SECRETIONS 105 



absorbed, are oxidized to indoxyl and skatoxyl. From this 

 decomposition of the proteid in the intestine arise aromatic (L. 

 oxyacids (oxyphenyl- acetic acid and oxyphenyl-propionic acid), 

 which are absorbed from the intestine and excreted by the kidneys. 



(c) In small quantities there are present in the urine 

 urinary pigments, among which is sometimes found urobilin, 

 which is supposed to be identical with hydro-bilirubin. 



Finally there are present in urine, gases, chiefly carbon 

 dioxide, traces of nitrogen and oxygen. . 



In certain diseases, e.g. diabetes, the urine contains 

 grape-sugar, aceton, oxybutyric acid and diacetic acid; in 

 inflammation of kidneys it contains proteids ; in icterus, bile 

 pigments and acids; and in haematuria, blood pigments. 



2. The structure of the kidneys. 



The kidneys are composed of a uniformly darkly stained cortex 

 and a radially striated medulla consisting of a large number of 

 Malpighian pyramids. 



In the cortex the uriniferous tubules of the gland are coiled 

 (convoluted tubules); in the medulla they are straight (tubuli 

 recti). Each uriniferous tubule begins in the cortex (R, Fig. 7) 

 as a spherical dilation, the Malpighian body (g), from which pro- 

 ceeds the convoluted tubule (/").' This proceeds downward into 

 the medulla, M, as a straight tubule (e), then turns and forms the 

 loop of Henle. The ascending limb' of Henle's loop joins the 

 intercalated part (c), which, turning downward towards the pelvis, 

 forms a straight collecting tubule (b). The collecting tubules join 

 to form a duct (a), which at the apex of the pyramids empties into 

 the pelvis of the kidneys. 



The Malpighian bodies are composed of a knot of blood vessels, 

 the glomerulus (g), which is placed in the blind, sac-like ending 

 of the uniferous tubule (the Bowman capsule) so that it is almost 

 entirely surrounded by the capsule. The fold of the capsule 

 bordering on the glomerulus is composed, in young individuals, 

 of cuboidal cells, while in older persons the cells are flattened. 

 The outer fold of the capsule is made up of flat polygonal cells. 

 It is continued downward, forming the walls of the convoluted 

 tubule whose cells are radially striated and have granular proto- 

 plasm. The cells in the walls of Henle's loop, of the intercalated 

 portion, and of the collective tubules are cylindrical epithelial cells. 



The uriniferous tubules are surrounded by connective tissue, in 

 which are found the blood vessels. 



The branches of the renal artery proceed from the hilus to the 

 boundary between the cortex and medulla (a, Fig. 8). Here they 



