SIGHT. 581 



saw objects only situate to the left of the centre of vision, and, 

 as at the same time there was an outward deviation of the axis of 

 this eye, through a paralysis of the nerve of the third pair, when 

 he employed both eyes, he saw objects double ; but, what was 

 still more singular, the right eye being closed, he saw with the 

 left eye the objects removed from twenty to twenty-five degrees 

 to the right of their real position. 



If the fifth pair, which gives sensibility to the face, is divided, 

 the eye, nose, and tongue, lose their sense of touch, ordinary 

 sensibility, in common with the skin, and are not excited by me- 

 chanical or acrid stimulus as before. x In this experiment, the 

 pupil becomes greatly contracted in rabbits and guinea-pigs, 

 and dilated in cats and dogs. y The retina has very little ordinary 

 sensibility, as Dr. Magendie showed by pricking and tearing it 

 with little or no pain ; whence contraction of the pupil does not 

 follow the application of any stimulus excepting light. The third 

 pair, which is a nerve of motion, supplies, in common with the 

 fifth pair, the iris, and therefore Mr. Mayo found that division of 

 it, at least in cats and pigeons, causes dilatation of the pupil, like 

 division of the optic nerve ; the dilatation arising in the former 

 case from the cerebral influence being no longer conveyed, and 

 in the latter from the cerebral influence being no longer excited. 

 On stimulating the ocular end of the third pair, divided in pigeons, 

 after removing the brain, he says that the iris suddenly acts: and, 

 on stimulating not the ocular, but the cerebral, end of a divided 

 optic nerve, the same thing happens. M. Fleurens says that 

 irritation of the superior quadrigemina causes contraction of the 

 iris, and extirpation of them its dilatation. 



Judgment of colour. Persons, all having excellent eyes, and see- 

 ing perfectly well, differ much in their powers of recognising per- 

 sons, finding their way, &c. In none of these points is the difference 

 so striking as with respect to judging of colours. It is by no means 

 uncommon to meet with individuals whose eyes appear excellent, 

 and whose sight is excellent, and who may judge of form and dis- 

 tance correctly, but who cannot distinguish certain colours. Dr. 

 Nicholl describes a boy who confounded green with red, and called 



x Dr. Magendie, I.e. 



y Dr. Magendie, 1. c. Mr. Mayo infbrms us that, after death, in the cat and 

 pigeon the pupil is always dilated, and in the rabbit contracted. Outlines, <$;c, 



Q Q 4 



