768 THE THYMUS. [BOOK n. 



these are much less abundant in and more easily fall out of the 

 medulla, so that in sections the medulla appears more transparent 

 than the cortex. It will be observed that this arrangement is 

 almost the reverse of that obtaining in the alveolus of a lymphatic 

 gland, in which the finer gland 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 form of globulin or a proteid allied to globulin 

 which, like the corresponding bodies 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 

 globulin-like body from the thymus, injected into the veins, will 

 give rise to extensive intravascular 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. 



