CHAP. XVII.] ADAPTATION TO DISTANCE. 47 



in miud. Distinctness of vision will depend on the size of objects, 

 as well as on their distance from the eye; perfection of vision, on 

 their distance alone. 



This leads to the consideration of one of the most admirable pro- 

 visions for the extended utility of the organ; viz. its capacity of 

 adaptation, under the influence of the will, to distinct vision at every 

 distance beyond that of a few inches. We have the power of pro- 

 ducing some change in the eye by which its focal length is modified 

 to suit the varying angle at which rays from surrounding objects fall 

 upon it. Many different explanations have been attempted of the 

 mode in which this adaptation is effected, of which may be mention- 

 ed that of Jurin, Ramsden, and Home, that the cornea undergoes a 

 change in its curvature, becoming more convex for near objects : 

 and that of Des Cartes, Albinus, Hunter, and Dr. Young, who con- 

 sidered the lens muscular, and to possess within itself the power of 

 changing its curvature. 



Others, again, ascribe this power of adaptation to the iris, the mo- 

 tions of which might, as Knox supposed, alter the curvature of the lens; 

 or, according to Sir David Brewster, cause the lens to change its 

 place, and come forwards during contraction of the pupil. A change 

 in the position of the lens has also been supposed to occur from con- 

 tractions in the ciliary processes or zonula, and many have con- 

 tended that the entire eyeball may alter its relative dimensions 

 by the action of its muscles. 



It is conceivable that any of these changes, could they be proved 

 actually to take place, might be sufficient to account for the effect; 

 but, in estimating their relative value, the greatest importance is to 

 be attached to the anatomical evidence by which they may be sup- 

 ported. In the eye of the bird, the ciliary muscle, from its posi- 

 tion and attachments, must necessarily approximate the lens to the 

 cornea; and the reasons for considering the same part muscular in 

 mammalia, and, if so, for ascribing to it the same function as in birds, 

 have been already mentioned, and appear to us conclusive. We, 

 therefore, on anatomical grounds alone, adopt this view, ably ad- 

 vocated by Porterfield,* conceiving that when the eye is intent on 

 near objects, the ciliary muscle is contracted, the lens advanced to- 

 wards the cornea, and the latter membrane, perhaps, rendered more 

 convex by the traction of the muscle on its border by means of the 

 cordage of the posterior elastic lamina; while in vision of remote ob- 

 jects the lens is carried back towards the retina by the elasticity of the 

 neighbouring parts. It is interesting to notice that this adjusting 

 * Treatise on the Eye. Vol. i. p. 446. 



