654 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



Renal Secretion, the activity of the skin as a secreting organ is, as a rule, 

 inverse to that of the kidnej^. As a consequence, in warm weather the 

 cutaneous blood-vessels relax, more blood circulates through the skin, 

 and the perspiration is, therefore, increased, while the amount of water 

 eliminated by the kidnej's is decreased. Hence, in warm weather the 

 urine is scanty and of high specific gravity. On the other hand, in cold 

 weather the cutaneous capillaries are contracted, the activity of the sweat- 

 glands diminished, while from the increased blood pressure in the internal 

 organs the activity of the kidney, especially of the glomeruli, is increased, 

 and the urine is now more abundant in quantity and of lower specific 

 gravity. 



The formation of sweat is a true secretion and is dependent upon 

 the secretory activity of the epithelial cells of the sweat-glands. The 

 relation between the activity of the sweat-glands and of the blood supply 

 has been very clearty made out. Nearly all conditions which increase 

 the blood supply to a part will lead to an increased secretion of sweat. 

 This, as already mentioned, will probably explain the increased secretion 

 when the skin is subjected to a high temperature. 



Bernard also succeeded in producing secretion by the skin by 

 division of the vaso-motor nerves supplying the part. Thus, division of 

 the cervical sympathetic in the horse will cause an abundant secretion of 

 sweat on the corresponding side of the face. 



The sweat-glands are further governed by special secretory nerves. 

 This statement is supported not only by the production of sweat in 

 various pathological conditions and in the evident influence of the 

 emotions on the sweat secretion, even in the absence of increased circu- 

 lation, but also has been demonstrated by direct experiment. 



If in a dog or cat the peripheral end of the sciatic nerve be stimu- 

 lated with an interrupted current, a profuse secretion of perspiration is 

 produced on the balls of the toes. Such a secretion is evidently not 

 produced by modifications of the blood supply ; for stimulation of the 

 sciatic nerve, as a rule, may be said to lead to the constriction of the 

 blood-vessels in this part, and the secretion may even be produced after 

 ligation of the blood-vessels of the limb or even after its amputation. 

 Moreover, the vaso-motor effects may be produced, as in the case of the 

 secretion of the saliva, and the secretory effects prevented by the injection 

 of atropine. The analogy, therefore, between the secretion of sweat and 

 that of saliva is clearly established, and we are warranted in stating that 

 the sciatic nerve, like the chorda tympani, contains special secretory 

 fibres whose stimulation leads to an increased activity of the secretory 

 epithelium. 



Moreover, experiment enables us to determine that the sweating 

 produced by exposure to high temperature is not solely due to the 



