322 PHILOSOPHICAL TRANSACTIONS. [aNNO 17Q3. 



art. 370) that such an eye will collect those rays on the retina, which diverge 

 from a point at the distance of Ti^ inches. This is a greater change than is 

 necessary for an ox's eye; for if it be supposed capable of distinct vision at a 

 distance somewhat less than 12 inches, yet it probably is far .short of being able 

 to collect parallel rays. The human crystalline is susceptible of a much greater 

 change of form. 



The ciliary zone may admit of as much extension as this diminution of the 

 diameter of the crystalline will require; and its elasticity will assist the cellular 

 texture of the vitreous humour, and perliaps the gelatinous part of the crystalline, 

 in restoring the indolent form. It may be questioned whether the retina takes 

 any part in supplying the lens with nerves; but, from the analogy of the olfac- 

 tory and auditory nerves, it seems more reasonable to suppose that the optic 

 nerve serves no other purpose but that of conveying sensation to the brain. 



Though a strong light and close examination are required, in order to see the 

 fibres of the crystalline in its entire state, yet their direction may be demon- 

 strated, and their attachment shown, without much difficulty. In a dead eye 

 the tendons are discernible through the capsule, and sometimes the anterior ones 

 even through the cornea and aqueous humour. When the crystalline falls, it 

 very frequently separates as far as the centre into 3 portions, each having a ten- 

 don in its middle. If it be carefully stripped of its capsule, and the smart blast 

 of a fine blow-pipe be applied close to its surface in different parts, it will be 

 found to crack exactly in the direction of the fibres above described, and all these 

 cracks will be stopped as soon as they reach either of the tendons. The applica- 

 tion of a little ink to the crystalline is of great use in showing the course of 

 the fibres. 



When first I observed the structure of the crystalline, I was not aware that 

 its muscularity had ever been suspected. We have however seen that Descartes 

 supposed it to be of this nature; but he seems to think that the accomodation 

 of the eye to a small distance is principally performed by the elongation of the 

 eye's axis. Indeed, as a bell shakes a steeple, so must the coats of the eye be 

 affected by any change in the crystalline; but the effect of this will be very in- 

 considerable; yet, as far as it does take place, it will co-operate with the other 

 change. 



But the laborious and accurate Leeuwenhoek, by the help of his powerful 

 microscopes, has described the course of the fibres of the crystalline, in a va- 

 riety of animals; and he has even gone so far as to call it a muscle;* but no one 

 has pursued the hint, and probably for this reason, that from examining only 



Now if the crystalline humour (which I have sometimes called tlie cryst. muscle) in our eyes, 

 &c. Phil. Trans., vol. 24, p. 17^y. — Crystallinura musculum, alias humorcni crystallinum dictumj 

 &c. Leeuwenb. op. ouin. i. p. 102. — Orig. 



