THE KIDNEY. 



211 



Vein 



lighter inner zone. The former is marked by coarse radial striations, the so-called 

 rods, produced by parallel rows of granules along the protoplasmic threads. The 

 narrow inner zone, next the lumen, is relatively clear, containing few granules and 

 showing a faint striation, due probably to the arrangement of the protoplasmic threads. 

 This "bristle border," as it is sometimes called, is seen only in very well preserved 

 tissue; since it is prone to disintegrate, the partially destroyed border may give rise to 

 appearances mistaken for cilia. During active secretion, the epithelial cells are 

 relatively low and the lumen 

 of the tubule is wide, these 

 relations being reversed dur- 

 ing periods of functional inac- 

 tivity. 



3. The loop of Henle be- 

 gins in the boundary zone 

 between the cortex and me- 

 dulla by the passage of the 

 end-segment of the convoluted 

 tubule into the descending limb, 

 which is distinguished not only 1 

 by the conspicuous reduction 

 in its diameter (12-15 /O. being 

 the narrowest part of the entire 

 renal tubule, but also by the 

 character of its epithelium. 

 The latter consists of low flat- 

 tened elements, in which the 

 ellipsoidal nuclei equal or sur- 

 pass the thickness of the cells, 

 whose cytoplasm is clear or 

 slightly granular. The ascend- 

 ing limb differs from the de- 

 scending in its increased diam- 

 eter (24-28;*), thicker epitheli- 

 um, which is dark and striated, 

 and extension into the cortex. 

 Since the cuboidal cells are 

 often irregular in height, the 

 lumen correspondingly varies, 

 in places being almost oblite- 

 rated. The length of Henle' s 

 loop is influenced by the level 

 of the corresponding Malpighi- 

 an body within the cortex the 

 nearer the medulla the body 

 lies the greater the descent of 

 the loop towards .the papilla, 

 and vice versa. On entering 



the cortex the ascending limb rises to the immediate vicinity of its Malpighian body, 

 around or over which it curves to end in the succeeding distal convoluted tubule. The 

 usual position of the sudden transition from the narrow into the wider part of Henle's 

 loop is in the descending limb a short distance above the loop, although the change 

 may occur beyond the turn, or even within the bend itself. 



4. The distal convoluted or intermediate tubule, from 40-45 // in diameter, pur- 

 sues a moderately tortuous path, marked by a number of abrupt changes in direction, 

 but in a general way is enclosed by the arch of the proximal convoluted segment 

 which it finally crosses. Its epithelium, which at first resembles that of the ascending 

 limb, becomes clearer and less distinctly striated, the cells having a more definitely 

 defined low cylindrical or pyramidal form, although presenting local variations in 

 height. 



FIG. 263. Longitudinal section of renal pyramid, showing 

 general structure of medulla with Henle's loops and collecting 

 tubules. X 45. 



