H2 HUMAN PHYSIOLOGY 



creased curvature of the anterior surface of the lens. This 

 may be proved by examining the images formed from the 

 three refracting surfaces when it will be found that the 

 image from the anterior surface of the lens becomes smaller 

 and brighter (Sanson's images). The examination of these 

 images is facilitated by the use of the Phacoscope. (Experi- 

 ment.) 



Positive accommodation is brought about by contraction 

 of the ciliary muscle, which pulls forward the ciliary pro- 

 cesses 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. 49). 



FIG. 49. Mechanism of Positive Accommodation. rpu: r V,imr o f nos j 

 The continuous lines show the parts in . P 



negative accommodation, the dotted lines tlV6 accommodation is 



the positive accommodation. accompanied by a con- 



traction of the pupil 



due to contraction of the sphincter pupilhe muscle. By this 

 means the more divergent peripheral rays which would 

 have been focussed behind the central ones are cut off, and 

 spherical aberration is prevented. 



The muscles acting in positive accommodation the ciliary 

 and sphincter pupilLe (Fig. 50, C.M. and S.P.) are supplied 

 by the third cranial nerve (///.), while the dilator pupilLr 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. 107). 



The sphincter centre is refiexly 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 eye. At the same time the centre for the 

 ciliary muscle is also called into play. 



The centre for dilatation of the pupil is situated in the 



