THE ORGANS OF SENSE. 



I2Q 



traction of the pupil, and stimulation of the distal end leads to dilata- 

 tion. As section of the third nerve is followed by dilatation and section 

 of the sympathetic by contraction of the pupil, it is obvious that tonic 

 impulses are constantly passing along both nerves. The fact that 

 dilatation of the pupil is due to active contraction of the radial muscle 

 fibres of the iris, and not merely to relaxation of the sphincter, is proved 

 by two experiments. (1) Localised stimulation of the periphery of the 

 iris leads to contraction of that part 

 of the iris only, and (2) if a sector 

 of the iris be separated by two radial 

 cuts it will contract, either on local 

 stimulation or on stimulation of the 

 sympathetic nerve in the neck. The 

 sympathetic root of the ciliary gang- 

 lion contains vaso - constrictor fibres 

 for the vessels of the eye. 



In man, and in other animals in 

 which there is a partial decussation of 

 the optic nerves, the reflex contraction 

 of the pupil to light is bilateral, that 

 is, light falling on one eye leads to 

 contraction of both pupils. This is 

 due to the fact that by means of the 

 decussation each optic nerve forms 

 connections with both superior corpora 

 quadrigemina, and thus with the 

 nuclei of both third nerves. If one 

 optic nerve is atrophied, the pupil 

 of that eye will contract when light 

 falls on the other eye, but not when 

 light falls on the affected eye. 



Contraction of the pupil occurs 

 not only as a result of the light 

 reflex, but also from other causes. 



The various factors which bring about diminution in the size of the 

 pupil are : 



1 . Light falling on the retina, giving the reflex effect already described. 



2. (a) Accommodation, (6) Sleep. In both of these conditions the 

 contraction of the pupil is an associated condition. In accommodation, 

 it occurs simultaneously with the contraction of other muscles supplied 

 by the same nerve ; in sleep the eyes are rotated upwards and inwards, 

 and the pupils contract in association with the convergence. 



9 



c 



FIG. 40. Scheme of the nerves 

 the eye (after Foster). 



Sympathetic fibres red. a, third nerve ; 



b, sympathetic root of ciliary ganglion ; 



c, nasal branch of fifth nerve ; d, ciliary 

 ganglion; e, e, long ciliary nerves ; /, 

 short ciliary nerves. 



