THE SENSES 791 



motor ataxia the light-reflex sometimes disappears, while the 

 constriction of the pupil in accommodation still takes place 

 (Argyll-Robertson pupil). Artificial stimulation of the optic 

 nerve has the same effect on the pupil as the ' adequate ' 

 stimulus of light ; and in many animals (including man), 

 though not in those whose optic nerves completely decus- 

 sate, both pupils contract when one retina or optic nerve 

 is excited. This should be remembered in using the pupil- 

 reaction as a test of the condition of the retina. For 

 although the absence of contraction may show that the 

 retina of the eye on which the light is allowed to fall is 

 insensible (unless there is some physical hindrance to its 

 passage, such as opacity of the lens or cataract), the occur- 

 rence of contraction does not exclude insensibility of the 

 retina unless the other eye has been protected from the light. 

 But not only is the iris under the control of constrictor 

 nerve-fibres, it is also governed by dilator nerves ; and the 

 size of the pupil at any given moment depends on the play 

 of two nicely-balanced forces. 



The dilator fibres pass out by the anterior roots of the first three 

 thoracic nerves (dog, cat,, rabbit), accompanied apparently by vaso- 

 constrictor fibres for the iris. Reaching the sympathetic chain 

 through the corresponding rami communicantes, they traverse the 

 first thoracic ganglion, the annulus of Vieussens, the inferior cervical 

 ganglion and the cervical sympathetic. They end by arborizing 

 around some of the cells of the superior cervical ganglion, whose 

 axons eventually arrive at the Gasserian ganglion, and running along 

 the ophthalmic division of the trigeminal to the eye, reach the iris by 

 its ciliary branches. 



Stimulation of the cervical sympathetic causes marked 

 dilatation of the pupil (Practical Exercises, p. 861), even 

 when the third nerve is excited at the same time. All the 

 evidence at our command goes to show that the pupillo- 

 dilator fibres do not act by constricting the bloodvessels of 

 the iris. For dilatation of the pupil can be caused in a 

 bloodless animal by stimulating the sympathetic. And even 

 when the circulation is going on, a short stimulation of the 

 sympathetic causes dilatation of the pupil without vaso-con- 

 striction, while with longer excitation the dilatation of the 

 pupil begins before the narrowing of the bloodvessels. Nor 

 does it seem possible to accept the view that the sympa- 

 thetic fibres are inhibitory for the sphincter muscle of the 



