1118 



VISION. 



being seen nearly in a straight line drawn per- 

 pendicularly to the retina from that point of 

 it where its image falls and this law of visible 

 direction may be regarded as exercising an 

 important influence on the production of 

 single vision. 



In reference to this phenomenon, it is ne- 

 cessary to advert to the remarkable structure 

 of the commissure of the optic nerve. The 

 chiasma results from the junction of the optic 

 tracts, in front of and inferior to the tuber 

 cinereum. The fibres which constitute the 

 inner margin of each tract, B, are continued 



Fig. 885. 



across from one side of the brain to the other, 

 forming no connection with the optic nerves, 

 and existing where those nerves do not exist, 

 as in the mole. These fibres may be con- 

 sidered as commissural between the thalami of 

 opposite sides. The remaining fibres of the 

 tracts go to form the optic nerves, the cen- 

 tral passing across to the nerve of the oppo- 

 site side, and the outermost, c c, passing to 

 the optic nerve of the same side. Besides 

 these, the two retinas are brought into direct 

 connection by the fibres, A, which form the 

 anterior border of the chiasma. From this 

 arrangement it appears that corresponding 

 parts of the two retinas are brought into 

 relation one to the other, in the same manner 

 as corresponding parts of the cerebral con- 

 volutions are linked together by the various 

 commissures : the right side of each retina 

 appears also to be continuous with the right 

 optic tract, and the left side of each with the 

 left ;thus each side of the central apparatus 

 is brought into communication with its 1 own 

 side of both retinal images, which may be 

 supposed to favour their conception as one. 



The learned Arabian Alhazen supposed 

 that when corresponding points of the two 

 retinas are affected, the mind perceives one 

 image ; this opinion has found favour ; but Dr. 

 Todd and Mr. Bowman, and also Dr. Alison 

 have especially pointed out the importance of 

 the decussation of the commissural fibres. 

 Dr. Alison arrives at the conclusions : first, 

 that certainly in some, and probably in all 

 animals, the structure of the optic nerve 

 brings the impressions which form inverted 

 images on the retina into the same order on 

 the sensorium as those which might result 

 from the touch of the same objects ; secondly, 

 that in those animals which can direct both 

 eyes to one point, the partial decussation of 

 the optic nerves, generally if not universally 

 present, enables the images produced by an 

 object on the corresponding parts of the 

 retinas of the two eyes to co-operate in pro- 



ducing one impression on the sensorium, and 

 one sensation in the mind; and lastly, that 

 the decussation which takes place in the cor- 

 pora pyramidalia affords correct information 

 as to objects of sight from impressions made 

 on them simultaneously in both optic lobes, 

 that is tr> say, on both sides of the senso- 

 rium, notwithstanding that the impression 

 on each side of the sensorium comes from 

 the opposite side of the object in view. Thus 

 it will be seen that Dr. Alison refers the con- 

 nection of sight with touch to the decussation 

 of the corpora pyramidalia. We may fairly 

 conclude that there must be some structural 

 provision in the organisation of the cerebral 

 portion of the visual apparatus, which favours 

 the perception of a single image from a double 

 impression, and keeps in perfect harmony the 

 important senses in question : how far the 

 corpora pyramidalia may be concerned, is 

 matter of speculation. The precise action of 

 the mind, by which the single image only is 

 appreciated, is of course unknown to us, but 

 an additional argument in favour of the part 

 performed by the chiasma is afforded by the 

 occurrence of cases where only half an object 

 or word is seen when both eyes are directed 

 to it. Thus, as has been humorously illustrated 

 by Dr. Hull, the word patriot becomes riot; 

 and of matrimony, only many is seen. Such 

 phenomena are (as remarked by Dr. Todd and 

 Mr. Bowman) readily explained by supposing 

 the anatomical arrangement of the sides of the 

 retina, with regard to the optic tracts, to be 

 such as has been described, since any derange- 

 ment of one optic tract would then affect the 

 same part of both optic images. 



For the production of single vision, it is 

 necessary that the muscles which move both 

 eyes should act in perfect concert. The 

 effect of this is, that the axes of the eyes 

 converge towards the object to which they 

 are adjusted, and the image falls on corre- 



Fig. 886. 



spending parts of the two retinse. Let A n 

 be the two eyes, and c any object before 

 them. Then AC, uc are their axes, which 

 meet in c. An image is consequently pro- 

 duced in each eye which will correspond with 

 the perspective projection of the object from 

 the points A and B. If the two images are un- 

 symmctrically placed, so that the axes meet 

 either before or behind the object, a double 

 sensation is excited, as happens in strabismus, 

 and the double vision of intoxication. The 

 tipper sides of both retinas correspond, as clothe 

 lower; and the outer side of each corresponds 

 with the inner side of the other ; but the objects 

 and corresponding points of the retinas should 



