CLASS I. 2. 2. 12. OF IRRITATION. 69 



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

 is likewife defcribed with nearly fimilar marks. 



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

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

 gave caufe to the whitenefs of the hair on thcfe parts by delaying 

 or impeding its growth. 



12. Callus. The callous (kin on the hards and feet of laborious 

 people is owing to the extreme vefTels coalefcing from the per- 

 petual prefiure they are expofed to. 



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

 aclion, and their fides grow together ; hence the palenefs of the 

 ftins of elderly people, and the lots of that bloom, which is ow- 

 ing to the numerous line arteries, and the tranfparency of the 

 | flctn, that enciofes them. 



M. M. Warm bath. Paring the thick fkin with a knife. 

 Smoothing it with a pumice (lone. Cover the part with oiled 

 filk to prevent the evaporation of the perfpirable matter, and 

 thus to keep it moid. 



13. Catarafta is an opacity of the cryftalline lens of the eye. 

 It is a difeaie of light-coloured eyes, as the gutta ferena is of 

 dark ones. On cutting oft' with fciflars the cornea of a calf> 

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

 er light, the artery, which fupplies nutriment to the cryftalline 

 humour, is eafily and beautifully feen ; as it rifes from the cen- 

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

 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 fenfibility to light ; as 

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

 in diameter, againft a wall about a foot diftant from each other, 

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

 with one eye, and retreating till you lofe fight of one of the exter- 

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

 vifible, and perhaps carries only a tranfparent fluid, and atlengthin 

 fome fubjecls I fuppofe ceafes to be pervious ; then it follows, that 

 the cryftalline lens, lofmg fome fluid, and gaining none, becomes 

 dry, and in confequence opaque ; for the fame reafon, that wet or 

 piled paper is more tranfparent than when it is dry, as explained in 

 Clafsl. 1.4. i. Thewantof moifture in the cornea of old people, 

 when the exhalation becomes greater than the fupply,is the caufe 

 of its want of tranfparency ; and which like the cryftalline gains 

 rather a milky opacity. The fame analogy may be ufed to explain 

 the whitenefs of the hair of old people, which lofes its peliucidity 

 along with its moifture. See Clafs I. 2. 2. 1 1. 



M. M. Small electric (hocks through the eye. A quarter of 



a 



