THE THYMUS GLAND. 



21 I 



By means of connective-tissue septa, the thymus is divided into 

 larger lobes, and these again into smaller lobes, until finally a 

 number of small, irregularly spheric structures are formed the 

 lobules of the gland. These are, however, connected by cords of 

 lymphoid tissue, the so-called medullary cords. The lobules of the 



Fig. 169. A small lobule from the thymus of child, with well-developed cortex, 

 presenting a structure similar to that of the cortex of a lymph-gland ; X 6 : a, 

 Hilus ; b, medullary substance ; <r, cortical substance ; d, trabecula. 



thymus gland consist of a reticular connective tissue much more deli- 

 cate at the periphery than at the center of the lobule. The reticulum 

 supports branched connective -tissue cells, with relatively large nuclei. 

 In the meshes of the reticular tissue are cellular elements, in structure 

 similar to the lymphocytes, which are more numerous at the periph- 

 ery of the lobule than at its center, so that we may here speak of 



Fig. 170. Hassal's corpuscle and a small portion of medullary substance, sliowing 

 reticulum and cells, from thymus of a child ten days old. 



the lobule as divided into a cortical and a medullary portion. 

 Leucocytes with polymorphous nuclei, also leucocytes with eosino- 

 phile granules, are also found. The medullary portion is usually 

 entirely surrounded by the cortical substance, but may penetrate to 

 the periphery of the lobule, allowing the blood-vessels to enter and 



