Structure of the eye in Fishes.

Fig. 321. Structure of the eye in Fishes.

(1809). But the focus of the crystalline will be short in proportion as its power is increased: every arrangement has therefore been made to approximate the retina to the posterior surface of the lens; the eyeball is flattened, by diminishing the relative quantity of the vitreous humour; and a section of the eye (fig. 321, b, c) shows that its shape is very far from that of a perfect sphere. This flattened form could not, however, have been maintained in fishes, had not special provision been made for the tmrpose in the construction of the sclerotic: the outer tunic of the eye, therefore, generally contains two cartilaginous plates imbedded in its tissue, which are sufficiently firm in their texture to prevent any alteration in the shape of the eyeball; and in some of the large fishes the sclerotic is actually converted into a cup of bone presenting orifices at the opposed extremities - one for the insertion of the transparent cornea, the other for the admission of the optic nerve.

(1810). The vitreous humour and crystalline lens in many fishes are kept in situ by a ligament placed for the purpose. This is a delicate falciform membrane derived from the retina (fig.321,B,c), which plunges into the vitreous humour, and, being continued along the internal concavity of the eye, is fixed to the capsule of the lens. In some fishes, as the Salmon, this ligament is of a dark colour; and in the Conger there are two such bands, by which the crystalline is suspended as by its opposite poles.

(1811). Another peculiarity in the structure of the visual apparatus of osseous fishes is the existence of a vascular organ placed at the back of the eyeball, and interposed between the choroid tunic and a brilliant metallic-coloured membrane which invests the choroid externally. This organ, generally called the "choroid gland" by the older anatomists (fig. 321, A,gg), is of a crescentic form, and always of a deep-red colour. It is principally made up of blood-vessels, which run parallel to each other; and from it issue other vessels, frequently very tortuous, and always much ramified, which form a vascular network in the choroid. The nature of this organ it is not very easy to determine. Some have believed it muscular; but the striae perceptible in it are vascular, and not fibrous: others have thought it to be glandular; but it has no excretory duct. Most probably it is an erectile tissue analogous to that of the corpus cavernosum, and has some influence in accommodating the form of the eye to distances, or to the density of the surrounding medium*.

(1812). The pupil of the eye in the animals we are describing is very large, so as to take in as much light as possible, but generally motionless. In some genera the shape of the aperture is curious: thus in the Rays a broad palmate veil hangs in front of the pupillary aperture; and in one case, the Anableps, there are two pupils to each eye.

(1813). The eyes of osseous fishes are lodged in the bony orbits of the face, imbedded in a soft glairy cellulosity; but in many of the cartilaginous tribes, such as the Sharks and Rays, each eyeball is moveably articulated to the extremity of a cartilaginous pedicle fixed to the bottom of the orbital cavity (figs. 322, i, & 321, c).

(1814). Six muscles serve to turn the eye in different directions: namely, four recti, arising, as in Man, from the margin of the optic foramen; and two oblique muscles, derived from the anterior part of the orbit, and inserted transversely into the globe. These muscles are well represented in fig. 322, wherein the reader will observe that the superior oblique does not pass through a pulley, as is the case in the human subject.

* Cuv. et Val. op. cit p. 338.

Brain and cerebral nerves of the Skate: a, olfactory ganglion.

Fig. 322. Brain and cerebral nerves of the Skate: a, olfactory ganglion; b, c, cerebrum; d, cerebellum; e, medulla spinalis; g, the eyeball; i, its cartilaginous pedicle; k, olfactory sac; 1, distribution of the olfactory nerve.

(1815). It is extremely remarkable that even in fishes the muscles of the eye have special nerves appropriated to them, and those precisely the same as in the highest Mammalia. The third pair of nerves animates them all, except the external rectus and the superior oblique, and also sends off filaments to be distributed to the choroid, although no ophthalmic ganglion has yet been discovered. The fourth pair is exclusively appropriated to the superior oblique; and the external rectus, or abductor muscle, invariably receives its supply from the sixth pair.

(1816). To animals whose eyes are constantly washed by the water in which they live, any lacrymal apparatus would obviously be superfluous; and consequently, in the class before us, neither lacrymal gland nor lacrymal puncta, nor even eyelids, properly so called, are ever met with.

(1817). Behind the optic lobes of a fish's brain the ganglia from which the other cerebral nerves emanate become confused into one mass, so that they are no longer distinguishable from each other. The nerves themselves, however, are easily recognized, and, with the exception of the ninth pair (the Ungual or hypoglossal nerves), which are not met with in fishes, both in their distribution and number precisely accord with those with which the human anatomist is familiar. We have already traced the third, fourth, and sixth pairs to the muscles of the eye. The fifth issues through the great ala of the sphenoid, and divides, as in Man, into an ophthalmic branch (fig. 319, a), which runs through the orbit to be distributed to the parts about the nose; a superior maxillary branch (β), that supplies the parts about the upper jaw; and an inferior maxillary branch (0), destined to the lower jaw: the general distribution of the nerve, as far as regards the face, is in fact exactly similar to that of the same nerve in Man; but in fishes it is found to give off other branches not met with in the human subject, one of which (μ) is destined to the operculum. Another (ξ) takes a very remarkable course: it mounts up to the top of the skull, joins a large branch of the eighth pair (φ), and, issuing from the cranium through a hole in the parietal and interparietal bones, passes along the whole length of the back on each side of the dorsal fin, receiving twigs from all the intercostal nerves, and supplying the muscles of the fin and the fin-rays themselves.