THE SECRETION OF THE URINE 491 



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 plashed 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 continuous 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. 189). 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 tubs joins by means of the short connecting 

 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 go, until at length 

 they open as ducts of Bellini on the apex of a papilla. The two convo- 

 luted tubules (with the spiral and zigzag tubules) are lined by similar 

 epithelial cells with granular contents, and the tendency of the granules 

 to be arranged in rows perpendicular to the basement membrane gives 

 them a striated or ' rodded ' appearance (Fig. 190). The granules are 

 eosinophile (p. 17), which is also a character of the granules of other 

 secreting cells. Towards the lumen the cells may show a brush of pro- 

 cesses, looking like cilia, but in mammals these are not motile. The 

 ascending part of Henle's loop also has cells of the same general char- 

 acter, with numerous granules, although the ' rodding ' may not be so 

 distinct. We shall see directly that the morphological resemblance is 

 the index of a f unction ,1 likeness. The blood-supply of the tubules, 

 especially of the convoluted portions, is exceedingly rich, the efferent 

 vessels of the glomeruli breaking up around them into a close-meshed 

 network of capillaries, from which the blood is collected into inter- 

 lobular veins running parallel to the interlobular arteries between the 

 pyramids of Ferrein. The straight tubules of the medulla are also 

 surrounded by capillaries given off from straight arteries (arteriae 

 rectae) running down into it partly from the arterial arches and partly 

 from efferent vessels of the glomeruli nearest the boundary layer, the 

 blood passing away by straight veins (venae rectas) which join the larger 

 veins accompanying the arterial arches. The greater part of the 

 blood going through the kidney has to pass through two sets of capil- 

 laries, one in the glomeruli, the other around the tubules. Even the 

 portion of it which does not go through the glomeruli has for the most 

 part a long route to traverse in narrow arterioles and venules to and 

 from its capillary distribution. And the mean circulation-time through 

 the kidney has been found to be longer than that through most other, 

 organs (p. 137). 



Theories of Renal Secretion. To come back to our problem of 

 the nature of renal secretion, the anatomical structure of the kidney 

 might be expected to throw light upon the question. And, indeed, 

 it was on a purely histological foundation that Bowman established 

 his famous ' vital ' theory of renal secretion. Impressed with the 



