4i8 



COMPARATIVE ANATOMY 



known as the descending limb of Henle's loop. Bending sharply, it 

 forms the ascending limb of Henle's loop, and returns to the cortex, where 

 it becomes convoluted again, the distal convoluted portion, and unites 

 with a collecting tubule. As an element in a cortical ray, each collecting 

 tubule passes from the cortex into a pyramid. Usually a group of col- 

 lecting tubules unite and open by a common aperture into the pelvis 

 of the ureter. There is evidence that the secretion of urine takes place 

 chiefly in the renal corpuscles and in the convoluted portions of the 

 tubules, while the limbs of Henle's loop are chiefly absorptive in function. 

 The high degree of concentration of the constituents of urine leads physi- 

 ologists to assume that the quantity of water excreted by the renal 

 corpuscles is many times greater than that which finds its way through 



PROXIMAL -CONVOLUTED TUBUL; 



COLLECTING TUBULE 



CAPILLARY ^, 



GLOMERULUS 



A. CORTEX B. MEDULLA 



Fig. 343.- — Sections — A and B — of the human kidney. Section A is taken from the 

 region of the cortex, B from the medulla. Glomeruli which are abundant in the cortex 

 are lacking in the medulla. (Redrawn from Bremer after Schaper.) 



the collecting tubules into the ureter, and consequently to conclude that 

 most of this water is reabsorbed by the tubules. Their great length is 

 assumed to be an adaptation to this absorptive function. (Fig. 341) 



Blood Supply. Arteries enter and veins leave the kidney by way of 

 the hilum on the median side of each kidney. Within the renal columns, 

 branches of the renal artery form interlobar arteries. Passing between the 

 medulla and cortex, these, in turn, become the arcuate arteries, and 

 give off branches both to the medulla and the cortex. In the cortex, 

 they are known as interlobular arteries, from which branches supply the 

 glomeruli. The arteriole which carries blood to a glomerulus is larger 

 than that which leaves the glomerulus. By this arrangement, blood 

 pressure in the glomerulus is considerably greater than that in capil- 

 laries generally, so that filtration through the glomerular capillaries is 

 increased. The renal veins parallel the arteries and receive the same 

 names. Lymphatics and sympathetic nerves are abundant. 



