!^05 Memoln of Erasmus Darivi/i, M.D. 



to ihe crvstalline humour is easily and beautifully seen ; as 

 it rises From the centre of the optic nerve through the vi- 

 treous huinour to the crystalline. It is this point, where 

 the arterv enters the eye throusrh the cineritious part of the 

 optic nerve, (which is iti part near tlje middle of the nerve,) 

 vvliich is without sensibility to light; as is shown by fixing 

 three pa])er.s, each of them about half an inch in diameter, 

 against a wall about a foot distant from each other, about 

 the heiiiht of the eye ; and then lt)i)king at the middle one, 

 with one fve, and retreating till you lose sight of one of the 

 external papers. Now as the animal grows older, the artery 

 becomefe 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, losingj 

 some fluid, and gaining none, becomes dry, and in con- 

 sequence opake; for the same reason, that wet or oiled pa- 

 per is more transparent than when it is dry, as explained in 

 Class I. 1.4. 1 . The want of moisture in the cornea of old 

 people, when the exhalation becomes greater than the sup- 

 ply, is the cause of its want of transparency ; and which, like 

 the crystalline, 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 mois- 

 ture. 



M. M. Small electric shocks through the eye. A quarter 

 of a grain of corrosive sublimate of mercury dissolved in 

 brandy, or taken in a pill, twice a day for six weeks. 

 Couching by depression, or by extraction. The former of 

 these operations is much to be preferred to the latter, though 

 the latter is at this time so fashionable^, that a surgeon is 

 almost compelled to use it, lest he should not be thought 

 an expert operator. For depressing the cataract is attended 

 with no pain, no danger, no confinement, and may be as 

 readily repealed, if the crystalline should rise again to the 

 centre of the eye. The extraction of the cataract is attended 

 with considerable pain, with long confinement, generally 

 with fever, always with inflammation, and frequently with 

 irreparable injury to the iris, and consequent dai>ger to the 

 whole eye. Yet .has this operation of extraction been trum- 

 peted 



