The membrane, of which we have just de scribed the reflections and extent, is regarded by Cuvier as analogous to the tunica conjunc tiva, but a difficulty arises in this mode of considering it, in consequence of the position of the cornea (o), which, in its structure and connection with the integument, bears a close analogy to the cornea in Fishes. The charac teristic difference which the cornea presents in the latter class, as compared with that of the Cephalopoda, is its adhesion to the margins of the anterior aperture of the sclerotica, by which the anterior, chamber of the eye is limited to a very small space ; while in the Sepia it would seem as if the membrane circum scribing the anterior chamber had over-passed its usual bounds in consequence of the absence of any such adhesion between the cornea and sclero tica. When we consider the nature of the membrane in question, .and the relations of the fluid it secretes to the cornea and crystal line, should we not be justified in considering it, notwithstanding its excessive development, as analogous rather to the membrane of the aqueous humour, than to the conjunctiva, the ratio of the development of which is as that of the eye-lids or folds of membrane ex ternal to the cornea, and of which we have only a slight rudiment in the Sepia? (v.) The space between the cartilaginous orbit and the posterior part of the eye is circum scribed by a membrane (e, e) which has the character rather of a condensed layer of cellular tissue than of a true serous membrane. In this space is contained the optic ganglion (f), its filaments (g), and the surrounding soft white substance (h), by some considered of an adi pose, by others of a glandular nature. This cavity is proportionally larger in the Octopus than in the Sepia.
The eye-ball of the Cuttle-fish is an irregular spheroid, flattened in the direction of its axis. The vertical diameter is less than the horizontal, but both exceed the diameter of the axis. The eye-ball is remarkable in all the Dibrauchiata for its considerable development as compared with the size of the body ; it is proportionally largest in the Calamaries, and smallest in the Octopods.
The exterior membrane covering the ante rior part of the eye-ball (i) receives the inser tions of the muscles of the eye, and seems as if it were formed by their aponeurotic expan sions ; it lies immediately beneath the reflected layer of the serous covering, is of a soft texture, and has a pinkish colour with a glistening silver lustre ; in the Poulp it is spotted like the skin. The entire eye-ball is surrounded by a second layer of membrane (k, k), having a similar texture and appearance ; these are analogous to the exterior or fibrous layers of the sclerotica in the eyes of Fishes. We next find a cartilaginous layer (/, /) corresponding to the internal cartilaginous sclerotica of the Pla giostomous Fishes. This coat is very thin, and almost membranous posteriorly, where the fibrils of the optic ganglion penetrate it, and where it presents a cribriform surface of consi derable extent, in which it may be observed that the orifices of the sieve are of consi derable size, and not very close together. Anterior to the cribriform surface the cartila ginous sclerotica increases in thickness, but more so on the lower than the upper side of the eye, and about the middle of the eye ball it terminates in a slightly thickened mar gin. A layer of fibrous membrane (m, m)
is continued from this margin, along with the external fibrous layer (/), and assists in forming the soft thick anterior part of the sclerotica, which forms the circumference of the pupillary aperture (n), or that by which light is admitted to the cavity of the eye. The supe rior part of this aperture is encroached upon by a bilobed curtain-like process, which we have observed to present a semi-transparent texture in the eyes of some Cuttle-fishes, as if it were an abortive formation of a sclerotic cornea: in position it resembles the _curtain-like process depending from the iris of the Ray.
The inner surface of that part of the sclero tica which lies anterior to the lens is lined with a dark pigment.
The tunic which immediately lines the car tilaginous sclerotic is not, as in Fishes, a membrana argentea, or a vascular choroid, but consists of an expansion of the ner vous fibres which are given off from the optic ganglion, connected together by a vascular and cellular tissue (o, o). The ganglion does not resolve itself into these fibres uniformly from the circumference to the centre, but sends them off from its exterior surface only, so that, on making a section of the part, the centre of the ganglion presents a homogeneous pulpy texture, separated by a distinct external layer from the origins of the fibrils, as in the figure, f.
The fibres, after perforating the cartilaginous sclerotica, and expanding into the post-pig mental retina, extend towards the groove of the crystalline, in a direction chiefly parallel to one another, the tunic formed by them be coming thinner as they advance forwards ; this is joined by a thin membrane, which extends from the anterior margin of the cartilaginous sclerotica, and forms, with that membrane, a ciliary plicated zone (p,p, where it is repre sented as left entire,) which penetrates the groove of the lens. The outer surface of this thick nervous tunic is fibrous and flocculent, and connected to the sclerotica by a fine cel lular tissue : the ant,erior or internal surface is perfectly smooth.
This surface of the nervous tunic is co vered by a tolerably consistent layer of a dark purple-brown pigment (q). Cuvier, who re gards the preceding tunic as the only part analogous to the retina in the eye of the Ce phalopods, expresses his surprise that this black layer is not an insurmountable obstacle to vision ;* and different theories have been proposed to account for the singular position of the pigment on that supposition. In the eyes of different Sepix which we had immersed in alcohol preparatory to dissection, we have, however, invariably found between the pig ment and the hyaloid coat a distinct layer of opaque livhite pulpy matter (r), of sufficient consistence to be detached in large flakes, and easily preserved and demonstrated in prepara tions. We confess, however, that we can discover no connection between this layer and the thick nervous expansion behind the pig ment ; but, nevertheless, we cannot but regard it as being composed of the fine pulpy matter of the optic nerve, and as constituting a true prx-piginental retina.