146 READINGS IN BIOLOGICAL SCIENCE 



Stalk to the base of the brain. In man it is about the size of a hazelnut and 

 weighs approximately 0.6 to 0.8 grams. It lies in a bony pit in the floor of 

 the skull. The forward part, called the miterior lobe, arises from the rear 

 of the embryonic mouth cavity; the posterior part, or posterior lobe, 

 originates as a down-growth chiefly from the brain. 



Although of small size the pituitary body liberates a number of hor- 

 mones which not only afi^ect the body in general but also influence most, 

 if not all, of the other endocrine glands. The secretions of some of these, 

 in turn, influence the pituitary gland. Because of its manifold secretions 

 and widespread influence it has been called "the master gland" of the body 

 — also, "the conductor of the endocrine orchestra." Following its removal, 

 atrophy of the thyroid, adrenals and sex glands occurs. Underdevelop- 

 ment of the pituitary gland is usually associated with sexual infantilism; 

 the germ glands, or gonads as they are termed technically, never produce 

 gainetes (ova or spermatozoa), and their endocrine activities do not 

 appear. The result is that the accessory reproductive structures and the 

 secondary sex characters of the male or female never reach the adult state. 

 If, in experimental anim-als, the pituitary gland is removed during early 

 pregnancy, abortion results. This can be prevented, however, by daily injec- 

 tions of pituitary extracts, as can the other changes which follow removal 

 of the gland. Thus it is evident that all the phenomena controlled by the 

 sex hormones are dependent indirectly upon the normal functioning of 

 the pituitary body. 



Two potent products have been obtained from posterior lobe substance. 

 One of these acts powerfully on uterine muscular contractions and is 

 sometimes used to hasten childbirth. The second substance aflFects the 

 smooth muscle of the arterioles; also, the volume of urine secreted. It has 

 been of use medicinally in control of diabetes insipidus. This form of 

 diabetes, which is unrelated to the commoner diabetes mellitiis, is char- 

 acterized by profuse secretion of dilute urine and excessive thirst. 



The anterior lobe secretes at least six specific hormones; a growth hor- 

 mone; one which stimulates the thyroid; one which induces lactation; and 

 three, called "gonadotropic hormones," which incite various gonad ac- 

 tivities. The first recognized and perhaps the best known is the one that 

 promotes general body growth, particularly growth of the skeleton. Over- 

 activity of this lobe leads to gigantism, underactivity, to dwarfism and 

 persistence of the infantile condition. Giant rats have been produced by 

 daily injection of anterior-lobe substance into the body cavity of either 

 normal animals or those with excised pituitary. Autopsies on various hu- 

 man giants have revealed tumorous and enlarged pituitary glands. If the 

 excessive secretion begins in youth while the growth zones of the long 

 bones are still unossified, lengthening of the bones, particularly of the 

 arms and legs occurs, and a form of gigantism is the outcome; but if such 



