486 



EXCRETION 



The distal convoluted tub: 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- 



Fig. 190. From a Vertical Section of Dog's Kidney to show the Structure of Different 

 Portions of the Renal Tubule (Klein), a, Bowman's capsule enclosing glomerulus, 

 the capillaries of which are arranged in lobules separated by a little connective 

 tissue. The capsule and glomerulus together constitute a Malpighian body or 

 corpuscle ; n, neck of capsule ; c, c, convoluted tubules, cut in various directions ; 

 b, irregular or zigzag tubule ; d, e, and/ are straight tubules, which take part in the 

 formation of a medullary ray or pyramid of Ferrein ; d, collecting tubule ; e, e, spiral 

 tubule; /, narrow part of ascending limb of Henle's loop-tubule; b, c, and e are 

 lined with rodded epithelium. 



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 functional 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 (arterise 



