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this peculiarity which makes a blind spot. The rest of the eye—the vitreous humor, the sclerotic, etc.—are formed by modification of the adjacent tissues.
It is seen, then, that in both the invertebrate and the vertebrate eye the retinal rods are transformed epithelial cells in which nerve fibers terminate ; but in the one case these are of epidermal origin, in the other they originate from the epithelial lining of the brain. But the difference between these two modes of origin is not so great as it at first seems. For of the three original layers of the embryo—the ectoderm, the endoderm, and the mesoderm—the nerve centers are formed by an infolding of the outer one-the ectoderm; and therefore the lining epithelium of the brain vesicle and of the optic vesicle is really an infolded part of the epidermal epithelium. This is shown in Fig. 149, A.
Transition from Invertebrate to Vertebrate Eye.
We see then that the line of evolution is continuous for the invertebrate eye, but how did the vertebrate eye come out of the invertebrate eye? There has been much discussion and many theories on this point, but the most probable one seems to be that of Beranek.* According to him the lens of the vertebrate eye is not homologous with the lens of invertebrates, but rather with the whole eye of invertebrates. The lens of the invertebrate eye is not formed by infolding of the epidermal surface, but by cuticular ingrowth at the point of closure of the optic vesicle. On the contrary, the lens of the vertebrate eye is formed by infolding of the epidermal surface, precisely as is the whole eye of invertebrates. Therefore, according to Beranek, in the primitive vertebrate, in fact before the vertebrate character was fully declared, the eye was formed after the manner of the invertebrate eye by epidermal infolding, but still in an imperfect condition, like c or d or e, Fig. 148, with the posterior part forming a retina, and fibers terminating in the usual way forward, but the optic vesicle (epidermo-optic vesicle) almost or quite touching the cephalic ganglion-i. e., with very short or no optic nerve (Fig. 149, E). Under these conditions direct stimulation of brain vesicle might well develop an additional optic vesicle (cerebro-optic vesicle) and an additional retina (cerebral retina). The new retina gradually replaced the old, the previous eye became the lens only, the retinal part being transformed into its posterior part, which is known to have a different structure from the anterior. The vitreous humor was of course afterward filled in between.
* Arch. des Sciences, vol. xxi7, p. 361, 1890.
The perfecting of the Vertebrate Eye. The gradual evolution of the invertebrate eye is satisfactory. The transition from the invertebrate to the vertebrate eye is doubtful.
is doubtful. But thenceforward the line of evolution is retaken and continues very regularly. We have already, in the previous chapter, some of these stages. We now give them briefly in the order of evolution.
In the lowest class of vertebrates--the Fishes--the eve, though formed on a different plan, is probably no better than a squid's. In fishes the eyes are placed well on the sides of the head, with their axes so widely divergent that their fields of view do not to any extent overlap. There is no consensual movement-each
eye moves for itself. There is no common field of vieweach eye
looks for itself. There is no common point
of sight, and therefore no corresponding points of the two retinæ, and therefore also no binocular vision and no accurate judgment of solid form and relative distance based on binocular perspective.
Leaving out amphibians and reptiles, of which we know little, in Birds, although their optic axes are still widely divergent, yet by a unique arrangement of corresponding points about a very excentral fove.?, binocular vision becomes possible for them, although their most perfect vision is still monocular.
Birds are a very highly specialized class of vertebrates in many respects. It is not strange that they should be so in vision also.
In mammals the eyes are brought more and more to the front; the optic axes more and more nearly parallel in a passive state ; the convergence of the axes on a point of sight becomes more and more easy; and with this comes the gradual development of corresponding points about a more highly organized central area, and thus all the phenomena of binocular vision and the judgments based thereon. But in mammals, generally, attentive observation and accurate perception of details at the point of sight is sacrificed to the greater advantages of an almost equal vision over a very wide field. The sight of mammals is no doubt keen, perhaps keener than ours in detection of objects, but not, I think, in determining their character.
Only in the anthropoid apes do we find the eyes brought fairly to the front with the optic axes parallel in a passive state, and a highly organized central fovea added, and vision thus made far more accurate at the point of sight. It is evident that this is the essential condition of attentive examination of the object looked at.
Finally, in man again, out of this there came thoughtful attention to the object looked at to the partial exclusion of other things, which seems to be a necessary condition of the emergence of the higher faculties of the mind. The existence of the fovea is necessary to the concentration of attention on the thing looked
For how could we attend to one thing if all other things were equally distinctly seen? The same law is carried up from the physical into the higher psychical field. Concentration of thought on the subject thought of is a necessary condition of effective thought-work. The mind's eye, too, must have its fovea, or we do no effective work. The mind's eye also must be binocular (page 178) or we get no true moral perspective.
29 Binocular vision, eyes of apes
35 vision, eyes of lower verte-
45 vision, fundamental phenom-
38 epa usually overlooked in....... 248
31 possessed by mammals in
varying degrees of perfection... 299
214 possessed by most birds. 300
vision, usual mode of repre-
vision, experiments illustrat-
171 ing the false mode of represent-
199 vision, comparative physiol-
eye-sockets incompatible with... 299
visual phenomena in ocular
visual phenomena in drowsi-
visual phenomena in intoxica-
Bird's head, section of....
57, 81, 302
experiments illustrating... 82-85
spot, size of
spot and its representative
in the field of view.............
its place in the field of view.. 87
161, 176 Central spot of the retina... 52, 55, 300
function of the...