KIDNEY AND SKIN AS EXCRETORY ORGANS. 839 



which lower general arterial pressure, such as general vascular dila- 

 tation of the skin vessels, may also depress the secretory action of 

 the kidney by diminishing the amount of blood flowing through it. 



In what way any given change in the vascular conditions of the 

 body will influence the secretion of the kidney depends upon a num- 

 ber of factors and their relations to one another, but any change 

 which will increase the difference in pressure between the blood in 

 the renal artery and the renal vein will tend to augment the flow 

 of blood unless it is antagonized by a simultaneous constriction in 

 the small arteries of the kidney itself. On the contrary, any vas- 

 cular dilatation of the vessels in the kidney will tend to increase 

 the blood-flow through it unless there is at the same time such a 

 general fall of blood-pressure as is suflJicient to lower the pressure 

 in the renal artery and reduce the driving force of the blood to an 

 extent that more than counteracts the favorable influence of dimin- 

 ished resistance in its small arteries. 



Hormone Stimulation. — The evidence for the existence of 

 secretory nerves to the kidney is either negative or unsatisfactory. 

 The secretion of urine is probably controlled through chemical 

 stimuli. Various foreign substances, or the normal constituents of 

 the blood when in excess of a certain concentration, are secreted, 

 presumably, on the general theory adopted, because they stimulate 

 in some way the activity of the kidney cells. 



The increased amount of urine that occurs when the blood- 

 flow through the gland is increased may be referred in a general 

 way to the greater amount of these chemical stimuli, excretory 

 products, etc., that pass through the organ. The general metabo- 

 lism of the body is constantly adding to the blood substances of 

 this kind, which manifest some special reaction with the kidney 

 cells that results in their secretion. But, in addition to these 

 general stimuli, it is possible that specific hormones may be pro- 

 duced that are adapted to stimulate the kidneys and correlate 

 their activity to conditions elsewhere in the body. Schaefer* 

 and Herring have shown that a substance is contained in extracts 

 of the posterior lobe of the pituitary gland which has this action, 

 and it is quite probable that this hormone may function normally, 

 although as yet we know nothing of the conditions under which it 

 acts. Cowf has stated that a similar diuretic hormone is formed 

 in the mucous membrane of the intestine, particularly of the duo- 

 denum and jejunum. When water is taken this hormone is carried 

 into the blood with the absorbed water and is responsible for the 



* Schaefer and Herring, "Philosophical Transactions" (London), B. 199, 1, 

 1906. 



t Cow, "Journal of Physiology," 48, 1, 1914. 



