NERVE 



147 



upon the retina. This, however, requires a change in the lens 

 arrangement of the eye, and this change, beginning in man when 

 the object comes within about 6 metres (the far point of accom- 

 modation), becomes greater and greater, till it can increase no 

 further when the near point is reached. The change is called 

 positive accommodation, and it consists in an increased 

 curvature of the anterior surface of the lens. This may be 

 proved by examining, in a dark room, the images of a candle 

 formed from the three refracting surfaces 'Sanson's images), 

 when it will be found that the image from the anterior surface 

 of the lens becomes smaller and brighter when the eye is 

 directed to a near object. The examination of these images 

 is facilitated by the use of the Phacoscope {Practical 

 Physiology). 



Positive accommodation is brought about by contraction 

 of the ciliary muscle (seep. 139), which pulls forward the 

 ciliary processes to 

 which the hyaloid 

 membrane is attached, 

 and thus relaxes the 

 suspensory ligament of 

 the lens and the front 

 of the lens capsule, 

 and allows the natural 

 elasticity of the lens to 

 bulge it forward (fig. 68). 



This change of posi- 

 tive accommodation is accompanied by a contraction of the 

 pupil due to contraction of the sphincter pupillae muscle. 

 By this means, the more divergent peripheral rays which would 

 have been focussed behind the central rays to produce a 

 blurred image are cut off, and spherical aberration is prevented. 



The muscles acting in positive accommodation — the 

 ciliary and sphincter pupilloe — (fig. 69, CM. and S.P.) 

 are supplied by the third cranial nerve {III.), while the 

 dilator pupillae is supplied by fibres passing up the 

 sympathetic of the neck. The centre for the third nerve is 

 situated under the aqueduct of Sylvius, and separate 

 parts preside over the ciliary muscle and the sphincter pupillse. 



■Fig. 68. — Mechanism of Positive Accommoda- 

 tion. The continuous lines show the parts 

 in negative accommodation, the dotted 

 lines in positive accommodation. 



