" If the image fall on identical points in both eyes it _will be seen single, and if the image does not fall on such identical points it will appear double.
" The two globes of the eyes are most mi nutely divided into degrees, minutes, and seconds of latitude and longitude; at all cor responding points they are identical, at all dif ferent points non-identical. The outer lateral portion of one eye is identical with the inner portion of the other eye ; the upper part of one retina is identical with the upper part of the other, and the lower parts of the two eyes are identical with each other.
" The left half of the retina A, from 1 to 5, however, (fig. 421,) is not, as a whole, identical 'with the left half of the retina B, from 1 to 5, but certain points only of the left halves of both retinw are identical, viz. those which in the two retinw occupy the same degrees of latitude and longitude: 1 is identical with 1, 2 with 2, and so on • but 1 in the one eye is not identical with 5 in tlie other eye.
" To explain the single vision, thorefore, it is necessary that not \ rperely each root of the optic nerve, but each primitive tibre of each root should in the chiasma divide into two branches for the two optic nerves, so that the identical fibres of the two nerves might com municate with the brain at one point only, viz. by one radical fibre, as in the annexed wood cut (fig. 422). But such a division of the fibres in the chiasma does not exist : Treviranus and Volkmann were unable to detect any division of fibres • in the chiasma,and I also was unsuc cessful in my search for such dividing fibres. ( Fig. 422.) 4. If single vision in ntan be explained on the assumption that certain parts of the two retinw are reciprocally identical, and that such identity depends upon a partial decussation in the chiasma, single vision in animals should of course admit of explanation upon the same principles; and if this be granted, the relative directions of the optic axes in the vertebrate classes ought to afford a good criterion of the extent to which the retinw are reciprocally identical ; for when the optic axes have a strictly lateral direction (as in many osseous fish), the same object can never be depicted on both retinw simultaneously, and consequently it may be inferred that, in such cases, no parts of the two retinx are reciprocally identical. Again, when the optic axes are very divergent, as in many quadrupeds, the respective fields of vision must comprise in great measure different objects, and under -such circumstances it may be presumed that the two retinw have but little identity. And INhen the eyes are so set that the
optic axes are parallel, or capable of becoming parallel, or convergent (as in man), the same objects, or nearly the same, will almost con stantly occupy the two fields of vision; and in such case the greatest amount of reciprocal identity may he assumed to occur in the two retinm.
Now, if the relative directions of the optic axes in animals bear relation to the amount of reciprocal identity in their retinw, and if this reciprocal identity depend upon the decussa tion in the chiasma, as has been assumed, the structure of the chiasma in animals generally should vary as the relative directions of their optic axes.
Such variation in the structure of the chiasma has not, however, been proved to occur gene rally throughout the animal series ; on the con trary, the chiasma usually conforms to the type prevalent in the class to which the animal belong,s, without evincing in its conformation much regard to the relative directions of the optic axes; and examples are not unfrequent in which the anatomy of the optic nerves is at variance with what the relative directions of the optic axes would require theoretically.
Thus in the pleuronectes fish, (fig. 409,) the optic axes are so directed that the two retime may be inferred to havea certain amount of identity, and nevertheless, in such of them as have been examined by the writer, the optic nerves are severally derived from opposite sides of the brain, and cross each other without forming a chiasma; or in other words, retinw evincing mutual identity are supplied by optic nerves which have no identity of origin ; the type pre valent in osseous fish being preserved, without respect to the directions of the optic axes. In many of the cetacea, the direction of the optic axes is such that the retinm can have no identity, and nevertheless a perfect chiasma, such as occurs in other mammalia, exists in these animals. And in tile oWl the eyes look more directly forwards than those of most other birds, from which it may be presumed that the amount of mutual identity in the two retinw is much greater in them than in thoie birds whose eyes have a lateral aspect ; but nevertheless the structure of the chiasma in the owl appears in nothing different from that which prevails in birds whose optic axes have a strictly lateral direction.