THE EXCRETION OP PERSPIRATION. 413 



even the major part of the water, is a ^genuine secretion, the 

 result of the metabolism of the cells. 



From experiments it is clear that nervous influences alone, 

 in the absence of any vascular changes, or in the total depriva- 

 tion of blood, suffice to induce the secretion of perspiration. If 

 the central stump of the divided sciatic be stimulated, sweating 

 of the other limbs follows, showing that perspiration may be a 

 reflex act. It is found that stimulation of the peripheral end of 

 the divided cervical sympathetic leads to sweating on the cor- 

 responding side of the face. 



Sweating during dyspncsa and fron^ fear, when the cutane- 

 ous surfaces are pale, as well as in the dying animal, shows also 

 the independent influence over the sudorific glands of the nerv- 

 ous system. Heat induces sweating by acting both reflexly and 

 directly on the sweat-centers we may suppose. Unilateral 

 sweating is known as a pathological as well as experimental 

 phenomenon. Perspiration may be either increased or dimin- 

 ished in paralyzed limbs, according to circumstances. It is 

 possible that there is a paralytic secretion of sweat as of saJiva. 

 The subject is very intricate, and will be referred to again on 

 account of the light it throws on metabolic processes generally. 



Absorption by the skin in man and other mammals is, under 

 natural conditions probably very slight, as would be expected 

 when it is borne in mind that the true skin is covered by sev- 

 eral layers of cells, the outer of which are hardened. 



Ointments may unquestionably be forced in by rubbing ; 

 and perhaps absorption may take place when an animal's tis- 

 sues are starving, and food can not be made available through 

 the usual channels. It is certain that abraded surfaces are a 

 source of danger, from affording a means of entrance for dis- 

 ease-producing substances or for germs. 



Comparative. — It is usually stated in works on physiology 

 that the horse sweats profusely, the ox less so ; the pig in the 

 snout; and the dog, cat, rabbit, rat, and mouse, either not at all 

 or in the feet (between the toes) only. That a closer observa- 

 tion of these animals will convince any one that the latter 

 statements are not strictly correct, we have no doubt. These 

 animals, it is true, do not perspire sensibly to any great extent ; 

 but to maintain that their skin has no excretory function is an 

 error. 



Summary.— The skin of the mammal has protective, sensory, 

 respiratory, and excretory functions. The respiratory are in- 



