CHAP, iv.] METABOLIC PROCESSES OF THE BODY. 803 



When an extract of the suprarenal bodies is injected, even in small 

 quantity, into the body of a sound animal distinct physiological 

 effects are produced, among them constriction of the blood vessels, 

 inhibition of the heart, and a prolongation of the contractions 

 of the skeletal muscles not unlike that characteristic of the action 

 of the drug veratrin. One fact, gained by clinical experience, is of 

 interest. 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 con- 

 nection 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 struc- 

 ture, 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 of trabeculse of the same nature the 

 latter dividing the organ into a number of irregular more or less 

 cylindrical anastomosing follicles or lobules, and sending 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 frame-work 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 anastomo- 

 sing 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 



