EXCRETION 



323 



We have learned that some glands are very clearly 

 under the command of the central nervous system while 

 others are influenced more particularly by the changing 

 volume of the blood-stream which penetrates their 

 vessels. The kidney is a gland of the latter type. 

 Its activity is usually in proportion to its blood-supply. 

 A general rise of arterial pressure will drive the blood 

 more rapidly than before 

 through the renal capillaries 

 and the response is likely to 

 be prompt and considerable. 

 The explanation of the in- 

 fluence of cold upon the 

 kidneys can probably be ex- 

 plained on this principle. 

 There is first a constriction of 

 the surface vessels. This may 

 lead to a rise of pressure in the 

 aorta and so to an accelerated 

 kidney circulation. It is also 

 possible that when the surface 

 vessels are narrowed those of 

 the deeper parts, including the 

 kidneys, are dilated through 

 nervous influence. In this 

 case there might be no rise of 

 aortic pressure, but the new 

 distribution of blood would 

 secure for the kidneys a larger share than before. 



It may be said, in passing, that the sweat glands are 

 under direct nervous control. While there is usually a 

 correspondence between the degree of their activity and 

 the blood-supply of the skin, this is not always true. 

 We know that there may be cold sweating when the skin 

 is pale and manifestly receiving but little blood. There 

 is other evidence to the same effect : that the glands may 

 produce much perspiration with a restricted allowance 



FIG. 74. A sweat gland 

 wrapped about with capillaries 

 is suggested. It is shown as 

 though there were no other 

 structures below the skin up to 

 which the sweat duct leads. 



