804 THE THYMUS. [BOOK iv. 



substance with its adenoid tissue crowded with leucocytes is 

 placed in the centre, surrounded by the more open network of 

 the lymph sinus. 



The blood vessels of the thymus running along the septa form 

 capillary networks which, though closer and more abundant in the 

 cortical than in the medullary portions of the lobules, have no such 

 special arrangement as obtains in lymphatic glands. 



Lymphatic vessels, abundant in the capsule and septa, are 

 undoubtedly in connection with the substance of the lobules. 



The medullary substance frequently contains bodies, known as 

 " concentric capsules," nests of concentrically disposed nucleated 

 flattened epithelial or epithelioid cells. They appear to arise from 

 a proliferation of the epithelioid cells lining small blood vessels, 

 and have been supposed to be connected with the degenerative 

 changes by which, with obliteration of the vessels, the whole organ 

 dwindles away soon after birth. 



501. From the thymus there may be extracted by means of 

 saline solution a nucleo-albumin which, like the corresponding 

 body from lymphatic glands or from leucocytes, seems to have 

 some special relations to the formation of fibrin. Thus, as has 

 already been said ( 22), a solution of this nucleo-albumin from the 

 thymus, injected into the veins, will give rise to extensive intra- 

 vascular clotting. 



The thymus, like the other bodies on which we are now dwelling, 

 is also rich in extractives. Thus xanthin, hypoxanthin, leucin, 

 lactic, succinic and other acids have been found in it. 



But of what really takes place in the body we have no exact 

 knowledge. Since the thymus is best developed before birth, 

 disappearing after birth at a rate which varies much in different 

 individuals and still more in different kinds of animals, and being 

 eventually replaced by fat and connective tissue, it is obvious that 

 its chief functions are in some way associated with events taking 

 place before birth or in early life. 



