(1587). The eyes of the Dibranchiate Cephalopoda are not less remarkable in their construction than those of the Nautilus, and from their greater complexity will require a more elaborate description. In order to simplify the details connected with this portion of our subject as much as possible, we shall describe separately, as forming distinct parts of the ocular apparatus met with in the common Cuttle-fish (Sepia officinalis), first, the orbit; secondly, the globe of the eye; thirdly, the chamber of the optic ganglion; and fourthly, the muscles of the visual organ.

(1588). The orbit differs from that of all other classes of animals, inasmuch as it is a cavity circumscribed on all sides and covering even the front of the eye*. The bottom of the orbital cavity is cartilaginous, being partially formed by a process derived from the cranial cartilage; but elsewhere it is made up of the common fleshy integument of the body (fig. 292, d d, e): becoming gradually attenuated, the skin (b) passes over the anterior portion of the eye, where, being transparent (f), it represents the cornea, although it has no connexion with the eye-ball itself. Beneath the cornea the integument again becomes opake, and forms a thickened fold (a), which might be considered as the rudiment of an under eyelid. The orbit therefore forms a complete capsule, enclosing the whole of the apparatus of vision.

(1589). The globe of the eye fills up the anterior part of the orbital chamber, and is remarkable from having no cornea properly so called; so that, on raising the transparent skin (f) which forms the exterior wall of the orbit and supplies the place of the cornea, the prominent surface of the crystalline lens (0) is found quite naked beneath it, neither an aqueous humour, nor an iris properly so called, being present. The outer coat of the eye (g g, i) represents the sclerotic tunic in Man: it is tough, fibrous, and of a silvery lustre; perforated anteriorly by a large round aperture, representing that which contains the cornea in the human eye, and pierced posteriorly by numerous foramina, through which the multitudinous branches derived from the optic ganglion (k) enter.

* Descriptive and Illustrated Catalogue of the Physiological Series of Comparative Anatomy contained in the Museum of the Royal College of Surgeons of England, vol. iii. part 1. pi. 52.

Anatomy of the eye of the Cuttle fish. (After Cuvier.)

Fig. 292. Anatomy of the eye of the Cuttle-fish. (After Cuvier).

(1590). The second tunic is usually regarded as the retina, occupying a singular situation and presenting a very anomalous structure. No choroid intervenes between this retina and the sclerotic, as is the case in the eye of Man; but numerous nervous branches given off from the optic ganglion (k), having penetrated into the interior of the eye through the cribriform sclerotic, immediately expand into a thick nervous membrane which lines the sclerotic tunic, and is continued forward to a deep groove in the substance of the crystalline lens, wherein it is implanted, so as to form a kind of ciliary zone (m), which is slightly plicated, and obviously assists in keeping the lens in situ.

(1591). Between the retina and the vitreous humour is interposed a thick layer of black pigment, which, being thus strangely situated, has very naturally puzzled all physiological inquirers, inasmuch as it would apparently form an insurmountable barrier between the rays of light and the retinal membrane. The researches of Professor Owen would seem, however, to have removed the difficulty presented by this hitherto incomprehensible and anomalous arrangement, as he has succeeded in discovering, in addition to the thick post-pigmental nervous expansion, a delicate lamella in front of the pigmentum nigrum, correspondent, in position at least, with the retina of vertebrate animals. "In the eyes of different Sepiae which we had immersed in alcohol preparatory to dissection, we have, however, invariably found, between the pigment and the hyaloid coat, a distinct layer of opake white pulpy matter, of sufficient consistence to be detached in large flakes and easily preserved and demonstrated in preparations.

We confess, however, that we can discover no connexion between this layer and the thick nervous expansion behind the pigment; but nevertheless we cannot but regard it as being composed of the fine pulpy matter of the optic nerve, and as constituting a true prae-pigmental retina*".

(1592). It has been already stated that there are no chambers of aqueous humour; and we are but little surprised that, in animals destined to see objects contained in water, the existence of a refracting medium scarcely at all differing in density from the surrounding element should be dispensed with. To compensate, however, for this deficiency, the crystalline, as is the case in all the aquatic Vertebrata, is of short focus and great power, being, in fact, not merely, as it is generally described, a double convex lens, which is the usual shape of this important piece of the optic apparatus, but exhibiting that form of a simple magnifier most approved of by opticians as being best adapted to ensure a large field of view. Whoever is conversant with the principles upon which the well-known "Coddington lens" is constructed, will have little difficulty in appreciating the advantages derived by introducing a precisely similar instrument in the eye of the Cuttle-fish. The Coddington lens is a sphere of glass divided into two portions by a deeply-cut circular groove, which is filled up with opake matter.

The lens of the Cuttle-fish is in like manner divided into two parts of unequal size (fig. 292, o o) by a circular indentation, wherein the post-pigmental retina, with its coat of dark varnish (m), is fixed, and thus a picture of the most perfect character is ensured. The crystalline penetrates deeply into the vitreous humour: the latter, enclosed in a delicate hyaloid membrane, fills up, as in Man, the posterior part of the eyeball; while the small space that intervenes between the posterior surface of the crystalline and the back of the ocular chamber sufficiently attests the shortness of the focus of so powerful a lens.