. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 774 OPTIC NERVES. rally throughout the animal series ; on the con- trary, the chiasma usually conforms to the type prevalent in the class to which the animal belongs, 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, (Jig. 409,) the optic axes are so directed that the two retina
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 774 OPTIC NERVES. rally throughout the animal series ; on the con- trary, the chiasma usually conforms to the type prevalent in the class to which the animal belongs, 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, (Jig. 409,) the optic axes are so directed that the two retinae may be inferred to have a 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, retinae 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 retinae can have no identity, and nevertheless a perfect chiasma, such as occurs in other mammalia, exists in these animals. And in the 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 retinae is much greater in them than in those 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. But further, when the optic axes are very divergent, as in some quadrupeds, any object which can be depicted upon both retinae simul- taneously, will throw its images on the outer parts of the two retinae, and in order to explain A single vision under such circumstances, the outer parts of the two retinae shou
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