552 



CEPHALOPODA. 



as the margin of the thick posterior cartila- 

 ginous orbit, to which it is attached, and is 

 thence reflected forwards (c, c) upon the mus- 

 cles of the eye-ball, also upon the long narrow 

 anterior and inferior ocular cartilage (d, d), and 

 upon the exterior fibrous layer of the sclero- 

 tica; it is reflected inwards over the anterior 

 thickened margin of the sclerotica, where the 

 large anterior aperture of that membrane re- 

 mains unclosed by the cornea, and consequently 

 passes along its inner surface like the mem- 

 brane of the aqueous humour; it seems to us, 

 however, not to pass over the anterior part of 

 the capsule of the crystalline lens, but into the 

 groove (p,p) which divides that body into two 

 parts. The serous layer above described can- 

 not be detached from the cornea, but ceases to 

 be demonstrable as a distinct membrane where 

 the external fibrous coat is attached to the 

 cornea. The space between the eye-ball and 

 its capsule, which is thus circumscribed, is 

 filled with a watery fluid, which is most abun- 

 dant in the Calamaries. The cornea is sepa- 

 rated by the same fluid from the eye-ball ; but 

 its tension and slightly convex figure is main- 

 tained by it, as by the aqueous humour in the 

 eye of the vertebrate animal. The motions of 

 the eye-ball are facilitated by the secretion of 

 the serous sac, as the movements of the heart 

 in the pericardium, and in other instances in 

 which serous membranes are developed. 



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 (_/), 

 its filaments (g), and the surrounding soft white 

 substance (A), 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 Dibranchiuta 

 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 (/c, /c), 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 (Z, /) 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 (w), 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. 



