406 



THE KIDNEYS. 



slightly diverging lines, and they continue thus to divide and subdivide 

 until they reach the sides and bases of the pyramids, whence they pass, 

 greatly augmented in number, into the cortical substance, where they 

 become convoluted. In the cortical part the straight tubules belonging 

 to a Malpighian pyramid are continued for some way, in several groups 

 or bundles, the tubules in the centre of which approach nearer the 

 surface than those at the sides, which sooner become convoluted, so that 

 conical bundles are formed which have been termed p>j)wmds of Fcrrein, 

 several of which therefore correspond to a single Malpighian pyramid. 



The tubes commence in the cortical substance by spherical dilatations 

 (fig. 287, I) enclosing like a capsule the vascular Malpighian tufts to be 

 afterwards described. Arising in this manner, the tubes are at first 

 much convoluted (ii) ; they then become smaller (iii), pass straight 

 down in the pyramid towards the papilla, and return again (iv), forming 

 the looped tubes of Henlc. Each of these tubes on returning to the 

 cortical substance becomes again convoluted (v), and joins one of the 

 straight collecting tubes (vi). This part is named the junctional tube. 

 The collecting tulles uniting together eventually form the excretory 

 tubes (IX) (often called duds of Bellini), which open on the papillaa. 



Structure of the tubules. — The tubules consist in every case of a 

 basement membrane and epithelium, but the form and character of the 



Fig. 288. 



Convoluted Tubules of Kidney. About 400 Diameters. 



or, transverse, a', longitudinal section of a tubule from the human kidney ; h, portion 

 of a tubule from the dog's kidney, showing the striated appearance of the base of the 

 cells, somewhat analogous to that observed in the tubules of the submaxillary gland. 



latter, as well as the size of the tubes, varies considerably in the different 

 parts. In the straight or collecting and excretory tubes the epithelium 

 is columnar in form, the cells are distinct, and the lumen of the tube 

 is of considerable size. These tubules are largest near their termination, 

 at a short distance from which within the papilla), their diameter varies, 

 according to Huschke, from Trloth to ^^oth of an inch. Further on in 

 the pyramid they become smaller, measuring about -^h^ih of an inch 

 in diameter, and then do not diminish as they continue to bifurcate, 

 but remain nearly of the same uniform average diameter. 



