co, Cornea; sc, sclerotica; o, nervus opticus; c, sclerotic capsule; a, membrana argentea; v, membrana vasculosa; u, membrana uvea; ch, choroid gland; r, retina; f, processus falciformis; h, humor vitreus; l, lens; i, iris.
The form of the bulbus (Fig. [48]) is subhemispherical, the cornea (co) being flat. If it were convex, as in higher Vertebrates, it would be more liable to injury; but being level with the side of the head the chances of injury by friction are diminished. The sclerotica (sc) is cartilaginous in Chondropterygians and Acipensers, fibrous and of varying thickness in Teleosteans, in the majority of which it is supported by a pair of cartilaginous or ossified hemispheroid cups (c). In a few fishes, as in Ceratodus, Xiphias, the cups are confluent into one cup, which possesses a foramen behind to allow the passage of the optic nerve (o). The cornea of Anableps shows an unique peculiarity. It is crossed by a dark horizontal stripe of the conjunctiva, dividing it into an upper and lower portion; also the iris is perforated by two pupils. This fish is observed to swim frequently with half of its head out of the water, and it is a fact that it can see out of the water as well as in it.
The membranes situated between the sclerotica and retina are collectively called choroidea, and three in number. The one in immediate contact with the sclerotic, and continued upon the iris, is by no means constantly present; it is the membrana argentea (a), and composed of microscopical crystals reflecting a silvery or sometimes golden lustre. The middle layer is the membrana vasculosa s. halleri (v), the chief seat of the ramifications of the choroid vessels; the innermost layer is the membrana ruyscheana or uvea (u), which is composed of hexagonal pigment-cells, usually of a deep brown or black colour.
In many Teleostei a rete mirabile surrounds the entry of the optic nerve; it is situated between the membrana argentea and vasculosa, and called the choroid gland (ch). It receives its arterial blood from the artery issuing from the pseudobranchia; the presence of a choroid gland always being combined with that of a pseudobranchia. Teleosteans without pseudobranchia lack a choroid gland. In the Palæichthyes, on the other hand, the pseudobranchia is present and a choroid gland absent.
The iris (i) is merely the continuation of the choroid membrane; its capability of contracting and expanding is much more limited than in higher Vertebrates. The pupil is generally round, sometimes horizontally or vertically elliptical, sometimes fringed. In the Rays and Pleuronectidæ a lobe descends from the upper margin of the pupil, and the outer integument overlying this lobe is coloured and non-transparent; a structure evidently preventing light from entering the eye from above.
In most Teleostei a fold of the Choroidea, called the Processus falciformis (f), extends from the vicinity of the entrance of the optic nerve to the lens. It seems to be constantly absent in Ganoids.
The retina (r) is the membrane into which the optic nerve penetrates, and in which its terminal filaments are distributed. It consists of several layers (Fig. [49]). The outermost is an extremely delicate membrane (a), followed by a layer of nerve-cells (b), from which the terminal filaments issue, passing through several granular strata (c, d, e), on which the innermost stratum rests. This stratum is composed of cylindrical rods (f) vertically arranged, between which twin fusiform corpuscles (g) are intercalated. This last layer is thickly covered with a dark pigment. The retina extends over a portion of the iris, and a well-defined raised rim runs along its anterior margin.
Fig. 49.—Vertical section of the Retina of the Perch, magn. X 350.
The vitreous humour (Fig. [48], h) which fills the posterior cavity of the eyeball, is of a firmer consistency than in the higher Vertebrates. The lens is spherical, or nearly so; firm, denser towards the centre, and lies in a hollow of the vitreous humour. When a falciform process is present, it is with one end attached to the lens, which is thus steadied in its position. It consists of concentric layers consisting of fibres, which in the nucleus of the body have marginal teeth, by which they are interlocked together. In Petromyzon this serrature is absent, or but faintly indicated.