112 INTERNAL SECRETION 



completely disappeared. In man, the medullary substance con- 

 tains larger or smaller groups of epithelial cells, which are studded 

 with leucocytes. The thymus also differs chemically from the 

 lymph glands. Ivar Bang showed that the thymus cells have a 

 different chemical reaction to that of the leucocytes, the bone- 

 marrow cells, and the cells of the spleen. That the thymus cells 

 are not identical with those of the lymph glands is proved by 

 the fact that the amount of the nucleinates, the substances which 

 are characteristic of the nuclear structure of the true glands, is 

 at least five times as large in the thymus as in the lymph glands. 

 The older idea, that the thymus was the seat of origin of the 

 leucocytes (Beard), is disproved by the investigations of Stoehr. 



INVOLUTION OF THE THYMUS. 



The thymus is an organ which maintains its structure (as 

 described above) for a certain period of life only, and its function 

 is necessarily exercised only during that period. After a short 

 period of progressive development and growth which, according 

 to Friedleben, lasts in man until about the second year of life, 

 retrogressive changes set in, which lead eventually to a complete 

 disappearance of the parenchyma. The adenoid tissue is gradually 

 replaced by adipose tissue, the original lobulated formation being 

 retained until old age, and the fatty substance frequently enclos- 

 ing small remnants of true thymus tissue. Involution of the 

 thymus in man coincides normally with adolescence. 



Hammar shows that, in addition to the involution of adoles- 

 cence, there is also an accidental involution, which may occur 

 at any period of life and leads to a rapid reduction in the size 

 of the gland. He found that the thymus gland of rabbits in 

 the fasting state fell, during the first three days, to one-half its 

 normal weight, and that toxic influences operated in the same 

 direction. This accidental involution is characterized histo- 

 logically by a diminution in the lymphocytes of the cortex, a 

 reduction in the bulk of the lobes, a disappearance of the dif- 

 ferentiation between cortex and medulla, and a remarkable richness 

 in blood. In the involution of adolescence, the microscopic 

 changes are very similar, though the process sometimes affects 

 certain portions only of the parenchyma and these may be of 

 varying extent. The formation of adipose tissue takes place in the 

 interlobular or vascular connective tissue. 



The process of involution may, however, be arrested at any 

 age, and a remarkably large and well-preserved thymus, the so- 

 called persistent thymus, is sometimes seen in man. In these 

 cases, according to Schaffer, the original organ is not preserved, 

 but there is a new formation of thymus tissue, the original medul- 

 lary substance assuming a fresh cortical layer. Friedleben and, 

 latterly, Basch, found that after partial extirpation of the thymus, 

 there was hypertrophy of the remaining portions. 



