THE THYMUS BODY. 1799 



throughout the cortex, particularly in the neighborhood of the capillaries. Nucleated 

 red blood- cells have been described within the cortex (J. Schaffer). 



The medullary substance, although varying in its details according to the gen- 

 eral condition of the organ, consists of a supporting framework, composed of 

 branching cells, within the meshes of which lie small mononuclear lymphocytes, less 

 frequently polymorphonuclear leucocytes. Occasional eosinophiles are seen along 

 the blood-vessels, as well as multinuclear giant cells. Islands or cords of flattened 

 elements, regarded by many as epithelial in nature and derivatives of the primary 

 entoblastic anlage, also occur. The medulla of the fully developed thymus, or of 

 the organ just entering upon its retrogression, contains numerous spherical or ellip- 

 soidal masses of concentrically disposed, flattened modified cells. These bodies are 

 the corpuscles of Hassall, which were regarded as the direct remains of the epithe- 

 lium of which for a time the thymus was composed. Found only in the medulla, 

 they vary greatly in form and size, sometimes being simple spherical masses (.012- 

 .020 mm. in diameter), at others composite bodies (.1 mm. or more in diameter) 

 consisting of aggregations of small groups. The centre of the concentric bodies 

 often consists of slightly glistening, homogeneous, or granular substance which is 



FIG. 1519. 



Lobule 



Blood- 

 vessels, 

 injected 



Medulla 



- 



"-Cortex 



Transverse section of thymus body of child, showing general arrangement of lobules. X 25. 



albuminous, not fatty, In nature. According to Hammar, the corpuscles of Hassall 

 arise from hypertrophied reticulum cells, the latter being directly derived from the 

 primary epithelium. 



Vessels. The arteries are chiefly from the internal mammaries, but small 

 branches may come from the thyroid as well as from the pericardial arteries. The 

 arteries gain the interior of the lobule, and break up into capillaries along the junc- 

 tion of the cortical and medullary zones. The cortex is provided with a rich capil- 

 lary net-work, the medulla being relatively poorly supplied. The veins between the 

 lobules, which chiefly drain the capillaries, unite to form the larger trunks carrying 

 the blood from the organ. These run in many directions, the most important being 

 tributary to the left innominate. The lymphatics are large and numerous, and empty 

 into nodes behind the sternum. Traced into the interior of the organ, the lym- 

 phatics follow the connective-tissue septa to the lobules, around which they form a rich 

 plexus. Although it is probable that the lymph-paths come into close relations with 

 the thymus-tissue, the existence of intralobular passages, corresponding to lymph- 

 sinuses, has not been established. 



