476 A MANUAL OF PHYSIOLOGY 



they form connections with ganglion cells ; then, as non-medullated 

 fibres, run along the grey rami, and are distributed to the foot in the 

 sciatic. 



The evidence of the direct secretory action of nerves on the 

 sweat-glands is singularly striking and complete, in contrast to 

 what we know of the kidney. In the latter, blood-flow is the im- 

 portant factor ; increased blood-flow entails increased secretion. 

 In the former, the nervous impulse to secretion is the spring 

 which sets the machinery in motion ; vascular dilatation aids 

 secretion, but does not generally cause it. It would, how- 

 ever, be easy to lay too much stress on this distinction, for in 

 the horse the mere dilatation of the bloodvessels of the head 

 after section of the cervical sympathetic has been found to be 

 accompanied by increased secretion of sweat, and urinary 

 secretion can certainly be affected by the direct action of various 

 substances on the secretory mechanism, independently of 

 vascular changes. But the broad difference stands out clearly 

 enough, and the reason of it lies in the essentially different 

 purpose of the two secretions. The water of the urine is in the 

 main a vehicle for the removal of its solids ; the solids of the 

 sweat are accidental impurities, so to speak, in the water. The 

 kidney eliminates substances which it is vital to the organism 

 to get rid of ; the sweat-glands pour out water, not because it 

 is in itself hurtful, not because it cannot pass out by other 

 channels, but because the evaporation of water is one of the most 

 important means by which the temperature of the body is con- 

 trolled. In short, urine is a true excretion, sweat a heat- 

 regulating secretion. No hurtful effects are produced when 

 elimination by the skin is entirely prevented by varnishing it, 

 provided that the increased loss of heat is compensated. A 

 rabbit with a varnished skin dies of cold, as a rabbit with a 

 closely-clipped or shaven skin does ; suppression of the secretory 

 function of the skin has nothing to do with death in the first 

 case any more than in the second (p. 276). 



PRACTICAL EXERCISES ON CHAPTER VI. 



Urine. 



For most of the experiments human urine is employed in the 

 quantitative work the mixed urine of the twenty-four hours. Urine 

 may also be obtained from animals. In rabbits pressure on the 

 abdomen will usually empty the bladder. Dogs may be taught to 

 micturate at a set time or place, or kept in a cage arranged for the 

 collection of urine. Or a catheter may be used (p. 609). 



