Chap, hi.] SIGHT. 857 



ture of the lens may be observed to follow upon stimulation of 

 the ciliary nerves. We may conclude, therefore, that the pos- 

 sible explanation given above is the actual one. 



One or two additional points are worth mentioning. During 

 accommodation for near objects the pupil is narrowed; we shall 

 speak of this presently. A narrowing of the pupil means that 

 the edge and inner part of the iris moves over the front surface 

 of the lens toward the centre of the pupil. In becoming more 

 convex, the front surface of the lens, especially the central por- 

 tion, projects further forward into the anterior chamber, and in 

 so doing carries with it the pupillary edge and inner part of the 

 iris ; for the iris lies close upon and indeed in contact with the 

 anterior surface of the lens. And when the eye is carefully 

 watched sideways this projection forwards of the pupillary 

 margin of the iris may be observed. While the edge of the 

 pupil thus moves forward, and the body of the iris increases in 

 a radial direction, becoming correspondingly thinner (cf. Fig. 

 146), the circumferential edge of the iris is carried slightly 

 backwards, owing to the giving way to a certain extent of the 

 elastic ligamentum pectinatum on which the ciliary muscle pulls; 

 and thus additional space is afforded in the anterior chamber 

 for the aqueous humour driven aside by the projection of the 

 anterior surface of the lens. 



The action of the circular, equatorial fibres of the ciliary 

 muscle (Fig. 145) and of the fibres intermediate between these 

 and the longitudinal meridional fibres, is not quite so clear. 

 We may, however, suppose that the circular fibres acting in con- 

 cert with the longitudinal fibres would bring the ciliary processes 

 nearer to the lens, and so assist in slackening the suspensory 

 ligament. But no very decisive explanation has been given 

 why the circular fibres are often largely developed in some eyes, 

 it is said hypermetropic or long-sighted eyes, and scantily 

 present in others, myopic or short-sighted eyes. And indeed 

 there are several points in the whole action of accommodation 

 which still require to be cleared up. 



Accommodation is in a certain sense a voluntary act ; we can 

 by looking at near or far objects bring about the change when- 

 ever we please. Since, however, the change in the lens is always 

 accompanied by movements in the iris, it will be convenient to 

 consider the latter before we speak of the nervous mechanism of 

 the former. 



The Changes in the Pupil. 



§ 536. Although by looking at near or far objects, and so 

 voluntarily bringing about changes in the accommodation 

 mechanism, we can call forth the accompanying changes in the 

 iris, and can thus at pleasure produce a constriction, narrowing, 

 or a dilation, widening, of the pupil ; it is not in our power to 



