CHAPTER V 

 THE DISEASES OF THE THYMUS GLAND 



It has not been long that the thymus gland has been classed with the 

 hormonopoietic system. Originally it was regarded as a constituent of the 

 lymphatic apparatus. 



Anatomy and Embryology. The thymus gland develops as a paired 

 organ from the ventral part of the third branchial cleft. (See illustration 3, 

 p. 53). -The pairs unite very early; at birth the organ lies behind the 

 sternum, backward as far as the pericardium, reaching above somewhat 

 higher than the jugular notch. In the human being there may occur ac- 

 cessory lobes of the thymus which may be either independent, or be united 

 with the thyroid gland, or even the thymus gland itself. The thymus gland 

 is of entodermal origin; although at present there is no unity of opinion 

 with regard to the origin of the pictures resembling lymphocytes found 

 in the completed thymus and especially in its cortex. Since the inves- 

 tigations of Hammar the entodermal origin of the polymorphic fixed 

 reticular elements lying chiefly in the medulla, and of the so-called Hassal's 

 corpuscles stands assured. Stohr regards the origin of the thymus lympho- 

 cytes as due to division of the epithelial cells while recently Hammar and 

 Maximow, following the older assumption of His and Stieda, adhere to the 

 assumption of a secondary ingrowth of mesodermal pictures. According to 

 the latter view, the thymus gland therefore belongs to the lymphatic ap- 

 paratus; there is found In it a "symbiosis of cells of the different germinal 

 layers;" it is concerned with the production of lymphocytes. According to 

 Nagele it is indeed the principal site in the formation of the lymphocytes in 

 early life. The same author states that this view is also favored by phylo- 

 genetic factors, for in the amphibians the thymus is the source of lymphocytes 

 when the lymph glands are still absent. The statement of Ivar Bang that the 

 thymus gland contains at least five or six times more nucleinate than the 

 lymphatic glands does not speak unconditionally against this assumption, 

 as the analysis included the epithelial elements. 



The weight of the thymus gland increases after birth, according to the 

 statement of Hammar, v. Sury, Schridde, Ronconi, Pappenheimer, and others, 

 the growth ceasing only with the beginning of sexual maturity; the thymus 

 tissue gradually atrophies and is partly replaced by fat, although appreciable 

 remnants of thymus tissue are retained until late life. In late life too were 

 found, by Hammar, mitotic proliferation of lymphocytes and new formation 



223 



