CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 637 



radially towards the medulla. In this part of its course it is 

 spoken of as the spiral tubule. Still continuing its radial course, 

 the tubule, suddenly diminishing very much in width, passes on 

 for some distance right down into the pyramid below, until, at a 

 level which differs with the different tubules but is always at 

 some distance from the apex of the pyramid, the tubule bends 

 sharply round, and pursues a backward nearly straight course, 

 parallel to its former one, until it finds itself back again in the 

 cortex at some distance from the medulla ; the tubule in fact, in 

 continuation of the spiral segment, makes a loop, the loop of Henle, 

 dipping down into the medulla for a certain distance, and consist- 

 ing of a descending and an ascending limb, both of them running 

 a radial course which is straight or nearly so. The descending 

 limb is as we have said very narrow, but either before it makes the 

 bend, or just at the bend, or at some little distance beyond the 

 bend when it has already become the ascending limb, it enlarges 

 somewhat and changes in character though not reaching the 

 diameter of the spiral or convoluted tubule. Having reached 

 some part or other of the cortex, in a more or less straight radial 

 line, the ascending limb of the loop of Henle changes again in 

 character, becomes still wider, and runs in the cortex a once 

 more distinctly twisted course ; the twists however are not round 

 but angular, giving the tubule a zigzag appearance; hence this 

 portion of the tubule is called the zigzag or sometimes the 

 irregular tubule. Very soon however the turns of the tubule 

 become rounded, and the tubule still running in the cortex 

 assumes characters almost identical with those of the initial con- 

 voluted portion; it now receives the name of the second con- 

 voluted tubule. After several turns of this kind, all confined 

 to the cortex, the tubule once more changes in character and, 

 running a second time in a straight radial course towards the 

 medulla, becomes a collecting tubule pursuing a straight radial 

 course directed towards the apex of a pyramid. The collecting 

 tubule, joining other collecting tubules, and changing slightly 

 in character, while by repeated junctions becoming larger, is 

 continued on as a discharging tubule which, joining other tubules 

 as it passes towards the apex of the pyramids, opens at last into a 

 calyx at or near the summit of the papilla of the pyramid. 



Thus each tubule starting from a Malpighian capsule becomes 

 in succession a first convoluted tubule, a spiral tubule, a descend- 

 ing and ascending limb of a loop of Henle, a zigzag or irregular 

 tubule, a second convoluted tubule, a collecting tubule and 

 finally a discharging tubule. The discharging portion, the lower 

 part of the collecting portion, and some part or other of the loop 

 of Henle lie in the medulla, and form part of one or other of the 

 pyramids. In all the rest of its course the tubule lies in the 

 cortex ; but from what has been said it is obvious that the part of 

 the tubule confined to the cortex can not be called, along the 



