STRUCTURE OF THE KIDNEY 199 



shape. Their bases present the appearance 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 epithelium for the most part. 



The division is somewhat arbitrary, but the secreting portion 

 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 hilum, 

 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 Malpighian 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 pyramid, from which arch branches go inward to 

 supply the medullary 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 mechanism of 

 secretion to be explained by (i) the vascular supply and by (2) 

 the "vital activity" of the cells both operating in conjunction 

 with osmosis. 



Irritation of a certain part of the floor of the fourth ventricle 

 occasions certain marked changes in the quantity and quality of 

 the urine; secretion of the upper dorsal cord temporarily arrests 

 the secretion; mental emotions, such as fright, anxiety, etc., also 

 modify the flow. All these circumstances, and many others, 



