580 VETERINARY PHYSIOLOGY 



possibly secretion may occur in the former and absorption in 

 the latter. 



The kidney responds readily to very small changes in the 

 concentration or composition of the blood. Dilution, even to an 

 amount insufficient to disturb the blood pressure, may lead to 

 increased secretion of water, and the increase of various salts in 

 the blood, and especially of anions, may bring about an increase 

 of secretion. Thus an arrangement is secured by which the 

 composition of the blood plasma is kept constant, and its carry- 

 ing capacity for carbon dioxide is regulated. Hence renal 

 disease may induce disturbance in the respirations (p. 527). 



The Influence of the Nervous System on the Kidneys. 



That renal secretion is fundamentally independent of the 

 control of the central nervous system is shown by the facts 

 (1) that it goes on after the nerves to the kidneys have been 

 cut ; (2) that it proceeds normally in a kidney which has 

 been excised and transplanted. 



As already indicated (p. 573), stimulation of the splanchnic 

 nerves causes a constriction of the renal vessels and a stoppage 

 of the formation of urine. Stimulation of the vagus by 

 inhibiting the heart and lowering the arterial pressure causes 

 a fall in the secretion of urine. Stimulation below the cardiac 

 branch, or after its cardiac endings have been poisoned with 

 atropine, seems to produce no definite result. 



The action of the nervous system is therefore probably 

 entirely through the vaso-motor mechanism. Vaso-constriction 

 may be brought about — 



1st. By direct stimulation of the vaso-constrictor centre as 

 in asphyxia. 



Ind. Reflexly by stimulation of many ingoing nerves, 

 e.g. (a) by the application of cold to the skin ; (6) by 

 irritation of the bladder or urethra after the use of the 

 catheter. 



Vaso-dilator effects on the kidney are produced by stimu- 

 lating the posterior roots of the lower dorsal nerves, which may 

 explain the beneficial action of warm applications over the 



