VII. Contributions to the comparative anatomy of the reptilian and the amphibian eye, chiefly based on ophthalmological examination

Abstract

Since the publication of my “Contribution to the Comparative Anatomy of the Mammalian Eye ”(‘Phil. Trans.,’ B, vol. 194, p. 194 (1901)), I have supplemented this paper by similar investigations on the eyes of the lower classes of the Vertebrata, thereby hoping to extend our knowledge of the higher orders by comparison with those of the lower groups. As it was reasonable to expect in them incipient and simpler stages, where Mammals show highly complex or vestigial conditions, the hope seemed justified to approach the solution of various morphological and physiological problems. Of course, the investigations should by rights begin with the Fishes, at least in theory. In practice, however, they present insurmountable difficulties to ophthalmological investigation, partly on account of the obstacles in the way of holding them in a fixed position, and of keeping them alive when removed from their natural element, and partly also on account of the refraction of their eyes, which are naturally adjusted for seeing under water, which causes their eyes to become enormously hyperopic when examined in air. Moreover, their crystalline lenses are almost invariably spherical, a condition which admits of only a minute portion of the fundus being examined at one time, whereby the difficulty of obtaining a complete view of the fundus becomes greatly increased. The examination of the eye after death, either by illuminating the interior with the ophthalmoscope, or by dividing the eye equatorially and viewing the fundus through a microscope, is by no means a satisfactory procedure, since the post-mortem changes occur so rapidly that the picture is altogether misleading. For our present purpose, however, the fish’s eye is of small importance.