ON THE MECHANISM OF THE EYE. 



599 



esj)eciiUIy behind, vvliere it is shorter: this 

 is observable in the partridge ; and, in the 

 same bird, the whole surface of the lens is 

 seen to be covered with points, or rather 

 globules, arranged in regular lines, (Plate 

 13. Fig. 99.) so as to have somewhat the ap- 

 pearance of a honeycomb, but towards the 

 vertex k^ss uniformly disposed. This regu- 

 larity is a sufficient proof that there could he 

 no optical deception in the appearance ; al- 

 though it requires a good microscope to dis- 

 cover it distinctly ; but the zone may be 

 easily peeled off under water, and hardened 

 in spirits. Its use is uncertain : but it may 

 possibly secrete the liquid of the crystalline; 

 and it as much deserves the name of a gland, 

 as the greater part of the substances usually 

 so denominated. In peeling it off, I have 

 very distinctly observed ramifications, which 

 were passing through it into the lens ; (Plate 

 12. Fig. 97.) and indeed, it is not at all dif- 

 ficult to detect the vessels connecting the 

 margin of the lens with its capsule; and it 

 is surprising that M. Petit should have 

 doubled of their existence. I have not yet 

 clearly discerned this crystalline gland in the 

 hunvan eye ; but I infer the existence of 

 something similar to the globules, from tlie 

 spotted appearance of the image of a lucid 

 point already mentioHed; for which I can no 

 otherwise account, than by attributing it to 

 aderangement of these particles, produced by 

 the external force, -and to an unequal impres- 

 sion made by them on the surface of the lens. 

 In birds and in fishes, the fibres of the 

 crystalline radiate equally, becoming finer 

 as they approach the vertex, till they are 

 lost in a uniform substance, of tiie same de- 

 gree of firmness, which appears to be perfo- 

 lated in the centre by a blood vessel. (Plate 



3 



13. Fig. 100.) In quadrupeds, the fibres at 

 their angular meeting are certainly not con- 

 tinued, as Leeuwenhoek imagined, across 

 the line of division: yet there does not ap- 

 pear to be any dissimilar substance inter- 

 posed between them, except that very minute 

 trunks of vessels often mark that line. But, 

 since the whole mass of the lens, as far as 

 it is moveable, is probably endued with a 

 power of changing its figure, there is no 

 need of any strength of union, or place of 

 attachment, for the fibres, as the motion 

 can meet with little or no resistance. Every 

 common muscle, as soon as its contraction 

 ceases, returns to its natural form, even 

 without the assistance of an antagonist; and 

 the lens itself, when taken out of the eye, 

 in its capsule, has elasticity enough to re- 

 assume its proper figure, on the removal of 

 a force that has compressed it. The capsule 

 is highly elastic ; and, since it is laterally 

 fixed to the ciliary zone, it must cooperate 

 ■in restoring the lens to its flattest foi'm. If 

 it be inquired, why the lens is not capable of 

 becoming less convex, as well as more so, 

 it may be answered, that the lateral parts 

 have probably little contractive power; and 

 if they had more, they would have no room 

 to increase the size of the disc, which they 

 must do, in order to shorten the axis ; and 

 the parts about the axis have no fibres so 

 arranged as to shorten it by their own con- 

 traction. 



I consider myself as being partly repaid 

 for the labour lost in search of the neives 

 of the lens, by having acquired a more ac- 

 curate conception ofllie nature and situation 

 of the ciliary substance. It had alrea<ly been 

 observed, that in the hare and in the wolf, 

 the ciliary processes are not attaclied to the 



