exocrine glands secrete directly into the cavities of the body or onto its surface. They consist of the salivary glands, the tear glands, the sweat glands, the mucous glands, the liver, the kidneys, the mammary glands, the sebaceous glands, and the many glands in the stomach and intestines that produce substances aiding in digestion.The ductless or endocrine glands discharge their secretions, called hormones, directly into the blood stream. Six of these glands are particularly important in psychology and psychiatry because of their effects on behavior as well as on the internal environment of the organism. These are (1) the pituitary: the anterior portion regulates growth and acts as a master gland, influencing the thyroid, pancreas, adrenals and gonads, while the posterior portion controls water metabolism; (2) the thyroid, regulating metabolic rate and thereby affecting activity level, fatigue, and body weight; (3) the parathyroids, controlling calcium metabolism and the excitability of the nervous system; (4) the pancreas, regulating sugar metabolism by means of secretion of insulin; (5) the adrenals: the cortex maintains life processes as well as salt and carbohydrate metabolism, while the medulla governs physiological changes in emotion; and (6) the gonads, which determine secondary sexual characteristics and maintain the functioning of the reproductive apparatus in both male and female. Others are the thymus, lying behind the breastbone, which is large in infancy and gradually shrinks: it is believed to play a role in building immunity and possibly in regulating growth and maturity; and the pineal gland, situated between the cerebral hemispheres, whose functions are still in doubt, although tumors in this gland are one of the causes of precocious sexual development.