420 A MANUAL OF PHYSIOLOGY 



it. It would, however, be easy to lay too much stress on this 

 distinction, for in the horse the mere dilatation of the blood- 

 vessels 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, perhaps, 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 controlled. 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 ; sup- 

 pression of the secretory function of the skin has nothing to 

 do with death in the first case any more than in the second. 



PRACTICAL EXERCISES ON CHAPTER VL 

 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 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. 526). 



i. Specific Gravity. Pour the urine into a glass cylinder, and 

 remove froth, if necessary, with filter-paper. Place a urinometer 

 (Fig. 130) in the urine, and see that it does not come in contact 

 with the side of the vessel. Read off on the graduated stem the 



