CLASS 1. 2. 2. 12. OF IRRITATION, 69 



can Philosophical Society, Vol. II. page 292, where a female one 

 is likewise described with nearly similar marks. 



The joining of the frontal bones, and the bregma, having been 

 later than that of the other sutures of the cranium, probably 

 gave cause to the whiteness of the hair on these parts by delaying 

 or impeding its growth. 



12. Callus. The callous skin on the hands and feet of laborious 

 people is owing to the extreme vessels coalescing from the per- 

 petual pressure they are exposed to. 



As we advance in life, the finer arteries lose their power of 

 action, and their sides grow together; hence the paleness oi the 

 skins of elderly people, and the loss of that bloom, which is 

 owing to the numerous fine arteries, and the transparency of the 

 skin, that encloses them. 



M, M. Warm bath. Paring the thick skin with a knife. 

 Smoothing it with a pumice stone. Cover the part with oiled 

 silk to prevent the evaporation of the perspirable matter, and 

 thus to keep it moist. 



13. Calaracta is an opacity of the crystalline lens of the eye. 

 It is a disease of light-coloured eyes, as the gutta serena is of 

 dark ones. On cutting off with scissors the cornea of a calf's 

 eye, and holding it in the palm of one's hand, so as to gain a pro- 

 per light, the artery, which supplies nutriment to the crystalline 

 humour, is easily and beautifully seen; as it rises from the cen- 

 tre of the optic nerve through the vitreous humour to the crys- 

 talline. It is this point, where the artery enters the eye through 

 the cineritious part of the optic nerve, (which is in part near the 

 middle of the nerve,) which is without sensibility to light; as 

 is shewn by fixing three papers, each of them about half an inch 

 in diameter, against a w r all about a foot distant from each other, 

 about the height of the eye; and then looking at the middle one, 

 with one eye v and retreating till you lose sight of one of the exter- 

 nal papers. Now as the animal grows older, the artery becomes less 

 visible, and perhaps carries only a transparent fluid, and at length 

 in some subjects I suppose ceases to be pervious; then it follows 

 that the crystalline lens, losing some fluid, and gaining none, be- 

 comes dry, and in consequence opaque; for the same reason, 

 that wet or oiled paper is more transparent than when it is dry, as 

 explained in Class 1. 1. 4. 1. The want of moisture in the cornea of 

 old people, when the exhalation becomes greater than the supply, 

 is the cause of its want of transparency; and which like the crys- 

 talline gains rather a milky opacity. The same analogy may be used 

 to explain the whiteness of the hair of old people, which loses 

 its pellucidity along with its moisture. See Class 1.2.2.11. 



M. M. Smnil electric shocks through the eye. A quarter of 



