634 THE SENSES. 



jects, the rays coming from any point of such an object arrive at the 

 cornea in a nearly parallel position, and are then refracted to such a de- 

 gree that they meet in a focus at the retina (Fig. 201). When the eye 

 is directed to a nearer point (Fig. 202), the lens increases its anterior 

 convexity ; and the divergent rays, being more strongly refracted, are 

 still brought to a focus at the retina, as before. It thus becomes possi- 

 ble to fix alternately, in distinct vision, objects at various distances in 

 front of the eye. 



Mechanism of the Change in Figure of the Lens in Accommodation. 

 The mechanism by which the lens is rendered more convex, in vision 

 for near objects, is far from being completely demonstrated. The rea- 

 sons have already been given which lead to the conclusion 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 iris certainly contracts in accommodation for near objects. This 

 is easily observed on examining by daylight the pupil of an eye which 

 is alternately directed to near and remote objects. The pupil visibly 

 diminishes in size when the eye is fixed upon a point near by, and again 

 enlarges when the sight is accommodated for the distance. The move- 

 ments of the ciliary muscle, on the other hand, are not subject to ob- 

 servation ; but the attachments and position of this muscle have led 

 many writers to attribute to it an important, if not the principal, part 

 in causing a change of form in the crystalline lens. 



So far as we are at present able to form a judgment on this question, 

 it may be said 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 thus employed. The con- 

 traction of the circular fibres of the sphincter papillae must, therefore, 

 have for its probable object to fix the inner border of the iris, thus 

 affording an internal point of attachment for the radiating fibres of the 

 same muscle. These fibres have for their external attachment the 

 elastic tissue at the inner wall of the canal of Schlemm (Fig. 189); and 

 from this circle also arise the fibres of the ciliary muscle, which radiate 

 outward and backward to their final attachment at the surface of the 

 choroid membrane. If the circular and radiating fibres of both these 

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

 may exert a pressure upon the borders of the lens, sufficient to cause 

 the protrusion of its anterior face at the pupil, where alone its advance 

 is not resisted. The aqueous humor, displaced by the protrusion of the 

 lens, may find room in the external parts of the anterior chamber, where 

 the outer border of the iris recedes, under the traction of the ciliary 

 muscle. These are the general features of the mechanical action in 

 accommodation, as it is generally supposed to take place. At the same 



