470 THE METABOLIC PROCESSES OF THE BODY. 



It consists of a capsule of connective tissue, plain muscular fibres being 

 absent, and septa or trabeculse of the same nature which divide the organ 

 into a number of irregular more or less cylindrical anastomosing follicles 

 or lobules, and send finer radiating septa into the interior of each lobule. 

 These lobules present the same characters throughout the whole mass of 

 the organ, there not being, as in a lymphatic gland, any distinction between 

 a cortex and a medulla of the whole body. The words are, however, 

 applied to each lobule, to distinguish the central from the peripheral part 

 of the lobule itself. Both the central medulla and the peripheral cortex 

 of each lobule consist of a framework of reticular connective tissue, which 

 in the cortex is identical with or closely allied to adenoid tissue, but in the 

 medulla is coarser and more open and to a larger extent composed of 

 branched anastomosing epithelioid cells. The meshes of the cortex are 

 crowded with leucocytes, but 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 leu- 

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

 the lymph sinus. 



The bloodvessels 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 arrangements 

 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 " con- 

 centric capsules," nests of concentrically disposed nucleated flattened epi- 

 thelial or epithelioid cells. They appear to arise from a proliferation of 

 the epithelioid cells lining small bloodvessels, 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. 



415. 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 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, suc- 

 cinic 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. 



THE HISTORY OF FAT. ADIPOSE TISSUE. 



416. Globules of fat of various sizes make their appearance in the 

 very elements of most of the tissues, in muscular fibres, in epithelial cells, 

 in nerve cells, in leucocytes, and so on ; and the medulla of medullated 

 nerves consists largely of a peculiar fatty material. Besides this certain 

 cells of connective tissue at various times, and in various places, become so 



