CHAPTER V.

COMPARATIVE PHYSIOLOGY OF BINOCULAR VISION.

THE cause of the remarkable law of corresponding points, on which all the phenomena of binocular vision depend, has not been traced with certainty to anatomical structure. It is probably in some way connected with the existence of an optic chiasm and the crossing of the fibers of the two optic nerves there, but in what way is not understood. We have already (page 102) alluded to a hypothesis, "the nativistic theory," which supposes that fibers from corresponding points unite into one fiber or end in one brain-cell; but even if this be true, it is undiscoverable. The optic chiasm doubtless is a sign of some kind of sympathetic relation between the two eyes; but whether this necessarily reaches the degree which produces corresponding points is uncertain.

The chiasm exists in nearly all vertebrates, but not in invertebrates. In vertebrates sometimes the fibers of the two nerve-roots (optic tracts) simply cross each other without uniting; this is the case in fishes. In others the fibers of the roots partly cross and partly do not, so that each nerve is made up of fibers from both roots; this is the case in mammals and birds, and probably to some extent in reptiles. It seems certain then that invertebrates do not enjoy binocular vision. It is proba

ble also, from anatomical structure alone, that osseous fishes do not enjoy this faculty. Whether in some still higher animals the sympathetic relation which certainly exists between the two eyes reaches the point necessary for their successful use of the two eyes as one instrument is also, I believe, very doubtful. I proceed to give some reasons for this belief, derived from the position of the two eyes.

In man the axes of the conical eye-sockets diverge about 25°, or each makes with the median line an angle of a little more than 12°. In these slightly diverging conical sockets the eyeballs are so placed, and the muscles so adjusted, that in the waking passive state their axes are parallel; and from this passive parallel condition they may be easily converged even upon very near objects. In man, then, though the eye-sockets still diverge considerably, the eyes are set in front with axes naturally parallel. This is evidently the position most suitable for binocular vision; for the eye-sockets could not be brought any nearer to parallelism without diminishing too much the interocular space, and thus the accuracy of binocular judgment of distance.

In monkeys the position of the eyes is much the same as in man. They are placed well in front, with their axes apparently parallel in the passive state, and therefore well adapted for binocular convergence on near objects. But as we go down the vertebrate scale, the eyes are placed wider and wider apart, then moved more and more to the side of the head; the axes of the eye-sockets are therefore more and more divergent, and the difficulty of convergence on a near point becomes greater and greater, until in some mammals, as cetacea, in many birds, and in all fishes, the eyes are placed no longer in front, but on the sides of the head, with their

optic axes inclined nearly or quite 180° with each other. It is evident that animals with eyes so placed can not converge the optic axes on a single point, especially a near point. In fact, it is well known that those birds. which have their eyes placed well on the side of the head, when they wish to look attentively, turn the head and look with one eye. It seems impossible that animals like the whale and fishes, in which the eyes are fairly on the side of the head, can enjoy a true binocular vision with its consensual movements of the two eyes, with its double and combined images, its stereoscopic effects, and its complex but accurate visual judgments based on these effects. It seems impossible that, for such animals, the law of corresponding points could have been developed, or can now exist; for if it did, it could only, as we have seen (page 260), lead to false judgments as to the direction of objects. They see with two eyes, but these do not act together as one instrument, as a single binocular eye; they are independent, and see each for itself. I have watched the motions of the eyes of fishes swimming in an aquarium, and they seem to me to move independently of each other. The same is true of all other senses, even in man: however much their organs may be multiplied, each organ perceives for itself. The property of corresponding points, from which all the phenomena of binocular vision are derived, is something peculiar to the eye of the higher animals. Nothing analogous exists in the other senses. Binocular vision in its perfection, as it exists in man and the higher animals, is the last result of the gradual improvement of that most refined of all the sense-organs, the eye, specially adapting it to meet the wants of the higher faculties of the mind.

There are, it is true, consensual movements and

sympathetic relations in the double organs of other senses s-e. g., the consensual movements of the hands. There is even a kind of binaural audition,* by means of which we judge imperfectly of direction of sound. But these are not only infinitely inferior in degree of perfection to, but they are essentially different in kind from, that consensual movement and that sympathetic relation which we find in the eyes, and which slowly in the process of evolution gave rise to the wonderful property of corresponding points and the phenomena of binocular vision.

Binocular vision, then, is certainly wanting in invertebrates, for the eyes in these are either immovably fixed, as in insects and many crustaceans, or, if movable, as in snails, etc., their movements are not consensual. The most perfect eyes among invertebrates are found in cephalopods. These have true recti muscles for turning them about, but from their position they can not move consensually. There is also no optic chiasm invertebrate.

in any

Teleost fishes do not enjoy binocular vision, for there is in them no optic chiasm, and the position of their eyes makes it impossible for them to converge their axes on objects, especially near objects. The movements of their eyes also seem to be independent. Sharks and selachians generally have an optic chiasm, and therefore probably more sympathetic connection between the eyes than osseous fishes. It is possible that binocular effects begin first to be developed in these. Yet not only in these, but even in reptiles and some birds, binocular seems to be at least subordinate

*

Thompson, "Philosophical Magazine," vol. iv, p. 274 (1877); vol. vi, p. 383 (1878); "American Journal of Science and Arts," vol. xix, p. 145 (1880); Steinhauser, "Philosophical Magazine," vol. vii, p. 261 (1879).

to monocular two-eyed vision (if I may be allowed the expression). The carnivorous birds and all mammals except cetacea seem to enjoy binocular vision very much as man does, though I believe in a less perfect degree.

There is another peculiarity of the human eye, probably closely connected with the highest effects of binocular vision, which still more quickly disappears as we go down the vertebrate scale. I refer to the existence of the central spot of the retina. We have already seen that this spot, situated exactly in the center of the retinal concave, and therefore just where the visual line pierces the retina, is the most highly organized and sensitive portion of the retina. It is not more than a millimetre in diameter. Now every spot of the retina has its representative in the field of view. The representative of this is the point of sight and a very small area about that point, viz., the area of very clear vision. At the ordinary reading distance of 12 inches, this area is not more than three quarters of an inch in diameter. If, while gazing steadily and attentively at one point, we observe the relative distinctness of points in other portions of the field of view, we shall find that these become rapidly less and less distinct as the point is more distant from the line of sight. In other words, there is a regular gradation of distinctness, from the point of sight, where it is greatest, to the extreme margins of the field of view, where it is least. Now, as the retina corresponds to the field of view point for point, it follows that there is a regular gradation in keenness and definiteness of perception, and therefore in fineness of organization, from the central spot, where it is greatest, to the anterior margin of the retina, where it is least. This superior fineness of organization has not been demonstrated except for the central spot; but the gra