CHAP, iv.] METABOLIC PROCESSES OF THE BODY. 767 



suggest on the one hand that important metabolic processes take 

 place in them, some of which are probably connected with the 

 history of the pigments of the body at large. On the other hand 

 the unusually large nerve supply, and the derivation of part of the 

 body from the sympathetic ganglia, suggest peculiar nervous con- 

 nections. And the organ has often served as a starting point for 

 speculations in these two directions ; but our exact knowledge 

 concerning them is very limited. The results of experiment have 

 taught us little ; extirpation for example has been often followed 

 by the death of the animal operated upon, but the cause of the 

 death in such cases is by no means clear. 



One fact, gained by clinical experience, is the only real item of 

 knowledge which we possess. Disease of the suprarenal bodies, 

 apparently tubercular in nature and beginning in the medulla, 

 is so often associated with a change in the colour of, with an 

 increase of the pigment of the skin, 'bronzed skin', 'Addison's 

 disease ', that some connection between the two must exist ; but 

 the several links of the chain are as yet unknown. It is tempting 

 to associate the increase of pigment in the bronzed skin with the 

 chromogen or colour-yielding substance spoken of above ; but we 

 have no warrant for doing so, such for instance as any indication 

 of ties between the suprarenal bodies and changes either in 

 haemoglobin itself or in bilirubin, which two bodies we have 

 reason to regard more particularly as mothers of pigment. 

 Moreover the bronzed skin is only one of the symptoms of 

 Addison's disease, failure of nutrition and nervous symptoms 

 being also present. 



500. The Thymus, This, though it arises in the embryo 

 as a paired outgrowth from the epithelial walls of a pair of 

 visceral clefts, and thus begins as an epithelial structure into 

 which mesoblastic elements subsequently intrude, soon puts on 

 such characters as to appear essentially a lymphatic structure, and 

 indeed might be regarded as a part of the lymphatic system. 



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 radiat- 

 ing 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 



