452 A MANUAL OF PHYSIOLOGY 



understand them, cannot explain the proportions in which they 

 occur in the secretions, or the conditions under which they are 

 separated ; while other constituents and these the more specific 

 and important are manufactured in the gland-cells. 



In the kidneys the conditions seem at first sight favourable 

 to physical separation, as opposed to physiological secretion. 

 Urine has been described as essentially a solution of urea and 

 salts, and both are ready formed in the blood. The arrange- 

 ment of the bloodvessels, too, suggests an apparatus for filtering 

 under pressure. 



FIG. 169. LEUCIN CRYSTALS. FIG. 170. TYROSIN CRYSTALS. 



Bloodvessels and Secreting Tubules of Kidney. The renal artery 

 splits up at the hilus into several branches, which pass in between 

 the Malpighian pyramids, and form at the boundary of the cortex 

 and medulla vascular arches, from which spring, on the one side, 

 interlobular arteries running up into the cortex between the pyramids 

 of Ferrein, and, on the other, vasa recta running down into the 

 boundary layer of the medulla (Fig. 171). The interlobular arteries 

 give off at intervals afferent vessels ; each of these soon breaks up 

 into a glomerulus or tuft of vascular loops, which gather themselves 

 up again into a single efferent vessel of somewhat smaller calibre than 

 the afferent. The glomerulus is fitted into a cup or capsule (of 

 Bowman), which is closely reflected over it, except where the afferent 

 and efferent vessels pass through, and forms the beginning of a 

 urinary tubule. If we suppose the tuft pushed into the blind end of 

 the tubule so as to indent it, it will be easily understood that the 

 single layer of flattened epithelium reflected on the glomerulus is 

 continuous with that lining the capsule, which in its turn is con- 

 tinuous with the epithelial layer of the rest of the urinary tubule. 

 This has been divided by histologists into a number of parts which 

 it is unnecessary to enumerate here, further than to say that the 

 urinary tubule proper begins in the cortex in Bowman's capsule 

 and the proximal convoluted tubule (with its continuation, the spiral 

 tubule), and ends in the cortex with the distal convoluted tubule, 

 the connection between the two being made by a long loop (Henle's) 

 with a descending and an ascending limb (Fig. 172). Between the 

 ascending limb and the distal convoluted tube is interposed the 

 zigzag tubule. The tubule throughout its length is bounded by a 

 basement membrane lined by a single layer of epithelium, which 

 differs in its character in different parts of the tubule. 



The distal convoluted tube joins by means of the short con- 

 necting tubule one of the straight tubules which form the pyramids 

 of Ferrein or medullary rays in the cortex, and which run down into 

 the medulla, always uniting into larger and larger tubes as they 



