OF SENSATION, AND THE ORGANS OF THE SENSES. 

 Fig. 43. 



A longitudinal section of the globe of the Eye; 1, the sclerotic, thicker behind than in front; 2, the cornea, 

 received within the anterior margin of the sclerotic, and connected with it by means of ajbeveled edge; 3, 

 the choroid, connected anteriorly with (4) the ciliary ligament, and (5) the ciliary processes; 6, the iris; 7, 

 the pupil ; 8, the third layer of the eye, the retina, terminating anteriorly by an abrupt border at the com- 

 mencement of the ciliary processes; 9, the canal of Petit, which encircles the lens (12); the thin layer in 

 front of this canal is the zonula ciliaris, a prolongation of the vascular layer of the retina to the lens' 

 10, the anterior chamber of the eye, containing the aqueous humour; the lining membrane by which the 

 humour is secreted is represented in the diagram; 11, the posterior chamber; 12, the lens more convex 

 behind than before, and enclosed in its proper capsule; 13, the vitreous humour enclosed in the hyaloid 

 membrane, and in cells formed in its interior by that membrane ; 14, a tubular sheath of the hyaloid mem- 

 brane, which serves for the passage of the artery of the capsule of the lens ; 15, the neurilema of the optic 

 nerve ; 16, the arteria centralis retinae, imbedded in its centre. 



be in the focus of the converging rays ; or, the distance of the screen remain- 

 ing the same, he may vary the convexity of his lens, in such a manner as to 

 adapt it to the distance of the object. It is not improbable, that both of these 

 methods are employed in the eye, though no distinct evidence has been obtained 

 of the operation of either. Several hypotheses have been proposed, to account 

 for the phenomenon ; it is easily proved tjiat no one of them can alone be 

 true ; but it cannot be readily shown that any of them is entirely false : and it 

 would not seem unlikely, therefore, that all may participate, in various degrees, 

 in the effect. The following are the principal of these. 1. An alteration in 

 the form of the globe of the eye by the action of the muscle^, so that its antero- 

 posterior diameter may be increased or diminished.* 2. A change in the con- 

 vexity of the cornea. This might be very well connected with the last ; since, 

 if the globe were converted into a spheroid, of which the antero-posterior 

 diameter would be the longest, the curvature of the cornea would be increased ; 

 whilst, if the antero-posterior diameter were shortened, the curvature would 

 be diminished. 3. Change of position of the crystalline lens, by means of 

 the ciliary processes. 4. Change of figure of the lens itself. That one or 

 both of the last two are concerned in the effect, would appear from the fact, 

 well known to every Oculist, that, after the removal of a cataract, the power of 

 adapting the eye to distances is greatly diminished. 5. Change in the aper- 

 ture of the pupil; the mode in which this could assist in accommodating the 

 eye to variations of distance, is not very obvious. 



330. Some curious circumstances relative to the connection between the 

 optical adaptation of the eye to distances, and the changes in the direction of 

 the axes of the two eyes, have been pointed out by Miiller. When both eyes 



* The influence of the muscles in altering the form of the globe may be better com- 

 prehended, now that we know the mode in which this is kept in its place in the front of 

 the orbit, by a fascia passing behind it, and attached anteriorly to the lids. 



