THE SENSES. 543 



directed to the distant object, the pupil recedes and the edge of the iris 

 disappears behind the sclerotic. 



The accommodation of the eye for near objects is therefore produced 

 by increased refractive power of the lens, from the greater bulging 

 of its anterior face. This increases the convergence of rays passing 

 through it, and compensates for their greater divergence beforehand. 

 In the condition of ocular repose, with the eye directed to distant 

 objects, rays coming from any one point arrive at the cornea nearly 

 parallel, and are so refracted as to meet in a focus at the retina. When 

 the eye is directed to a nearer point, the lens increases its anterior con- 

 vexity ; and the divergent rays, being more strongly refracted, are still 

 brought to a focus at the retina, as before. It thus becomes possible to 

 fix alternately, in distinct vision, objects at various distances. 



Mei-hrmixm of Accommodation. The means by which the lens is 

 rendered more convex, in vision for near objects, is not fully demon- 

 strated. Reasons have already been given for the belief that it is 

 accomplished, in some way, by muscular action ; and the two muscles 

 which, separately or together, undoubtedly produce this change, are 

 the iris and the ciliary muscle. 



The pupil certainly contracts in accommodation for near objects. 

 This is easily observed on examining by daylight an eye which is 

 alternately directed to near and remote objects. The ciliary muscle, on 

 the other hand, cannot be inspected in this way ; but its attachments 

 and position have led many writers to consider it as the principal 

 agent in changing the form of the lens. 



It appears that the diminution in size of the pupil is not by itself an 

 efficient cause of accommodation ; since, according to Helmholtz, if the 

 observer look through a perforated card, the orifice of which is smaller 

 than the pupil, near objects still appear indistinct when the sight is 

 directed to the distance, and vice versa, notwithstanding the invariable 

 dimensions of the artificial pupil employed. The contraction of the 

 sphincter pupillae probably serves to fix the inner border of the iris, 

 as a point of attachment for its radiating fibres. These fibres are 

 attached externally to the elastic tissue at the posterior wall of the 

 canal of Schlemm (Fig. 130); and from this circle also arise the 

 fibres of the ciliary muscle, which radiate thence to their attachment 

 at the choroid membrane. If the circular and radiating fibres of 

 both muscles contract together, they will form a connected system, 

 which may exert a pressure on the borders of the lens, sufficient to 

 cause the protrusion of its anterior face. The details of this mechan- 

 ism are by no means clearly understood ; and explanations, varying 

 more or less from the above, have been proposed by observers of high 

 authority. The direction and degree in which pressure would be exerted, 

 by muscular fibres attached like those in the interior of the eye, are too 

 imperfectly known to warrant a positive statement in this respect. 



Limits of Accommodation for the Normal Et/e. The normal eye is 

 so constructed that rays emanating from a single point, though coming 



