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The Minute Anatomy of the Thymus. [Feb. 16. 



The cysts are formed by the vacuolation and degeneration of the 

 central cells of the small concentric corpuscles ; they are filled with 

 degenerated epithelium, or with haemoglobin masses. The borders of 

 the cysts are at first formed of epithelioid cells, but there is a gradual 

 transition from these flattened cells to ciliated epithelium. 



The process of involution is very gradual ; the first steps are begun 

 in foetal life. The main factors are : — (1) The formation of fibrous 

 tissue within the follicle ; this arises by means of the granular cells, 

 of the giant cells, and of the connective- tissue corpuscles ; (2) the 

 increase in the inter-follicular connective- tissue, and the deposit of 

 plasma cells in this tissue ; the plasma cells are here the forerunners 

 of fat cells ; they are identical with many of the colourless cells of the 

 medulla of bone, and stain in a very characteristic manner in indigo- 

 carmine and carmine solutions ; (3) the invasion of the inter-follicular 

 connective-tissue and plasma cells into the follicle. There is no fatty 

 degeneration of the cells of the thymus. 



There is considerable difference in size between the thymic corpuscles 

 and the colourless blood-corpuscles in amphibia and reptiles, and still 

 greater difference in the bird and the fish. The differences between the 

 thymus of birds, reptiles, and fishes, and that of mammals are also 

 pointed out in the paper. 



Haemoglobin is found in the thymus contained either in cysts, or in 

 cells which are situated near to, or form part of, the concentric 

 corpuscles. The haemoglobin in the cells varies from granules to 

 masses exactly resembling coloured blood-corpuscles ; these masses 

 are oval in tbe bird, reptile, and fish, but circular in all mammals 

 except the camel. 



The lymph issuing from the thymus was obtained by tying the 

 vessels ; it contains more colourless cells than do the large lymphatics 

 of the neck, and, in addition, contains cells inclosing haemoglobin in the 

 form of granules, or in masses resembling coloured blood-corpuscles. 



The physiological conclusions arrived at are: that the thymus forms 

 one source of colourless blood-corpuscles, and that the cells containing 

 haemoglobin masses form coloured blood-corpuscles. This view is sup- 

 ported by the inclosed masses being identical in form with, though 

 often smaller in size than, the coloured blood-corpuscles ; is further 

 supported by finding such cells in the lymphatics, in the blood, in the 

 lymphatic glands, the medulla of bone, and the spleen ; and is still 

 further supported by the development of the blood in the chick, and 

 in the connective-tissue of young mammals. 



