VARIATIONS IN DIAMETER OF THE URINARY TUBULES. 87 



immediately before it forms the loop, though it is here always 

 materially smaller than in the tortuous portion of the canal. This 

 alteration in diameter continues unaltered to near the point 

 where the canal becomes continuous with the intermediary por- 

 tion on arriving at which it usually becomes slightly narrowed. 

 This, however, is but temporary, and it speedily enlarges again, 

 in the windings of this intermediary portion, to nearly the 

 same diameter that it presented in its tortuous portion. In 

 some few kidneys this dilated part df the intermediary portion 

 offers the remarkable appearance of its hitherto cylindrical 

 calibre, presenting irregular dilatations varying in number and 

 size. In the final curve, by which the intermediary portion 

 joins the collecting tube, the canal once more becomes tempo- 

 rarily narrowed. 



In its course as an isolated tube the canal is thus seen to 

 change its diameter no less than seven times ; viz., 1. The 

 constriction at the neck of the capsule ; 2. The dilatation 

 extending from the neck of the capsule through the tortuous 

 portion ; 3. Constriction as it extends towards the loop of 

 Henle ; 4. Dilatation occurring in the open or ascending limb 

 of the loop ; 5. Constriction where it becomes continuous with 

 the windings of the intermediary portion ; 6. Dilatation in these 

 latter ; and lastly, 7. Constriction in the portion that extends 

 from the intermediary portion towards the collecting tubes, 



We now revert to the consideration of the collecting tube. 

 This, as has already been mentioned, is formed at the cortical 

 extremity of a medullary ray, by the junction of several canals 

 previously running separately into a common trunk, just as 

 the branches of a tree spring from, or are united into, one stem. 



After the collecting tube has been once formed, it still con- 

 tinues to receive a few branches just below the crown, and 

 from thence runs separately, and in a straight direction, to the 

 papillary portion of the medulla; it consequently always 

 remains in the line of a medullary ray. When the several 

 collecting tubes have reached the papillary region, they 

 gradually coalesce, until at length only a few of the originally 

 very numerous tubes remain. In every instance two vessels 

 only join together. In this way all the collecting tubes of one 

 medullary ray coalesce into a few chief tubes, and then the 



