THE ENDOCRINE SYSTEM 131 



vaginae are fused to an unpaired tube. In the rabbit, as in many 

 lower placentals, there are two complete uteri, and as an organ the 

 whole structure represents the stage of uterus duplex (Fig. 78, A). 

 A partly fused condition existing in some mammals, for example 

 sheep, is known as uterus bicornis (Fig. 78, B), while the com- 

 pletely fused condition in man is known as uterus simplex (Fig. 

 78, C). It is characterized by the independent opening of the two 

 uterine tubes into a single uterine cavity. The successive stages 

 of coalescence are doubtless associated with progressive reduction 

 of the number of young, the success of the species being determined 

 by greater perfection of the placental apparatus. 



The Endocrine System 



In contrast to the organ-systems usually recognized, which have 

 a structural continuity and are associated with contributory but 

 more or less separate functions, digestive, nervous, and the like, 

 there are certain organs which have a detached distribution, belong 

 structurally and embryonically to different systems and body- 

 layers, and yet have common general functions in chemical and 

 physical regulation, including growth. They are described as 

 internal secreting, ductless, or endocrine glands. Their effects 

 are exerted through relatively small quantities of very active 

 substances, hormones, thrown into the circulation. In a physio- 

 logical sense, they constitute an internal secreting or endocrine 

 system. They include the suprarenal bodies, the thyreoid, the 

 parathyreoids, the thymus, the hypophysis, the pineal body, and 

 portions of the male and female gonads and of the pancreas as 

 well as the epithelial lining of the duodenum. The occurrence of 

 such endocrine organs is a feature peculiar to the vertebrates, in 

 which they supplement nervous regulation, providing a duplicate 

 mechanism for the maintenance of the all-important balance among 

 the activities of the parts of the individual organism. 



Both the testis and the ovary produce substances which have 

 a pronounced effect upon metabolism and growth, especially the 

 development of secondary sex characters in the young, and, in the 

 adult female, changes connected with pregnancy. A more specific 

 action is shown, for example, by the pancreas, imbedded in which 

 are microscopic groups or islands of cells which are quite distinct 



