THE THYMUS BODY. 203 



leucocytes. The centres of the bodies may enclose vacuoles containing fat, 

 or mucus-reacting or colloid substance the latter by some being regarded 

 as representing the special production of the organ. 



The significance and genetic relations of the thymic bodies have long been the 

 subject of discussion. Formerly they were regarded as the remains of the original 

 epithelial elements of the organ, all other parts being the products of the invading 

 mesoclerm. In the light of present embryological data, it is probable that the cor- 

 puscles result from the aggregation of hypertrophied and otherwise modified reticulum- 

 cells and, therefore, that they are new and special formations, distinctive of the organ, 

 and not merely the atrophic remains of the original epithelial elements. It is evident, 

 however, that they are indirectly, through the reticulum-cells, derivatives of the primary 

 epithelium. Since new corpuscles are being continually formed and those existing 

 increase in size, during the active period of the thymus, it is possible that they are 

 concerned in producing a substance serving some particular purpose during early life. 



Thymus 

 tissue 



FIG. 252. Section of thymus body of man of twenty-eight, showing invasion and replacement of thymus- 



tissue by fat. X 20. 



The assumed importance of the thymus as the producer of the first lymphocytes, 

 credited to the organ by some, is doubtful. There is no convincing evidence that the 

 thymus is the seat of red blood-cell formation after birth. 



In addition to numerous capillaries, leucocytes and eosinophiles are present 

 among the constituents of the medulla. As age advances, the small cells become 

 less numerous and the cortex markedly diminishes in thickness, so that the medullary 

 substance comes into relation with the surrounding vascular interlobular connective 

 tissue with increasing frequency and extent. At a variable time, in some cases before 

 the second year and in others not until much later, an active general regression and 

 atrophy of the thymus becomes established. The general process, however, is often 

 antedated by reduction in the thymus, probably associated with impaired nutrition, 

 whereby the number of the small lymphoid cells is greatly decreased and the distinction 

 between cortex and medulla disappears. The medulla is the seat of occasional small 

 cysts, of uncertain form and size, lined with epithelial elements that often bear groups 

 of cilia-like processes. Ti.e thymus is not essential to life. 



Coincident with the atrophy of the thymus-tissue, fat-cells progressively 

 appear in the interlobular tracts, which latter, in consequence, become in- 



