76 



PHYSIOLOGY OF FARM ANIMALS 



[CH. 



lining the convoluted tubules and those of the ascending linih of 

 the loop of Henle are granular and striated and generally large, 

 while those of the collecting tubules and the descending limb of 

 the loop of Henle are cubical and free from granules; 



The Mechanism of Renal Secretion. There appear to be two 

 main factors in the secretion of urine, (1) the supply of blood, and 

 (2) the composition of the blood flowing through the kidney. 

 There are no secretory nerves going to the kidney. 



l^jjA J> CORTEX 



> MEDULLA 



Fig. 41. Diagi'ammatic view of the course of the tubules in the 

 kidney. A, Malpi^hian capusle containing a glomerulus ; 

 B, first convoluted portion of tubule ; C, descending limb 

 of loop of Henle; D, ascending limb of loop of Henle; 

 E, second convoluted portion ; F, collecting tubule ; G, apex 

 of pyramid; S, spiral tubule ; ra represents a small branch 

 of the renal artery, which breaks up into a bunch of looped 

 capillaries, the glomerulus finally joining up again to form 

 the vein re. 



That the secretion of urine is affected by the blood supply to 

 the kidney is rendered obvious when we consider the result of 

 .different degrees of temperature upon the skin. Cold causes a 

 constriction of the vessels in the skin and consequently lessens 

 the supply of blood to the skin, while the supply going to the 

 kidney is correspondingly increased. Conversely when the skin 

 is hot and flushed, and when evaporation of moisture from the 



