CHAP. IIL] ELIMINATION OF WASTE PRODUCTS. 669 



whole length of its course, a twisted or contorted tubule. The 

 upper part of the collecting tubule though still lying in the cortex 

 runs nearly straight; the beginning of the descending limb and 

 the end of the ascending limb of the loop of Henle though lying 

 in the cortex are nearly straight ; and even the spiral tubule is not 

 far removed from being straight. So that the cortex does not 

 consist of convoluted tubules only but in part of tubules more or 

 less straight. These however are not dispersed uniformly among 

 the convoluted tubules, but are gathered into bundles which run 

 in a radial direction from the bases of the pyramids through the 

 cortex towards the capsule. The bundles, of which there are 

 several to each pyramid, are called medullary rays or pyramids 

 of Ferrein (the large pyramids of the medulla being then 

 distinguished as the pyramids of Malpighi). 



Between and surrounding the several medullary rays are 

 masses of cortex, seen in radial sections as columns between two 

 adjacent rays, consisting of convoluted tubules, both first and 

 second, of zigzag tubules, and as we shall see of Malpighian 

 capsules ; all the tubules in the column are most distinctly twisted 

 and contorted, since the column contains only the very beginnings 

 of the spiral tubule, and the collecting tubule. The spiral tubule 

 beginning in the column of cortex between the medullary rays 

 makes at once for a medullary ray down which it runs to become 

 a descending limb of the loop of Henle; the ascending limb 

 coming up from the medulla runs in a medullary ray and only 

 leaves it to become a zigzag tubule ; and each collecting tubule 

 runs straight into a medullary ray and thence away into the 

 medulla. Hence each ray consists of spiral tubules, descending 

 and ascending limbs (especially the latter) of the loops of Henle, 

 and collecting tubules. 



Since each medullary ray receives spiral tubules and collecting 

 tubules, and gives off zigzag tubules at different levels above the 

 bases of the pyramids, it must be thicker below, where it holds all 

 the tubules which it has received or is about to give off, than 

 higher up, where it has already given off some tubules and has not 

 yet received all the tubules which it will receive. It diminishes 

 in fact, pyramid fashion (hence the name pyramid of Ferrein), 

 towards the surface of the kidney; and indeed just below the 

 capsule there is a layer of some little thickness consisting entirely 

 of cortical substance, that is of convoluted tubules, the medullary 

 rays not having as yet begun. 



It is obvious that the upper part of each pyramid of the 

 medulla differs from the lower part, in so far as that while the 

 latter contains straight tubules only, and these mostly discharging 

 tubules, the former contains, besides collecting and discharging 

 tubules, the ends of the loops of Henle, which are really parts 

 of the tubules in what we have called generally their twisted, or 

 devious course. Hence the upper part of the medulla contiguous 



