THE SENSE OF VISION 



615 



dation is the translation of aqueous fluid from tha front to the 

 circumferential edge of the lens. The excised lens, in itself, does 

 not possess elasticity. It is a jelly-like mass. The whole eyeball 

 is distended with fluid aqueous, vitreous, and blood in the blood- 

 vessels at a pressure of about 30 mm. Hg. The fluid is 

 secreted by the cells which line the ciliary processes, and the 

 intra-ocular pressure is regulated by their sesretory power. The 

 pressure of this fluid keeps the eyeball distended, and maintains 

 its shape constant in accordance with the requirements of an 

 optical instrument. It keeps the suspensoty ligament taut and 

 the lens flattened. Accepting the importance of the fluid, we must 

 suppose that the circular fibres of the ciliary muscle and the radial 

 fibres taking the attachment of these to the suspensory ligament 

 as the fixed point by their contraction, pull 

 open the meshes of the cribriform ligament, and 

 allow aqueous fluid to pass therein. The ciliary 

 body, in its outer part, is thus distended with 

 fluid, and its inner part approximated to the 

 circumference of the lens, and the suspensory 

 ligament relaxed. The lens then takes on a 

 more globular shape, and becomes more mobile 

 in its watery bed. By the passage of more, or 

 less, fluid from the aqueous chamber in front 

 into the meshes of the cribriform ligament at 

 the circumference, the curvature of the lens is 

 controlled with the greatest nicety. As the 

 fluid is distributed all round the circumference 

 of the eyeball, the actual expansion of the spaces 

 in the cribriform ligament requires to be very 



small in order to contain the fluid which is Fm - 346 -- DlAGRAM _. T0 . 

 translated and determine the necessary change 

 in the convexity of the lens. If the meridional 

 fibres of the ciliary muscle relax when the radial 

 and circular fibres contract, they will be bowed 

 inwards. If they contract when the other fibres 

 relax, they will restore the accommodated 

 eye to its resting condition. It is conceivable, and probable, that 

 these fibres act as antagonists to the circular and radial fibres, 

 and that accommodation is brought about by the balanced action 

 of antagonist muscles. During the act of accommodation, the pupil 

 contracts and the eyeballs are rotated inwards. All the muscles 

 concerned in the act of accommodation are supplied by the third nerve. 



The Near Point of Vision. The range of accommodation in the 

 adult is such as to allow clear vision of objects held up to, and slightly 

 within 6 inches of, the eye. This can be demonstrated by making in 

 a piece of cardboard two holes separated by less than the diameter 

 of the pupil. Holding these close to the eye, a needle held inside 

 5 inches is seen as two, but as it is gradually moved farther away 

 the images fuse, and one needle only is seen (Fig. 346). 



SHOW HOW THE 

 IMAGES OF NEAR AND 

 FAR Pixs FALL oy 

 RETIXA. 



A, Near pin; B, far 

 pin. 



