964 THE INTERNAL SECRETIONS 



an external portion or cortex. The medulla presents itself as a coarse 

 network of connective tissue in which are embedded lymphoid cells 

 and the concentric corpuscles of Hassall. The latter are of endodermic 

 origin, and have been formed from an outgrowth of the third pharyngeal 

 cleft. The former, on the other hand, appear to be of mesodermic ori- 

 gin. The cortex is made up of a similar reticulum of connective tissue, 

 the different nodules of which contain numerous lymphoid cells. 

 Although derived from epithelial tissue, the cortical substance even- 

 tually acquires the general characteristics of a lymphatic gland, but 

 this transformation is not complete, because it contains a much larger 

 amount of nuclear material than the ordinary glands of this type. 

 The blood-supply of the thymus ,is derived from the internal mammary, 

 inferior and superior thyroid, subclavian and carotid arteries. 



The Function of the Thymus. While no absolutely definite 

 statements can be made at this time regarding the function of this 

 gland, it is obvious that it exercises a metabolic influence which attains 

 its greatest importance at about the time of maturation. In support 

 of this view might be cited the involution of this organ after puberty, 

 and secondly, the fact that its removal gives rise to a more rapid 

 development of the testes. 1 Correspondingly, the removal of the 

 latter (castration) delays the atrophy of the former. 2 It is surmised 

 that this close relationship of the aforesaid organs is brought about 

 wholly by chemical means, because even pieces of the thymus of rab- 

 bits, when transplanted to other regions of the body, are affected 

 in precisely the same way by castration and sexual stimulation.* 



In accordance with Klose and Vogt 4 it has usually been supposed 

 that the thymus is essential to life and that its complete removal 

 proves fatal to young animals within a very short time. While these 

 results have not been substantiated by the work of Pappenheim, 

 Rowland and Vincent, 5 it appears that thymectomy nevertheless 

 produces certain metabolic disturbances, chief among which are a 

 retardation of the growth of the bones, mental deterioration, and a 

 tendency to adiposity. In connection with this point, attention should 

 .be called to the experiments of Gudernatsch which have shown that 

 the feeding of extract of thymus to young tadpoles stimulates their 

 growth, but retards their differentiation or metamorphosis. Some 

 authors, in fact, recognize a condition of hyperthymusism which may 

 be a complicating factor in Graves' disease. 



C. THE LIVER 



The Internal Secretory Power of the Liver. The carbohydrates 

 are absorbed in largest part through the intestinal radicles of the portal 



1 Paton, Jour, of Physiol., xxxii, 1905, 28, and xlii, 1911, 267. 



2 Goodall, Jour, of Physiol., xxxii. 1905, 191, and Pappenheimer, Jour. Exp. 

 Med., xix, 1914, 319. 



3 Marine and Manley, Jour. Lab. Clin. Med., iii, 1917, 48. 



4 Klinik and Biol. der Thymusdr. Tubingen, 1910. 

 1 Ergebn. der Physiol., 1911.- 



