206 GENERAL ZOOLOGY 



olism. The fact that the various endocrine organs have definite 

 functional relations with each other makes it impossible to define 

 the functions of one type of endocrine organ without considering 

 the functions of all of them. This interlocking relationship 

 complicates the problem of understanding the system as a whole, 

 since new discoveries may make it necessary to correct and restate 

 what were seemingly well-founded conclusions. While this 

 might be said to be true of all scientific work, it is particularly 

 true of endocrinology because of the intense activity in this field 

 of study at the present time. For this reason only a general out- 

 line of the subject will be presented in these pages, the discussion 

 being confined to the known effects produced by the ductless 

 glands individually and collectively. These glands include the 

 pituitary, thyroid, parathyroid, thymus, adrenal, pancreas, and 

 gonads. The pineal gland is also classified as a ductless gland, 

 but since little is known of its function, it will not be considered 

 here. 



Anterior Pituitary Gland. — The pituitary gland, attached to 

 the floor of the diencephalon, consists of two principal regions: 

 (1) the anterior lobe, or hypophysis, derived from the embryonic 

 oral epithelium, and (2) the posterior lobe, or infundibulum, 

 derived from the floor of the forebrain. A third region, the 

 intermediate lobe (Fig. 131), a band of tissue between the first 

 two, is derived from the hypophysis. That the anterior lobe of 

 the pituitary plays a dominant role in the endocrine complex is 

 indicated by the fact that it produces at least five distinct 

 endocrines or hormones, as follows: (1) somatotropic or growth- 

 stimulating; (2) gonadotropic, or gonad-stimulating; (3) thyro- 

 tropic or thyroid-stimulating; (4) lactogenic, or milk-producing; 

 and (5) adrenotropic, having a relation with the cortex of the 

 adrenal gland. Reciprocal functional relations exist between 

 the anterior pituitary lobe and the organs affected by its hormones. 



Somatotropic Hormone. — The somatotropic hormone is neces- 

 sary for the normal growth and development of the body, its 

 absence or reduction resulting in some form of dwarfism. This 

 has been demonstrated experimentally by causing an arrest of the 

 growth of bones and other body tissues of mice and rats as a 

 result of removing the anterior pituitary lobe. 



Overproduction of the growth hormone of the anterior pituitary 

 lobe in children produces gigantism and in adult life, acromegaly. 



