648 FUNCTIONS OF ANIMALS. 



when the strength is unimpaired, and also when enfeebled by 

 scanty food. 



i 



295 



CHAPTER IV. 



OF PERSPIRATION. 



IT is well known that considerable quantities of matter in the 

 state of vapour are constantly emitted from the skin. This va- 

 pour is called perspirable matter or perspiration. When, by the 

 sudden application of cold, the exhalents by which this va- 

 pour is thrown out are shut, the system becomes deranged, 

 and what we call in common language a cold is the conse- 

 quence. All the facts respecting the quantity and nature of 

 this perspired matter at present known have been stated under 

 the title of perspiration and sweat in a preceding chapter of this 

 work, to which the reader is referred. Nothing is known re- 

 specting the nature of the process. The exhalent vessels are 

 situated in the skin, and, according to modern anatomists, are 

 twisted in the form of a cork-screw. They are exceedingly small 

 in diameter, and their open mouths terminate just under the epi- 

 dermis. The process of perspiration is very similar to respira- 

 tion. Whether the external air has any thing to do with it has 

 not been ascertained ; but it is probable that it has. Water, car- 

 bonic acid, lactic acid, and an oily matter having a peculiar 

 smell, are thrown out from the blood-vessels of the skin, and 

 doubtless in considerable quantity. 



It has been supposed that the skin has the property of absorb- 

 ing moisture from the air, but this opinion has not been confirm- 

 ed by experiments, but rather the contrary. 



The chief arguments in favour of absorption of the skin have 

 been drawn from the quantity of moisture discharged by urine, 

 being, in some cases, not only greater than the whole drink of 

 the patient, but even than the whole of his drink and food. But 

 it ought to be remembered that, in diabetes, the disease here al- 

 luded to, the weight of the body is continually diminishing, and 

 therefore part of it must be constantly thrown off. Besides, it is 

 scarcely possible in that disease to get an accurate account of the 

 food swallowed by the patients ; and in those cases where very 



