268 HUMAN PHYSIOLOGY 



centripetal nerve of this reflex is the optic nerve. By this 

 reflex contraction of the pupil the retina is saved from too 

 strong illumination, since the smaller the pupil the less the 

 light that enters the eye. 



Normally both pupils are of the same size ; the change in 

 the pupil takes place in both eyes even when the light 

 entering one eye is changed (consensual pupil reflex). The 

 centres for this reflex lie in the cervical cord (see page 233). 



The contraction of the pupil also takes place during 

 accommodation and convergence of both eyes. The centre 

 for this process is situated in the corpora quadrigemina (see 

 page 242). 



The pupil is (i) contracted by sleep and by poisons 

 (physostigmin) ; (2) dilated by stimulation of many sensory 

 nerves, muscular exercise, dyspnoea, poisons (atropin). 



IV. Ophthalmoscope. Only light falling upon the most 

 anterior part of the eyeball enters the eye. Light falling 

 upon the side does not enter the eye because of the opacity 



FIG. 31. 



of the choroid and iris. The light which has entered the 

 eye is reflected by the retina back to its source. Hence we 

 cannot see the background of another person's eye because 

 no light emanates from our own eye by which the observed 

 eye is illuminated. But if a mirror, S, having an aperture 

 in its centre (Fig. 31) is placed between two eyes, A and 

 B y in such a position that it throws rays of light coming from 



