CLASS I. 2. 2, 1 1. OF IRRITATION. 67 



which neverthelefs uncoloured water, or fpirits, or quickfilver s 

 will permeate. The fame occurs in the filtration of fome col- 

 oured fluids through paper, or very fine fand, where the colour- 

 ing matter is not perfectly diflblved, but only diffufed through 

 the liquid. This has led fome to imagine, that the caufe of the 

 whitenefs of the hair in elderly people may arife from the dimi- 

 nution, or greater tenuity, of the glandular veffels, which fecrete 

 the mucus, which hardens into hair ; and that the fame differ- 

 ence of the tenuity of the fecerning veflels may pofTibly make the 

 difference of colour of the filk from different lilk- worms, which 

 is of all (hades from yellow to white. 



But as the fecreted fluids are not the confequence of mechan- 

 ical filtration, but of animal felection ; we muft look out for 

 another caufe, which muft be found in the decreafing activity of 

 the glands, as we advance in life ; and which affects many of 

 our other fecretions as well as that of the mucus, which forms 

 the hair. Hence grey hairs are produced on the faces of horfes 

 by whatever injures the glands at their roots, as by corrofive blif- 

 ters; and frequently on the human fubject by external injuries 

 on the head ; and fometimes by fevers. And as the grey col- 

 our of hair confifts in its want of tranfparency, like water con- 

 -verted into fnow -, there is reafon to fuppofe, that a defect: "of fe- 

 creted moifture (imply may be the caufe of this kind of opacity, 

 as explained in Cataracta, Clafs I. 2. 2. 13. 



M. M. Whatever prevents the inirritability and infenfibility 

 of the fyftem, that is, whatever prevents the approach of old age> 

 will fo far counteract; the production of grey hairs, which is a 

 fymptom of it. For this purpofe in people, who are not corpu- 

 lent, and perhaps in thofe who are fo, the warm bath twice or 

 thrice a week is particularly ferviceable. See Sect. XXXIX. 5. 

 i. on the colours of animals, and Clafs I. i. 2. 15. 



As meohanical injury from a percuffion, or a wound, or a cauf- 

 tic, is liable to occafion the hair of the part to become grey; fo 

 I fufpect the compreffion of parts againft each other of fome an- 

 imals in the womb is liable to render the hair of thofe parts of 

 a lighter colour ; as feems often to occur in black cats and dogs. 

 A fmall terrier bitch now (lands by me, which is black on all 

 thofe parts, which were external, when fhe was wrapped up in 

 the uterus, teres atque rotunda ; and thofe parts white, which 

 were moft conftantly preffed together ; and thofe parts tawny, 

 which were generally but lefs conftantly preffed together. Thus 

 the hair of the back from the forehead to the end of the tail is 

 black, as well as that of the fides, and external parts of the legs, 

 both before and behind. 



As in the uterus the chin of the whelp is bent down, and lies 



