220 On the accommodation of the Eye to distances. 



It was the opinion of Porterfield, that by the contraction of the cil- 

 iary Ugament, the crystalhne was drawn forwards, that the distance 

 of the cryrtalline from the retina was farther increased by the com- 

 pression of the vitreous humor behind the Ugament, and that the 

 aqueous humor pressed against the cornea made it more convex. 

 Porterfield uses the. term ciliary Ugament synonymously with ciliary 

 processes. The reason that his opinion did not receive more atten- 

 tion, may have been that the ciliary processes alone did not seem ad- 

 equate to the purpose. 



The opinion that the ciliary ligament or annulus alhus, is the mus- 

 cle by which the eye adapts itself to the perception of distant objects, 

 has been recently advanced by Dr. Knox. The annulus alhus has 

 not the least appearance of muscularity. The muscular fibres at the 

 roots of the ciUary processes seem to be meant, which are insufficient 

 to account for the phenomena. 



In fishes the iris does not seem capable of much motion, yet their 

 vision contrary to the usually received opinion seems to me to be 

 very acute. I have sometimes over a small lake watched the motions 

 of trout. They appeared to sport with each other and to enjoy life 

 as much as animals that live in air, and there was no error in their 

 judging of the position of a fly, nor in their aim at obtaining it. The 

 speed with which they elude the grasp of the hand that darts towards 

 them is very great, and can be accounted for only by the quickness 

 of their vision. Those who attempt to catch fishes with their hands, 

 find it no easy task to secure them. 



From the apparent structure of the triangular body already descri- 

 bed, from its origin, its attachment to the crystalline lens, its passing 

 through the loop in the iris and its insertion into the vitreous humor, 

 it appears to be the means of adjusting the eyes of fishes to different 

 distances. As the crystalline lens in these animals appears a perfect 

 sphere, a slight rolling motion will produce no change ui the direction 

 of the rays. A very small force is thus advantageously applied, it is 

 increased by the passing of the muscle over the puUy, and by its fa- 

 vorable insertion into the vitreous humor the power of moving the 

 crystalline is still farther inceased. 



It should be observed that in some fishes as the poigee, the mus- 

 cle does not pass through a loop in the iris, but is only attached to 

 it. In the herring, the mackerel and the shad, the falciform process 

 described by Cuvier, is evidently constructed for the motion of the 

 crystalline. 



