CLASS I. 1. 2. 14. OF IRRITATION. 



manner as the secretion of tears is designed to preserve the cor- 

 nea of the eye moist, and in consequence transparent; yet has 

 this cutaneous mucus been believed by many to be an excrement; 

 and I know not how many fanciful theories have been built on its 

 supposed obstruction. Such as the origin of catarrhs, coughs, 

 inflammations, erysipelas, and herpes. 



To all these it may be sufficient to answer, that the ancient 

 Grecians oiled themselves all over; that some nations have 

 painted themselves all over, as the Picts of this island; that the 

 Hottentots smear themselves all over with grease. And lastly, 

 that many of our own heads at this day are covered with the 

 flour of wheat and the fat of hogs, according to the tyranny of a 

 filthy and wasteful fashion, and all this without inconvenience. 

 To this must be added the strict analogy between the use of the 

 perspirable matter and the mucous fluids, which are poured for 

 similar purposes upon all the internal membranes of the body; 

 and besides its being in its natural state inodorous; which is not 

 so with the other excretions of feces, or of urine. 



The quantity of perspirable matter being greater than that of 

 the excrementitious matters voided by stool and urine, has been 

 used as an argument in favour of its being an excrement; the force 

 of which I do not see: but can readily understand, that there 

 must of necessity be a great exhalation of a fluid which is diffused 

 over the whole external surface of the warm skin, and perhaps 

 warmer lungs, for the purpose of keeping them moist and pliant, 

 and which is perpetually renewed as it evaporates; but, if it be 

 conceived to be an excrement, there seems to have been no ne- 

 cessity for its quantity being so great. 



The evaporation of this great quantity of fluid secreted on the 

 surface of the skin and lungs, must carry off much heat from the 

 body; and as both this secretion and consequent evaporation will 

 be in proportion to the activity of the cutaneous vessels, and the 

 heat occasioned by their increased secretion, it would seem, that 

 this evaporation of perspirable matter is the cause which preserves 

 the animal body at the uniform degree of heat of 98; in the 

 same manner as the evaporation of boiling water preserves it at 

 212 degrees of Fahrenheit's scale. 



The peculiar use of the perspirable matter in preserving the 

 membranes moist, which line the air pipes of the lungs, appears 

 from the curious discovery of Dr. Priestley, that the oxygen of the 

 atmosphere will pass through moist animal membranes, but not 

 through dry ones, so that if the membranes of the trachea were to 

 become dry, the animal must as immediately perish as if he was 

 to breath azotic gas alone. See Sect. XXVIJI. 2. of the pre- 

 ceding volume, 



VOL. n, E 



