H 



DIOPTKIC MECHANISMS OF THE EYEBALL :,i:, 



and from the fibres of the ligamentum pectinatum to be attached behind to 

 the whole extent of the ciliary processes. 



(c) The circular bundle, which forms a ring-muscle, composed of fibres 

 running around the circumference of the eye in the inner part of the ciliary 

 processes. This bundle is best 

 marked in hypermetropic eyes 

 and is almost absent in myopic 

 eyes. 



When this muscle contracts 

 it draws the anterior part of 

 the choroid and the ciliary pro- 

 cesses forwards and inwards, 

 while the ring-fibres approxi- 

 mate the ciliary processes to 

 the margin of the lens. By 

 this approximation of the cili- 

 ary processes to the lens the 

 suspensory ligament is relaxed, 

 and the anterior surface of the 

 lens bulges, i.e. becomes more 

 convex as a result of its inherent 

 elasticity (Fig. 272). 



This explanation of accommoda- 

 tion, which was first put forward 

 by Helmholtz, is almost universally 

 accepted. According to some the 

 change of shape of the lens during 

 accommodation is brought about by 

 the actual pressure of the ciliary 

 processes on its margin by which 

 the middle of its anterior surface 

 is pressed forwards. According to 



Tscherning this effect is produced, not by relaxation, but by a tightening of the 

 suspsnsory ligament through the contraction of the ciliary muscle. There is no 

 doubt, however, that during forced accommodation, such as can be brought about 

 by instillation of eserine into the eye, which produces spasm of the ciliary muscle, the 

 suspensory ligament is so relaxed that the lens lies loosely in the eyeball. Bending 

 the head downwards causes therefore an actual change in the position of the lens, 

 which may drop as much as 1mm. forwards towards the cornea. Under the same 

 conditions a quick movement of the head causes the lens to shake, and the quiver of the 

 lens can be seen by an external observer and proved by the subjective oscillation of 

 external objects which is noticed after such a movement. Moreover, if a needle be 

 passed through the sclerotic so that its point lies in the ciliary processes, stimulation 

 of the ciliary muscle causes a movement of the outer part of the needle backwards, 

 showing that the point of the needle which is in the ciliary processes has been moved 

 forwards (Fig. 273). The loosening of the lens during spasm of accommodation is well 

 shown in rare cases where there is congenital absence of the whole iris. In such case* 

 shaking of the patient's head is seen to cause an oscillation of the lens within the eyeball. 



During accommodation the increased curvature of the anterior surface 

 of the lens causes an approximation of this surface to the cornea, which may 



FIG. 272. Diagram of mechanism of accommoda- 

 tion. (TiGERSTEDT after SCHON.) 



The dotted line shows the form of the lens during 

 accommodation for near objects. 



