CHAP, in.] SIGHT. 1171 



compressed elastic lens to bulge forward. And experimental 

 evidence shews that this is what does take place. The ciliary 

 muscle is governed, as we shall presently see, by the ciliary nerves. 

 If in a living animal (dog) or in an eye immediately after removal 

 from the body, an opening be made in the sclerotic in order to 

 watch the choroid, it may be seen that when the ciliary nerves are 

 stimulated the choroid does move forward at the same time that 

 the front surface of the lens becomes more convex ; a needle, the 

 point of which is carefully lodged in the choroid, moves in such 

 a way as to shew that the choroid moves forward, though no 

 appreciable movement can be seen in a needle thrust into the front 

 part of the ciliary muscle itself. If the cornea be cut away so as 

 to leave only at one place a small fragment still connected to the 

 junction of the sclerotic and cornea, this piece moves backward 

 when the ciliary nerves are stimulated, shewing that the ciliary 

 muscle does pull 011 the point of junction of the sclerotic with the 

 cornea. When, however, the cornea is intact, or even when a 

 sufficiently large part of it is left, the junction becomes a fixed 

 point, at least relatively to the moveable choroid. Moreover not 

 only the contraction of the ciliary muscle and movement of the 

 choroid, but the actual slackening of the suspensory ligament and 

 change in the curvature of the lens may be observed to follow 

 upon stimulation of the ciliary nerves. We may conclude, there- 

 fore, that the possible 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 towards the centre of the pupil. In becoming more convex, 

 the front surface of the lens, especially the central portion, 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 corre- 

 spondingly thinner (cf. Fig. 141), 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, 

 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 concert with the longitudinal 

 fibres would bring the ciliary processes nearer to the lens, and so 



F. 75 



