THE EYE AND THE PHYSIOLOGY OF VISION. 



563 



attached to the sclerotic coat, the other ending loosely in 

 the choroid coat where this runs backward around the eye- 

 ball. It is evident that when these ciliary muscles contract 

 they will tend to pull the choroid coat forward; that is, 

 towards the lens. As the hyaloid membrane lies immedi- 

 ately underneath the choroid coat it will be pulled forward 

 along with the choroid coat and so thereby produce a slack- 

 ening of the tension of the hyaloid membrane, where it 

 encloses the lens. As soon as the slack is produced, the lens 

 by its inherent elasticity becomes more rounded, and by its 

 increased convexity the focus of the eye is changed for a new 

 distance. To maintain this focus it is evident that the cil- 

 iary muscles must remain contracted and keep this slack, 

 for as soon as the ciliary muscles relax the choroid coat and 

 the hyaloid membrane with it move back to their original 

 position under the pressure of the vitreous humor inside. 



Fig. 177. TO SHOW THE RELATIVE SHAPES OF LENS WHEN AT REST (LEFT HALF) AND 

 WHEN ACCOMMODATED FOR NEAR OBJECTS (RIGHT HALF). 



For explanation of other structures see the text. 



To recur to the analogy above, let us imagine the foot- 

 ball between the cloth and the lining of the coat. The coat 

 represents, therefore, the hyaloid membrane. To represent 

 the choroid coat around this hyaloid membrane imagine an 

 overcoat put over the coat. It will be readily seen that if 

 the overcoat could by means of the hands or any agency be 

 pulled forwards towards the chest, it would tend to pull the 



