824 THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS 



nated animals when the corpus luteum is made to develop by artificial 

 means such as puncturing the Graafian follicle. Furthermore, destruc- 

 tion of the corpora lutea in a pregnant rabbit arrests development of 

 the mammary glands. The corpus luteum has also an important func- 

 tion in connection with the formation of the uterine decidua and the 

 fixation of the embryo. Thus, after destruction of the corpus luteum at 

 an early period in pregnancy, the embryo fails to become adherent to 

 the uterus. 



THE THYMUS* 



The structure of the thymus is of a lymphoid nature consisting of a 

 cortex composed of closely packed masses of lymphocytes and a medulla 

 made up of a cellular reticulum, in the meshes of which are seen large 

 bodies possessing a concentric configuration the corpuscles of Hassel. 

 The gland is developed from outgrowths of the third branchial pouch 

 on either side, which, meeting in the midline, unite to form a solid block 

 of cells, this later, becoming hollowed out and branched. In the walls of 

 the tubules, so formed, lymph nodes appear which ultimately form the 

 cortex. The walls of the tubules themselves break up, their cellular 

 elements subsequently forming the reticulum and concentric corpuscles 

 of the medulla. Though prominent in early childhood the thymus un- 

 dergoes progressive involution with advancing years, until in adult life 

 it is more or less vestigial in character. It is still a question whether this 

 body should be included with the organs of internal secretion, and though 

 many views, mostly of a more or less speculative nature, have been ad- 

 vanced to justify its consideration as an endocrine organ, the proof that 

 it possesses an internal secretion, in the generally accepted sense, is 

 wanting. It is wiser perhaps to state that its functions are obscure, and 

 that we do not know what role it plays in the animal economy. The re- 

 sults of feeding the gland to animals or of ablation experiments are con- 

 flicting and of little help in arriving at any conclusion in this regard. 

 Attention, however, should be drawn to certain significant facts regarding 

 its behavior. First, its involution is arrested or retarded after castration, 

 a fact suggestive of an endocrine function ; secondly, it is believed to be a 

 source of the lymphocytes, and possibly also of the granular leucocytes. 

 Examinations of the blood, with a view to the lymphocytic counts, show, 

 from infancy to puberty, a declining curve, the gradient of which follows 

 closely that of thymic involution. On this account it is believed by some 

 that "the thymus functions as a lymphoid organ in infancy and child- 

 hood when a large number of lymphoid cells and leucocytes are needed 

 to combat infection." (Hoskins, E. R.). 



'For a review of the literature, the reader is directed to a recent article by Blatz. 57 



