VISION 985 



Changes in the Pupil produced by Light. It is not only by 

 accommodation that the size of the pupil may be affected. In 

 the dark it dilates, at first rapidly, then gradually, and it main- 

 tains the width it has reached for several hours. This has been 

 shown by taking photographs of the eye with the magnesium flash- 

 light. In this way the width of the pupil is recorded before it has 

 time to alter. Or a longer exposure to ultra-violet light, which 

 affects the pupil but little, may be employed. When ordinary 

 light falls upon the retina the pupil contracts, and the amount of 

 contraction is roughly proportional to the intensity of the light. 

 Contraction of the pupil to light is brought about by a reflex 

 mechanism, of which the optic nerve forms the afferent and the 

 oculo-motor the efferent path, while the centre is situated in the 

 floor of the aqueduct of Sylvius. The relation of this centre to 

 that which controls the changes in the pupil during accommodation 

 has not as yet been sufficiently elucidated; but this we do know, 

 that one of the paths may be interrupted by disease, while the other 

 is intact. For in tabes (locomotor ataxia), and in dementia para- 

 lytica (general paralysis), the light-reflex sometimes disappears, 

 while the constriction of the pupil in accommodation and conver- 

 gence still takes place (Argyll-Robertson pupil). Artificial stimula- 

 tion 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 decussate, there 

 is a consensual light-reflex i.e., 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 occurrence of contraction does not 

 exclude insensibility of the retina unless the other eye has been 

 protected from the light. 



Stimulation of the cervical sympathetic causes marked dilata- 

 tion of the pupil, even when the third nerve is excited at the same 

 time. 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 sympa- 

 thetic causes dilatation of the pupil without vaso-constriction, 

 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 sympathetic fibres are inhibitory for the 

 sphincter muscle of the iris. They act directly upon dilator muscu- 

 lar fibres. It has, indeed, long been known that in the iris of the 

 otter and of birds a radial dilator muscle exists; and it has been 



