vi DIOPTRIC MECHANISM OF THE EYE 295 



remains practically unaltered, while the anterior surface, on the 

 contrary, bulges with the increase in curvature, and also pushes 

 the central margin of the iris forward and constricts the aperture 

 of the pupil. This is obvious on watching the eye of any one who 

 looks alternately at far and near objects. In distant vision the 

 aperture of the pupil is seen as a long black line ; in near vision 

 the margin of the iris moves forward, and the pupil is narrowed 

 (Fig. 133). According to Helmholtz the displacement of the 

 anterior capsule of the lens in accommodation varies between 0'36 

 and 0'44 mm. 



Another very important factor in accommodation is the lateral 

 displacement of the lens, which can only be observed when an 

 effort is made to accommodate as fully as possible, or when a 

 persistent spasm of the ciliary muscles follows the introduction 

 of physostigmine solution into the coujunctival sac. This effect, 

 which was carefully investi- 

 gated and explained by Hess s 

 (1897-99), is due to the fall 

 of the lens by its own weight, 

 towards either the nasal, the 

 temporal, or the infra-orbital 

 part of the ciliary body, 

 according to the position of 

 the head. The lens only 

 remains centred and motion- 

 less in relation to the edge of 



i i ,1 i ,q : Fir;. 133. To show forward displacement of pupillary 



the pupil Wlien tne neaa IS margin of tbe iris during accommodation. (Helm- 

 liolrl cr> i-Viaf flip nlarip nf t,bp holtz.) A, prolile view of eye in distant vision; 



D, in accommodation for near vision. 



iris is horizontal. At the 



least movement of the eyes to the sides the lens shifts in one 

 direction or the other. This displacement can be accurately 

 measured. Hess found that it may vary from 0-25 to 0-30 mm. in 

 forced voluntary accommodation, and as much as 1 mm. in physo- 

 stigmine poisoning. 



A further effect can be seen during accommodation, or after 

 applying physostigmine, in eyes of which the iris is partially 

 defective, owing to iridectomy or trauma. According to the 

 accurate observations of Hess such eyes show during accommoda- 

 tion a bulging of the ciliary processes towards the equator of the 

 lens without any thickening, a fact that can only be explained by 

 assuming that in accommodation or in physostigmine poisoning 

 the ciliary muscles move forward in the direction of the cornea. 

 This is in agreement with the experiment of Hensen and Vblkers 

 (1873), who ran a very fine needle into the equator of a freshly 

 enucleated human eye, and applied electrical stimulation near the 

 ciliary processes-- the needle then moved in a manner that 

 indicated a forward displacement of the choroid. If the needle 



