9 io A MANUAL OF PHYSIOLOGY 



Voelckers). This can be observed either through a window in the 

 sclerotic in a dog or by following the movements of a needle thrust 

 into the eyeball. By carefully localized stimulation near the junc- 

 tion of the aqueduct with the third ventricle, it is possible to bring 

 about the forward bulging of the lens without any change in the 

 iris ; but the normal and voluntary act of accommodation cannot be 

 disjoined from the corresponding alterations in the size of the pupil. 

 Inward rotation of the eyes accompanies contraction of the pupil 

 in accommodation, and the question may be raised whether the 

 pupillary change is associated with the action of the extrinsic 

 muscles of the eyeball which cause convergence or with the action of 

 the intrinsic muscles which determine the changes in the curvature 

 of the lens. It is usually considered to be associated with both. In 

 any case, actual convergence is not necessary for the reaction, since 

 it may still be obtained on accommodation when convergence is 

 impossible on account of paralysis of the internal recti. 



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 

 flashlight. 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 eluci- 

 dated ; 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 paralytica (general paralysis), 

 the light-reflex sometimes disappears, while the constriction of 

 the pupil in accommodation and convergence 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 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. 



