STRUCTURE OP THE KIDNEY 197 



Uriniferous Tubules. These begin at the glomeruli and end 

 at the apices of the Malpighian pyramids. From their tortuous 

 course in the cortical portion they are there called convoluted 

 tubules, in contradistinction to the straight tubes of the medullary 

 portion. This, however, is only a general division; further 

 divisions are to be noted. 



The constricted portion of the tube where it leaves the glomer- 

 ulus is the (i) neck; passing away from the neck the tubule be- 

 comes very tortuous and is known as the (2) primary convo- 

 luted tubule, which, having run for a variable distance, becomes 

 narrow near the base of the pyramid, and taking a comparatively 

 straight course downward enters the pyramid under the name of 

 the (3) descending limb of Henle's loop; some of these run nearly 

 as far as the apex, but most of them near the base or middle of the 

 pyramid turn upward forming thus (4) Henle's loop and be- 

 ginning the (5) ascending limb of Henle's loop; the tube having 

 reentered the cortical substance becomes convoluted again, 

 (6) secondary convolution, which, by a less tortuous continua- 

 tion, the (7) intermediate tube, communicates with the collecting 

 tubules, or the (8) straight tubes of Bellini; these last beginning 

 in the cortex, and receiving in their course large numbers of inter- 

 mediate tubes, enter the base of the pyramid and run in a nearly 

 straight direction toward the apex. About 100 of these straight 

 tubes entering at the base join in their course downward until 

 at the apex they are represented by a single tube. These collec- 

 tions constitute the pyramids of Ferrein; there are about 12-18 

 pyramids of Ferrein to each Malpighian pyramid, and as many 

 tubal orifices at the apex. The so-called zigzag and spiral 

 tubules are here considered parts of the first and second convo- 

 luted tubules. (See Fig. 58.) 



Before they reach the collecting tubules the tubes vary in diam- 

 eter from T oVo to ToVfr inch; the collecting tubules progressively 

 increase in diameter from -^ to -%fa inch. The cells lining the 

 convoluted and intermediate tubules are inclined to the pyramidal 



