THE THYMUS 



567 



its circumference, the medulla reaches the surface of the lobule^ and 

 at such locations a narrow column of medullary lymphoid tissue connects 

 it with the adjoining lobule. 



A close-meshed reticulum, within the narrow meshes of which are 

 closely packed lymphoid cells, composes the lymp > Md^jfeue T> oJJJb^.loJmle. 

 That of^Tte ' Coftex ancfthe medulla is alike, except for the fact that 

 the meshes of the reticulum in the cortex are much more crowded 

 with 'lymphocytes' than are those of the medulla. The medulla of 

 each lobule is also charac- 

 terized by thejpresence of 

 several groups of concen- 

 trically arranged epitheli- 

 oid cells, the thymic cor- 

 puscles (concentric corpus- 

 cles of H assail). 



Each thymic corpuscle^ 

 consists of a large central 

 cell or group of cells, which 

 is surrounded by two to, five 

 layers of concentrically ar- 

 ranged flat epithelioi_d cells. 



These groups *Tr cell nests FlG 4gg _ A THYMIC CORPUSCLE FROM THE 

 are strongly acidophil in THYMUS OF AN INFANT. 



their staining reactions, Hematein and eosin. X 665. 



and therefore stknd out in 



marked contrast to the basophilic nuqlei of the surrounding lymphoid 

 tissue. A substanie simulating the colloid of the thyroid may be present 

 among the cells, &ince no similar structure occurs elsewhere in the body, 

 the concentric corpuscles are absolutely characteristic of the thymus 

 lobule. 



The nature of the tSWrnic corpuscles is not satisfactorily understood. 



According to one hypot 

 has been obliterated by 

 theory regards them as 

 the organ arose in the 

 of hypertrophied cells o 



lesis they represent blood-vessels whose lumen 

 proliferation of its endothelial cells. Another 

 remains of the epithelial columns from which 

 imbryo. They are also interpreted as masses 

 the reticulum. 



The types of lymphoid corpuscle which are found in the thymus 



are similar to those of the lymph nodes, though polymorphonuclear 



leukocytes are rather more frequent here, and giant cells, mononuclear 



or multinuclear in form, may be readily found in the medulla of this 



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