SPECIAL FUNCTIONS

hunger, either by depressing the desire for food or by stimulating appetite. The drying of the membranes in the oral cavity influences the sensation of thirst. Although thirst may be due to a lack of water in body tissues, a reduced salivary flow can produce a sensation of thirst. SPECIAL FUNCTIONS SPEECH is controlled by the coordinated ac- tion of several nerve functions. The speech center is located deep in the brain, and from it nerve im- pulses pass out to the larynx, which contains folds of mucous membranes called vocal cords. When air is forced from the lungs past these folds, cer- tain sounds are produced, and, in conjunction with the movements of the throat, lips, tongue, and teeth, articulate speech results. SLEEP is a period of unconsciousness when the higher physical powers are quiet, although body activities continue. It is usually considered a period of rest in which constructive processes build up and repair the body. Certain changes take place during sleep: respiration is slowed; less blood is sent to the brain and greater amounts to to the extremities; digestion goes on, but at a slower rate; body temperature may drop somewhat; and heart action is slowed. THE ENDOCRINE SYSTEM Homeostasis depends on the nervous and en- docrine systems, since both are lines of com- munication for body functions. The endocrine system sends messages by chemical hormones that are carried in the blood stream. These messages aid in the control, development, and integration of body functions. The endocrine system is made up of glands of internal secretion. These are called ductless glands because they have no ducts to carry away their secretions. The secretion of an endocrine gland is called a hormone. It enters directly into the blood or lymph circulation and eventually reaches the gland, tissue, or organ it controls or in- fluences. Very small quantities of hormones are produced, since only a trace amount is needed to produce the desired effect. Most hormones can be extracted from the glands of animals, and some can be produced syn- thetically. Medical officers may prescribe these isolated or synthetic hormones for patients who are deficient in them or who might otherwise Figure 3-51.—The endocrine glands. benefit from their use. The hormone-producing glands include the pituitary, thyroid, parathy- roids, adrenals, gonads, and pancreas (fig. 3-51). HYPOTHALAMUS The hypothalamus, a structure in the brain, synthesizes chemicals that are secreted to the pituitary gland to stimulate the release of its hormones. PITUITARY GLAND The pituitary is a small, pea-sized gland located at the base of the brain in the sella tur- cica of the sphenoid bone. It is often called the master gland of the body, because it influences most other endocrine glands. It is divided into two lobes, an anterior and a posterior. The anterior lobe plays the more important role in influenc- ing body functions. The hormones it produces have a broad and significant range of effects. SOMATOTROPIN, the growth hormone, in- fluences body growth and development. During 3-40