128 THE INDIVIDUAL ORGANISM 



complicated by the fact that not only is it situated in one of the most 

 inaccessible regions in the whole body (within the skull and beneath the 

 brain stem) but it produces several hormones that, in large part, regulate 

 the output and functional rhythms of other endocrine glands located 

 in totally different regions of the body. Again, not all types of endocrine 

 control are in continuous operation. Some are particularly active in early 

 life, others show a rhythm associated with sexual changes, and still others 

 are largely quiescent except when stimulated by way of the autonomic 

 nervous system. 



In the account that follows, some of the more important endocrine glands and 

 the hormones that they produce are listed and briefly discussed. 



Pituitary. This gland (sometimes called the hypophysis) is a small mass of 

 tissue about the size of a pea, located at the base of the brain and enclosed in a 

 bony pocket in the floor of the skull. It has four main parts — the anterior lobe, the 

 pars nervosa, the pars intermedia, closely applied to the pars nervosa, and a stalk, 

 which attaches it to the brain. The parts are closely fused but are different in 

 structure and origin. The anterior lobe is entirely glandular, derived from a pouch 

 on the roof of the mouth of the developing embryo. The pars nervosa and the pars 

 intermedia together form the posterior lobe; the first is derived from the brain, 

 while the second arises in the same way as the anterior lobe. At least nine hor- 

 mones have been obtained from the anterior lobe, while the posterior lobe pro- 

 duces at least three. The more important of the pituitary hormones are the 

 following: 



Hormones of the Anterior Lobe. All of the hormones of the anterior lobe which 

 have been prepared in pure or nearly pure form are proteins. A number of them 

 exert effects upon the morphological development of other structures, and are 

 accordingly called trophic hormones (Greek, trophein, "to nourish"). These 

 include the growth (somatotrophic) hormone, two gonadotrophs hormones, and 

 the thyrotrophic, adrenocorticotropic, and lactogenic hormones; others have 

 been postulated. There is also good evidence for the existence of anterior pituitary 

 hormones which have direct effects upon metabolism and are not trophic hor- 

 mones; these include the diabetogenic hormone and the fat-metabolizing hormone. 



1. The somatotrophic or growth hormone stimulates body growth. A deficiency 

 of this hormone in childhood results in the development of a pituitary dwarf. If 

 such individuals are recognized early enough and given injections of anterior lobe 

 extracts, normal growth may be produced or approximated. Pituitary dwarfs 

 are mentally normal, become sexually mature, and are well-proportioned of body, 

 unlike the cretin dwarfs that result from lack of thyroid secretion. Overproduc- 

 tion of the growth hormone in childhood results in gigantism. When overproduc- 

 tion begins after adult stature is reached, it causes disproportionate development 

 of the jaws, arms, and legs, producing the condition called acromegaly. 



2. The thyrotrophic hormone controls the size and activity of the thyroid gland 

 and causes it to produce thyroxin. Hypothyroidism and hyperthyroidism are 

 physiological states caused by too little or too much secretion of this hormone. 



3. The adrenocorticotropic hormone (commonly abbreviated to ACTH) con- 



