PURKINJE-SANSON IMAGES 



281 



'J'Ik' kiis .sus|)<.'iRltcl in llu- resting eye lias not its natural shape : 

 it is kept somewhat flattened by the tension ol' the capsule. U' 

 this tension can he relaxed, the lens will heeoine more convex 

 on account ol" its iidierent elasticity. A mechanism for bringing 

 this about is })resent. The ciliary nuiscle is lixed at the cornco- 

 sclerotic junction. When its radial fibres contract they drag the 

 ciliary processes, with the adherent hyaloid membrane, forward. 

 Simultaneously the circular fibres by their contraction constrict 

 the circle round which the suspensory ligament is attached. The 



venosus 



Conjunctiva 



Retina 



Fig. 73. — IScction of anterior parts of eyeball, slinwing structures concerned in accommoda- 

 tion. (After Merl<el anil Ivallius.) 



result is to relax the ligament and lens capsule, and the elasticity 

 of the lens comes into play. The maximum alteration in radial 

 curvature, which affects the anterior surface almost exclusively, 

 is from 10 mm. resting to 6 nmi. fully accommodated. The 

 normal adult is thus enabled to see objects distinctly to within 

 10 or 15 cm. distance. 



The Purkinje-Sanson images during the process of accommoda- 

 tion provide a clear demonstration that the main surface affected 

 is the anterior surface of the lens. The central, larger but less bright 

 erect image is reflected from this surface. If, therefore, the two 

 other images remain almost unchanged, and only this image alters 

 in size and position when the eye of the subject, resting on a 



