2. In vision the object is known to be wholly reversed in the image on the retina ; for ex ample, that which is above in the object is below in the image, and vice versa.; and that which is to the right in the object is to the left in the image, And vice versil; now, although the decussation in question might possibly explain the correct appreciation of an object whose image is reversed in one particular direction (say frorn above downwards), it never can fully explain the correct impression made by an object of which the image is reversed in all directions simultaneously.
3. In order that this explanation may apply to human vision, the same interlacement of filaments must first be demonstrated in the optic nerve of man, a task which has not as yet been accomplished.
Optic nerves of the compound eyes of Insects.
These nerves are excessively large and appear proportional to the size of' the organs of vision in the Insect.
Each nerve on arriving at the eye swells out into a bulb, which is convex and varies con siderably in dimensions in different species ; this bulb in general represents a segment of a sphere, and froin its surface nervous filaments in immense numbers arise and diverge like the radii of a circle. Several thousand of these filaments have been counted in a single nerve ; each of them is connected by its distal extremity to the apex of a small conical transparent body interposed between the filament and the cor responding facet of the cornea, but the most striking pkuliarity of the nerve is found in the perfect isolation of each of these filaments by the interposition of pigment.
The pigment is variable in colour, being sometimes light, at others dark ; it may be nearly black, dark violet, dark blue, purple, brown, brownish yellow, light yellow, or green, and sometimes several layers of different colours lie one over the other ; the pigment extends from the bulb of the nerve to the cornea ; it sur rounds each filament and separates it com pletely from adjacent filaments.
The effect of this disposition is to isolate the rays of light incident on each filament, and to prevent the transmission of all rays except such as fall in the direction of the axis of the filaments ; since all oblique rays must of neces sity impinge upon the colouring matter and be in consequence absorbed.
The reader is referred to the article IbtsEeTA for further details concerning the optic nerve in the ie an i mals.
Plaited optic nerve.
The optic nerve in some animals exhibits a peculiar plaited appearance; this condition occurs in greatest perfection in certain fishes, and in many birds a somewhat similar organi zation may be detected, though not at all so perfectly as in the fish tribes.
When the plaiting prevails in perfection, the nerve consists essentially of a thin membmne folded on itself exactly like a closed fan or a plaited frill ; but the arrangement referred to is not at first sight apparent, particularly if the nerve be inspected in that part of its course only which is external to the cmnial cavity, for there the neurilema is so thick, dense, and tough, that a correct estimate of the disposition of the nervous structure cannot be formed until this investment has been removed.
Deprived of its neurilema, the nerve seems to consist of a number of parallel laminw placed in juxta-position ; on closer examination these laminm turn out to be continuous with each other at their edges, and by a little care the nerve can be unfolded into a membrane of which the breadth is proportional to the num ber and depth of the original laminm.
NVhen the plaited condition is perfect, it pre vails along the entire length of the optic nerve, becoming manifest at its cerebral attachment, and continuing to the eye-ball ; even the retina seems to participate in the same disposition, as folds or plaits are observable on the surface of that nervous expansion ; and in certain fish the optic lobes themselves present a similar organization, for the walls of the cavities which these tubercles contain are in some instances covered with laminm.
The plaiting must be considered an essential attribute of the nervous substance and totally independent of the neurilematous investment, for this disposition of the nervous material occurs occasionally in the optic lobes and re tina, structures which are devoid of neurilema. ( .4g. 425.) Fig. 425.
Lannnated optic nerve.
In certain birds the optic nerve is laminated and bears a close resemblance to the plaited condition just described.