200 



NORMAL HISTOLOGY. 



5. The Thymus. — This organ readies its fullest development at 

 about the second year of life, after which retrograde changes, end- 

 ing in the substitution of fibrous and adipose tissues for its proper 

 structure, take place. Its development begins as an ingrowth of 

 epithelium from the branchial clefts. This epithelium forms a 



Fig. 176. 



Two concentric corpuscles of Hassall, from the foetal thymus. (Klein.) 



branching, solid column of cells surrounded by embryonic connec- 

 tive tissue, which develops into lymphadenoid tissue. In the 

 meantime the epithelial strands are broken up and the whole organ 

 becomes converted into a structure resembling a collection of lymph- 

 follicles, but with this difference : that remnants of the epithelial 

 strands remain in the centres of many of the follicles, where their 



Fig. 17 



Lobule from the thymus of a child. (SchSffer.) tr, trabecula; o, nodule of denser lymph- 

 adenoid tissue at periphery ("cortex"); b, b, sections of vessels within the less dense 

 lymphadenoid tissue in the centre (" medulla") ; c, c, concentric corpuscles of Hassall. 



cells become flattened and imbricated. These epithelial masses 

 are known as the concentric corpuscles of Hassall (Fig. 176). 



The thymus is enclosed in a fibrous capsule, which penetrates its 

 substance, dividing it into lobes and lobules. Each of these lobules 

 closely resembles a lymph-follicle, but it is doubtful whether lymph- 



