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lar judgment of distance; a greater divergence would destroy parallelism of the optic axes in a passive state -i. e., would require voluntary effort to produce and maintain parallelism.
The apes are exactly like man in this regard. In them, too, the eyes are naturally parallel in a passive state, and are therefore perfectly adapted for binocular vision. But as soon as we go lower down the vertebrate scale the eyes are placed wider and wider apart, the axes of the eye-sockets become more and more divergent, and with them the normal passive position of the optic axes become also more and more divergent, until finally in fishes the eyes are placed on the sides of the head with their optic axes divergent nearly or quite 180°. It is evident that eyes so placed can have no common field of view, no common point of convergence, no point of sight, and no binocular vision. Each eye moves and sees independently of the other. This may be seen by watching fishes in an aquarium.
In all mammals, however, except perhaps the whales, the divergence of the eye-sockets is not so extreme but that by voluntary effort they may be made to converge with their optic axes on an object. They therefore have binocular vision in various degrees of perfectionmore perfect in carnivores, less perfect in herbivores, but in all less perfect because less important than
For them wideness of view is more important than attentive examination and accurate binocular judgment of distance. For example, in ruminants the eyes are placed on the extreme margins of a broad front, perhaps six inches apart, and are very protuberant. This together with the horizontal elongation of their pupils gives them a very wide field of view. There is no doubt that the view of a grazing ruminant sweeps the whole horizon without moving the eyes or turning the head. The advantages of easy convergence are sacrificed to the greater advantages of a wide view.
Birds usually have their eye-sockets widely divergent, often 90° to 100° (Fig. 147). Their optic axes also seem nearly or quite coincident with the socket axes. This divergence is far too great to admit of easy convergence of the optic axes on an object, especially a near object. Yet most birds certainly have binocular vision. To make this possible, however, there is a peculiar and unique retinal structure, of which we shall speak under the next head.
Fovea.—It will be remembered that in man this most highly organized spot is situated at the point where the optic axis pierces the retina. It is in the very center of the retinal concave, and about it the corresponding points of the two retinæ are symmetrically arranged. In every act of looking, the images of the object looked at are made to fall on these spots. This is the necessary condition of accurate vision. We have usually called it the central spot because of its central position. In speaking of the human eye this is well, but in comparative anatomy it is better to call it the fovea, because it is not always central.
Now in mammals—although there is usually a more highly organized central area-a true fovea is wanting, except in the anthropoid apes. In these latter the retinal structure is precisely like man's in this regard. In mammals, however, except in apes, extremely accurate vision of single objects is largely sacrificed to the greater advantages of a tolerably clear vision over a very wide field. Their safety depends on this latter. It will be remembered (page 79) that in man the parts of the retina at a little distance from the fovea are more
sensitive to light than the fovea itself, though accurate observation of outline and details of surface are seen only by the fovea. Mammals' eyes are as sharp as, perhaps sharper than, ours, to detect the presence but not to discern the nature of objects. This is probably the reason that they are so easily startled by unaccustomed objects.
The case of birds is peculiar. The wide divergence of their optic axes (Fig. 147) would make binocular vision impossible for them if their corresponding points were arranged like ours—i. e., symmetrically about a central fovea. But, strange to say, some birds—perhaps
V most—have two fovece in each eye, one centrali. e., in the optic axes (Fig. 147 a)—the other in the temporal half of the retina and excentral by about 60° (Fig. 147 b). These latter are so placed that lines drawn through them and through the
SECTION OF BIRD'S HEAD. (After Slonacenter of the pupils are ker) V V", monocular visual lines; parallel each to the me
v v', binocular visual lines; a a', 60',
central and temporal foveæ respectively. dian plane of the head and therefore to one another. Evidently these temporal foveæ are suitably placed for convergence on a common point of sight, and therefore for binocular vision. Evidently also corresponding points must be arranged about these as with us about the central foveæ. Evidently their central foveæ can be used only for monocular not for binocular vision. But it is this central fovea which is the most distinct and therefore most highly organized. Therefore their binocular vision is less perfect than ours, or even than their monocular vision. Hence it is that a bird when it wishes to look attentively turns the head and looks with one eye so as to bring the image on the central fovea. So far as the central fovea is concerned they use their eyes independently of one another--each eye looks for itself.
Neither of these two foveæ of birds can be regarded as homologous with that of man. Taking the entrance of the optic nerve-or the blind spot o as the term of comparison-our fovea is temporal, but it is central in regard to the optic axis. In birds the entrance of the optic nerve or blind spots is between the two foveæ. The one is central to the optic axis but nasal to the blind spot, the other is, like ours, temporal to the blind spot but excentral to the optic axis.
Fovee and corresponding points are probably developed together, and both their existence and their place is determined by the position of the eyes and the habits of the animal, especially in looking attentively.
Thus then, judging alike from the chiasm, the position of the eyes, or by the existence and position of the fovea, we conclude that binocular vision becomes less and less perfect as we descend the scale and finally disappears in the lowest vertebrates. In invertebrates we find nothing at all like a chiasm nor a fovea. In many of them the eyes are also immovably fixed. We are justified in thinking that the phenomena of binocular vision do not exist in them.
TIIE EVOLUTION OF THE EYE.
The history of the origin and gradual evolution of this most refined instrument has always been regarded as among the most insoluble of mysteries. Recently, however, some important light has been thrown on it. A brief outline of what is known is here given, as a fitting close of this little volume.
1. The Invertebrate Eye. General sensibility to light is coextensive with life. It is found in the lowest protozoa and even in plants. This is not special sense; but, in accordance with a general law, all useful functions are by evolution soon specialized and localized in separate organs. It is probable that the first beginnings of the origin of a lightperceiving organ was determined by the stimulus of light itself on the epidermal surface. Certain groups of epithelial cells are thereby modified by elongation ; a nerve fiber connects itself with each cell, and pigmentary matter is deposited at their base. This is the beginning of the light-perceiving part of the eye-viz., the bacillary and pigmentary layers. Such deposit of pigmentary matter for light-absorption and such specialization of nerve-terminals for light-perception, or response to ethereal vibration, may take place in any ex