CLASS I. 2. 2. 11. OF IRRITATION. 67 



which nevertheless uncoloured water, or spirits, or quicksilver, 

 will permeate. The same occurs in the filtration of some co- 

 loured fluids through paper, or very fine sand, where the colour- 

 ing matter is not perfectly dissolved, but only diffused through 

 the liquid. This has led some to imagine, that the cause of the 

 whiteness of the hair in elderly people may arise from the dimi- 

 nution, or greater tenuity, of the glandular vessels, which secrete 

 the mucus, which hardens into hair; and that the same difference 

 of the tenuity of the secerning vessels may possibly make the 

 difference of colour of the silk from different silk-worms, which 

 is of all shades from yellow to white. 



But as the secreted fluids are not the consequence of mechani- 

 cal filtration, but of animal selection; we must look out for 

 another cause, which must be found in the decreasing activity of 

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

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

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

 by whatever injures the glands at their roots, as by corrosive blis- 

 ters; and frequently on the human subject by external injuries 

 on the head; and sometimes by fevers. And as the grey colour 

 of hair consists in its want of transparency, like water converted 

 into snow; there is reason to suppose, that a defect of secreted 

 moisture simply may be the cause of this kind of opacity ? as ex- 

 plained in Cataracta, Class I. 2. 2. 13. 



M. M. Whatever prevents the inirrit ability and insensibility 

 of the system, that is, whatever prevents the approach of old age, 

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

 symptom of it. For this purpose, in people, who are not corpu- 

 lent, and perhaps in those who are so, the warm bath twice or 

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

 1. on the colours of animals, and Class I. 1. 2. 15. 



As mechanical injury from a percussion, or a wound, or a 

 caustic, is liable to occasion the hair of the part to become grey; 

 so I suspect the compression of parts against each other of some 

 animals in the wornb is liable to render the hair of those parts 

 of a lighter colour; as seems often to occur in black cats and 

 dogs. A small terrier bitch now stands by me, which is black 

 on all those parts, which were external, when she was wrapped 

 up in the uterus, teres atque rotunda; and those parts white, 

 which were most constantly pressed together; and those parts 

 tawny, which were generally but less constantly pressed to- 

 gether. Thus the hair of the back from the forehead to the end 

 of the tail is black, as well as that of the sides, and external parts 

 of the legs, both before and behind. 



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



