DIOPTRIC MECHANISMS OF THE EYEBALL 607 



(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 processes forwards and inwards, while the ring-fibres 

 approximate the ciliary processes to the margin of the lens. By this 

 approximation of the ciliary 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. 270). 



This explanation of accommodation, which was first put forward 

 by Helmholtz, is almost universally accepted. According to some 



B 



FIG. 271. Accommodation in the cat's eye. R, distance ; A, for near vision. 



(After BEER.) 



Two needles have been passed through edge of cornea into ciliary bodies, 

 to show forward movement of latter during accommodation. 



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 suspensory ligament through the con- 

 traction 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 there- 

 fore an actual change in the position of the lens, which may drop as 

 much as 1 mm. forwards towards the cornea. Under the same condi- 

 tions 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 





