STRUCTURE OF THE KIDNEY IQ9 



diameter from M.WO to ^ooo inch; the collecting tubules pro- 

 gressively increase in diameter from %oo to ^oo inch. The 

 cells lining the convoluted and intermediate tubules are in- 

 clined to the pyramidal shape. Their bases present the ap- 

 pearance of fibers at right angles to the basement membrane 

 (hence "rodded" cells), while their opposite extremities are 

 granular. The tubes of Henle are lined by flattened epi- 

 thelium for the most part. 



The division is somewhat arbitrary, but the secreting por- 

 tion of the tubules is supposed to be confined to the cortical 

 substance, while the tubes of the medullary substance only 

 carry away the fluid. 



Blood Supply. The renal artery, having entered the hi- 

 lum, divides into branches, two of which usually enter each 

 column of Bertin. Running upward in these columns the 

 branches give off small arterial twigs to the substance of 

 the column. When a point opposite the bases of the Malpig- 

 hian pyramids is reached each branch follows the convex 

 base of the pyramid to which it is adjacent, the one branch 

 going in an opposite direction to the other. Each meets a 

 corresponding branch from the other side of the pyramid, 

 and thus a convex arterial arch covers the base of the pyra- 

 mid from which arch branches go inward to supply the me- 

 dullary substance and outward to furnish branches to the 

 glomeruli. The arrangement of the vessels in relation to the 

 Malpighian bodies has been noticed. In the glomerulus the 

 capillaries do not form a true anastomosis, but this is not 

 true of the network surrounding the convoluted tubes. 



Mechanism of Urinary Secretion. Histologists have been 

 unable to demonstrate the presence of distinct secretory 

 fibers for the glomerular or tubal cells. This leaves the me- 

 chanism of secretion to be explained by (i) the vascular 

 supply and by (2) the "vital activity" of the cells both op- 

 erating in conjunction with osmosis. 



Irritation of a certain part of the floor of the fourth ven- 

 tricle occasions certain marked changes in the quantity and 



