vi DIOPTEIC MECHANISM OF THE EYE 321 



the cells of its nucleus. But in order to explain the normal 

 association of pupillary contraction with accommodation due to 

 contraction of the ciliary muscle and convergence of the visual 

 axes produced by contraction of the recti interni, it must be 

 assumed that the centres of these reflexes lie close together and 

 are probably all contained within the nucleus of the oculo-rnotor 

 nerves. 



To account for the co-ordination that exists between myosis 

 and mydriasis (pupillary constriction and dilatation) it must be 

 assumed that from these reflex centres, whether they be in the 

 corpora quadrigemina or in the nucleus of the oculo-motor nerves, 

 paths run into the cervical cord, and come into connection there 

 with the cilio-spinal centre, and that on their activity depends the 

 contraction of the dilatator muscle and the active relaxation of 

 the sphincter of the pupil. Anatomical research has not, however, 

 discovered the course of these internuclear paths. 



Besides the peripheral centres, and those lying in the cord, 

 medulla oblongata, and mid-brain, it is necessary to admit the 

 existence of cortical centres, both for myosis and for mydriasis. 



The cortical centres for contraction of the pupil lie in the 

 occipital lobe, in the second external convolution in rabbits and 

 dogs. On gently stimulating this area with the faradic current, 

 transitory contraction of the pupil results (Bechterew, Plitz, 

 Angelucci). The cortical centres for dilatation of the pupil lie 

 in the sensory motor area. Terrier, Katschanowski, Hensen, and 

 Volckers hold that this cortical centre, which when stimulated 

 produces mydriasis, is the point of origin of the cervical 

 sympathetic. Grlinhagen, Bochefontaine, and Braunstein, how- 

 ever, found that mydriasis may occur even when the cervical 

 sympathetic has been divided. Angelucci confirmed this, but 

 noted that the reflex is abolished immediately after section of the 

 sympathetic owing to the inhibition produced by traumatism. 



X. To avoid blurring of the outlines of images projected on the 

 sensitive plate of a photographic apparatus by the diffuse light 

 reflected from the walls of the camera, the latter is usually painted 

 black. In the eye sharpness of the retinal images is obtained by 

 the layer of black pigment that clothes the outer surface of the 

 retina and extends from the posterior pole of the eye to the iris, and 

 by the pigment of the uvea. Albinoes, who have no retinal and 

 uveal pigment, are dazzled even by moderate light, and are 

 consequently forced, in focussing illuminated objects, to close the 

 eyelids so as to leave only a narrow slit between. 



The pupil of an albino looks red, though the normal pupil 

 appears black. From this it seems logical to conclude that under 

 normal conditions the pigment of the retina and uvea absorbs all 

 the light that enters through the pupil, and that it is only when 

 there is no pigment that it is possible to look into the fundus of 



VOL. iv y 



