NEURO-MUSCULAR MECHANISM 



125 



FIG. 57. Mechanism of Positive Accommoda- 

 tion. The continuous lines show the parts 

 in negative accommodation, the dotted lines 

 the positive accommodation. 



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 the images formed from the three refracting surfaces 

 (Sanson's images), when 

 it will be found that the 

 image from the anterior 

 surface of the lens be- 

 comes smaller and 

 brighter when the eye 

 is directed to a near 

 object. The examina- 

 tion of these images is 

 facilitated by the use 

 of the Phakoscope. (Practical Physiology.} 



Positive accommodation is brought about by contraction of 

 the ciliary muscle (see p. 119), 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. 57). 



This change of positive accommodation is accompanied by a 

 contraction of the pupil due to contraction of the sphincter 

 pupilla? muscle. By this means the more divergent peripheral 

 rays which would have been focussed behind the central ones 

 to produce a blurred image are cut off, and spherical aberration 

 is prevented. 



The muscles acting in positive accommodation the ciliary 

 and sphincter pupillse (fig. 57, CM. and S.P.) are supplied by 

 the third cranial nerve (III.}, while the dilator pupillse 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 (see p. 180). 



The sphincter centre is rellexly called into action, and the 

 pupil contracted 1st, When strong light falls on the retina 

 and stimulates the optic nerve. In this way the retina is 

 protected against over - stimulation ; 2nd, When the image 

 upon the retina becomes blurred as the object approaches the 



