NEURO-MUSCULAR MECHANISM 



127 



dilator muscle fibres of the pupil have not been demonstrated 

 in all animals, it has been suggested that the nerve may act by 

 inhibiting the sphincter pupillse, but the evidence on this 

 point is not conclusive. 



A peripheral mechanism exists in the muscle of the iris 

 which may act independently of the central nervous system, as 

 may be seen in the eye of a decapitated cat, and various drugs 

 act directly upon it e.g. physostigmin causing a contraction of 

 the pupil by acting upon the nerve endings and pilocarpine by 

 acting on the muscle fibres. Adrenalin causes a dilatation by 

 acting on the nerve endings and atropine by acting in the same 

 way. 



Imperfections of the Dioptric Mechanism 



(1) Hypermetropia. The eye is sometimes too short from 

 before backwards, and thus, in 

 the resting state, parallel rays 

 are focussed behind the retina, 

 and in order to see even a 

 distant object the individual 

 has to use his positive accom- 

 modation. As the object is 

 approached to the eye it is 

 focussed with greater and 

 greater difficulty, and the near 

 point is further off than in the 

 emmetropic eye (fig. 58, (7). 



The long-sighted eye differs 



FIG. 59. To show the cause of Astig- 

 matism. A, a slight curvature of 

 the cornea in the vertical plane ; B, 

 more marked curvature in the hori- 

 zontal plane, leading to rays from 

 b a horizontal line being focussed 

 in front of the retina when a a 

 vertical line is looked at. 



from the slightly presbyopic 

 in the fact that not merely 

 divergent, but also parallel 

 rays, are unfocussed in the resting state. 



(2) Myopia. In certain cases the antero-posterior diameter 

 of the eye is too long, and as a result parallel rays rays 

 from distant objects are focussed in front of the retina, and 

 it is only when the object is brought near to the eye that 

 a perfect image can be formed. In such an eye no positive 

 accommodation is needed till the object is well within the 

 normal far point, and the near point is approximated to the eye. 



