This, however, is not more wonderful than that the undulations of the luminous particles should excite the sensation of light in the retina, or that vibrations acting on the auditory nerve should give rise to sound. We may heap conjecture on conjecture as to the final ' cause of these phenomena, but we must stop at the limits to the boundaries of human knowledge. The profound resources of the 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 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 retina 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 retinae 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 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 retinx 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 retina of the two eyes to co-operate in pro clueing 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 to 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 he 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.