The hyaloid coat, which is remarkably dis tinct in all the Cephalopods, completely sepa rates the vitreous humour from the internal white layer above described. It is perfectly transparent, z-tnd, -though thin, is strong. The vitreous humour does not lose its transparency when preserved in alcohol.
The crystalline lens is of large size, and is composed of two Completely separated portions : the anterior moiety is the segment of a larger sphere, but forms the smaller part of the lens ; the posterior is a segment of a smaller sphere, and forms the larger part of the lens. Two layers of transparent membrane are continued from the ciliary body between these segments. Each of the segments is composed, as in the lens of higher animals, of concentric lamin2e, which become denser towards the centre, where the nucleus resists further unravelling of its structure. It is of a brown colour, and pre serves its transparency in alcohol. The laminw are composed of denticulated fibres ; but the minute description of their texture and arrange ment will be given in another place.
The white substance (h ) which surrounds the optic ganglion is divided into lobes, but exhibits no distinguishable secerning structure ; the bloodvessels of' the eye ramify between these masses ; the smaller twigs accompany the nervous fibrils ; the larger ones pass forwards to the anterior soft margin of the sclerotica. We regard this substance as analogous to the so-called choroid gland in the eyes of Fishes. Cuvier assigns to it the function of defending the nervous ganglion and fibres from surround ing pressure ; and this is most probably the true final intention of the substance, since .it intervenes between the ganglion and the mus cles of the eye-ball.
Of these we find three straight muscles and one oblique. The illferior rectus of each eye arises from a small transverse tendon which adheres to the inferior and anterior border of the cranial cartilage, to which it runs parallel, and is attached at its two extremities to the muscles above mentioned, and also to the base or root of the anterior elongated cartilaginous orbital plate.
A second straight muscle arises from the posterior margin of the elongated cartilage above mentioned ; its fibres run parallel to those of the preceding, and are inserted into the external sclerotica. Both these muscles are thin, broad, and fleshy.
The oblique muscle arises from the inferior and posterior marain of the external orbital car tilage, and expands, as it proceeds outwards and forwards, to terminate in the external mem branous sclerotic. These muscles are readily
exposed by dissecting away the orbital capsule from the under part of the eye-ball.
A short and strong superior rectus the ten don of which is continuous with that of the opposite side, is inserted into the upper part of the sclerotic.
A few observations remain to.be made on the structures defending the anterior part of the eye ball. The cornea of the Cuttle-fish is appa rently entire ; it is thickest at its superior mar gin (t), where it is implanted in a groove of the integument ; it becomes gradually thinner towards the lower margin, where it is over lapped by the rudimental eyelid (v). This consists of a narrow semilunar fold of inte gument, the concavity of which is directed upwards and a little backwards.
In the small Cephalopod which Captain Ross discovered in the Arctic Ocean, and which has been named after that distinguished and scien tific navigator,'" the cornea is defended by a continuous circular fold of integument, which can be completely closed by an orbicular sphincter in front of the eye, a structure which is probably required in this species in order to protect the cornea against the spiculw of ice with which its native seas abound, especially in. the summer or thawing season. In the Calamary, on the other hand, there is no tegu mentary fold. Upon carefully inspecting the cornea of the Cuttle-fish, a minute foramen will be seen near the inner or anterior margin of the cornea, covered by the upper extremity ofthe fold ofintegument. The aperture leads ob liquely downwards and backwards, and if air be blown or fluid injected through it, the large cavity surrounding the anterior part of the eye ball will be distended, and the cornea ren dered convex. In the Poulp the corresponding aperture (o,fig. 216) is somewhat larger, and situated more in the axis of vision : its inferior and posterior margin is extended beneath the opposite margin, so as to form a semi-transpa rent curtain behind the external opening. In the common Calamary and the Onychoteuthis the corneal perforation is still larger, vertically oblong, and through it the capsule of the cry stalline lens, which projects through the scle rotic aperture, is immediately exposed to the external medium.