524 



THE HUMAN BODY. 



the image reflected from it is also altered, becoming smaller 

 when the radius of curvature of the mirror is lessened and 

 vice versa. If the three images be carefully watched while 

 the observed eye looks at a near object in the same line as the 

 distant point previously looked at, it is seen that the image 

 due to corneal reflection remains unchanged; that due to light 

 from the front of the lens becomes smaller and brighter; the 

 image from the back of the lens also becomes very slightly 

 smaller. The change in the curvature of the front of the 

 lens can be calculated from the change in size of the image 

 reflected from it when the eye changes from distant to near 

 accommodation. When a distant object is looked at the 

 radius of curvature is 10 mm. (f inch), when a very near 

 about 6 mm. (/-inch), and this change is sufficient to ac- 

 count for the range of accommodation of the normal eye. 



When the eye is focussed for seeing a near object the cir- 

 cular muscle of the iris contracts, narrowing the pupil, but this 

 has nothing directly to do with the accommodation. 



Accommodation is brought about mainly by the ciliary 

 muscle (Fig. 151). In the resting eye it is relaxed and the 

 suspensory ligament of the lens is taut, and, pulling on its 

 edge, drags it out laterally a little and flattens its surfaces, 



ch 



v, ,. F IG. 151. Diagram to illustrate the mechanism of accommodation; on the right 

 half ot the figure for a near, on the left for a distant, object: cs, canal of Schlemm; 

 cf, circular portion of ciliary muscle: rf. radial portion of ciliary muscle: ch, ciliary 

 process of choroid; si, suspensory ligament; i, iris. 



especially the anterior, since the ligament is attached a little 

 in front of the edge. To see a nearer object the ciliary muscle 

 is contracted, and according to the degree of its contraction 

 slackens the suspensory ligament, and then the elastic lens, 

 relieved from the lateral drag, bulges out a little in the centre. 



