790 A MANUAL OF PHYSIOLOGY 



without any change in the iris ; but the normal and voluntary act of 

 accommodation cannot be disjoined from the corresponding altera- 

 tions in the size of the pupil. Inward rotation of the eyes accom- 

 panies contraction of the pupil in accommodation. 



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 

 maintains 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 



FIG. 293. NERVES OF THE EYE. 



.Ill, third or oculo-motor nerve ; IV, fourth or trochlear nerve ; V, ophthalmic branch 

 of fifth nerve ; VI, sixth or abducens ; C, carotid artery with its plexus of sympathetic 

 fibres ; i, ophthalmic ganglion, with its motor root 2, its sympathetic root 3, and its 

 sensory root 4 ; 5, direct ciliary filament ; 6, ciliary muscle ; 7, iris ; 8, cornea ; 9, con- 

 junctiva ; 10, lachrymal gland ; n, frontal nerve ; 12, nasal nerve; 13, recurrent branch 

 of ophthalmic division of fifth. The thick white lines represent the motor nerves ; the 

 thin continuous lines the sympathetic fibres ; the dotted lines the sensory nerves. 



td 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. Con- 

 traction 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 inter- 

 rupted by disease, while the other is intact. For in ioco- 



