On the accommodation of the Eye to distances. '219 



Fig. 2, a. Choroid gland. — b. Muscle attached to the crystalline lens. — c. Fat. 



Fig. 3, Plan. — a. Fat. — h, Vitreous humor. — c. Crystalline lens.— rf. Muscle — 

 e. Choroid gland. — /. Optic nerve. 



Fig. 4. Section of the eye of a sheep. — a. Superior choroid muscle. — b. Ciliary 

 processes. — c. Inferior choroid muscle. • 



Fig. 5, Crystalline and vitreous humor of a sheep, shewing the unequal diame- 

 ter of Petit's canal and the impressions of the superior choroid muscle. 



On the accommodation of the Eye to distances. 



Notwithstanding all the attention that has been paid to the eye, 

 there is much discrepancy of opinion about the method by which it is 

 accommodated to distances. We still read in the works of highly 

 respectable authors about the change being effected by the alteration 

 of the size of the pupil, by the muscularity of the crystalline lens, by 

 a greater or less degree of convexity of the cornea, or by the muscu- 

 larity of the ciliary ligament, without either of these theories being 

 sufficiently proved. 



It is evident that the change in the size of the pupil, can have no 

 more effect in the adjustment of the eye, than the increasing or di- 

 minishing of the aperture at the end of a telescope, can have in bring- . 

 ing it to a focus. When it is dilated by belladonna, the power of ac- 

 commodation, in my own case, to a certain extent, continues. 



The crystalline lens bears no analogy to muscular structure. If 

 it did, there is no attachment to its capsule, from which the fibres 

 could act. In a certain species of hawk, it is a piano convex, so ex- 

 quisitely cut, if I may use the expression, that if changed by muscular 

 action, its delicate edge would be acted upon, and its figure would 

 not likely return. 



The hypothesis of Mr. Travers, that the radiated fibrous processes 

 connected with the iris, bear upon the circumference of the crystal- 

 line lens and elongate its axis, admits of no proof but is liable to many 

 objections. 



The theory of Dr. Hosack, accounting for the phenomena by the 

 degree of convexity of the cornea, will not apply in every case. In 

 the eye of the sturgeon a thick cartilage encases the globe, as far as 

 the circumference of the cornea. It is so firm that no ciiange of form 

 can be produced by the external muscles, nor have I been able to 

 perceive that any alteration of the laminae of the cornea, can be made 

 by their action. A structure similar to that of the sturgeon exists in 

 many fishes. In animals that live in air, no change of form in the 

 cornea is, to unassisted vision, perceptible. In attempting proofs with 

 the microscope the difficulties in adjusting it have probably been sour- 

 ces of error. 



