CEPHALOPODA. 5o3 



The fibres, after perforating the cartilaginous exhibits no distinguishable secerning structure ; 



sclerotica, and expanding into the post-pig- the bloodvessels of the eye ramify between 



mental retina, extend towards the groove of the these masses; the smaller twigs accompany the 



crystalline, in a direction chiefly parallel to nervous fibrils ; the larger ones pass forwards 



one another, the tunic formed by them be- to the anterior soft margin of the sclerotica. 



coming thinner as they advance forwards ; this We regard this substance as analogous to the 



is joined by a thin membrane, which extends so-called choroid gland in the eyes of Fishes, 



from the anterior margin of the cartilaginous Cuvier assigns to it the function of defending 



sclerotica, and forms, with that membrane, a the nervous ganglion and fibres from surround- 



ciliary plicated zone (p, p, where it is repre- ing pressure ; and this is most probably the 



sented as left entire,) which penetrates the true final intention of the substance, since it 



groove of the lens. The outer surface of this intervenes between the ganglion and the mus- 



thick nervous tunic is fibrous and fiocculent, cles of the eye-ball. 



and connected to the sclerotica by a fine eel- Of these we find three straight muscles and 



lular tissue : the anterior or internal surface is one oblique. The inferior rectus of each eye 



perfectly smooth. arises from a small transverse tendon which 



This surface of the nervous tunic is co- adheres to the inferior and anterior border of 



vered by a tolerably consistent layer of a dark the cranial cartilage, to which it runs parallel, 



purple-brown pigment (g}. Cuvier, who re- and is attached at its two extremities to the 



gards the preceding tunic as the only part muscles above mentioned, and also to the base 



analogous to the retina in the eye of the Ce- or root of the anterior elongated cartilaginous 



phalopods, expresses his surprise that this black orbital plate. 



layer is not an insurmountable obstacle to A second straight muscle arises from the 



vision;* and different theories have been posterior margin of the elongated cartilage 



proposed to account for the singular position above mentioned ; its fibres run parallel to 



of the pigment on that supposition. In the those of the preceding, and are inserted into 



eyes of different Sepue which we had immersed the external sclerotica. Both these muscles are 



in alcohol preparatory to dissection, we have, thin, broad, and fleshy. 



however, invariably found between the pig- The oblique muscle arises from the inferior 

 ment and the hyaloid coat a distinct layer of and posterior margin of the external orbital car- 

 opaque white pulpy matter (r), of sufficient tilage, and expands, as it proceeds outwards 

 consistence to be detached in large flakes, and and forwards, to terminate in the external mem- 

 easily preserved and demonstrated in prepara- branous sclerotic. These muscles are readily 

 tions. We confess, however, that we can exposed by dissecting away the orbital capsule 

 discover no connection between this layer and from the under part of the eye-ball, 

 the thick nervous expansion behind the pig- A short and strong superior rectus, the ten- 

 men t ; but, nevertheless, we cannot but regard don of which is continuous with that of the 

 it as being composed of the fine pulpy matter opposite side, is inserted into the upper part of 

 of the optic nerve, and as constituting a true the sclerotic, 

 prae-pigmental retina. A few observations remain to be made on the 



The hyaloid coat, which is remarkably dis- structures defending the anterior part of the eye- 

 tinct in all the Cephalopods, completely sepa- ball. The cornea of the Cuttle-fish is appa- 

 rates the vitreous humour from the internal rently entire ; it is thickest at its superior mar- 

 white layer above described. It is perfectly gin (t) t where it is implanted in a groove of 

 transparent, and, though thin, is strong. The the integument ; it becomes gradually thinner 

 vitreous humour does not lose its transparency towards the lower margin, where it is over- 

 when preserved in alcohol. lapped by the rudimental eyelid (v). This 



The crystalline lens is of large size, and is consists of a narrow semilunar fold of inte- 



composed of two completely separated portions : gument, the concavity of which is directed 



the anterior moiety is the segment of a larger upwards and a little backwards, 



sphere, but forms the smaller part of the lens ; In the small Cephalopod which Captain Ross 



the posterior is a segment of a smaller sphere, discovered in the Arctic Ocean, and which has 



and forms the larger part of the lens. Two been named after that distinguished and scien- 



layers of transparent membrane are continued tific navigator,* the cornea is defended by a 



from the ciliary body between these segments, continuous circular fold of integument, which 



Each of the segments is composed, as in the can be completely closed by an orbicular 



lens of higher animals, of concentric laminae, sphincter in front of the eye, a structure which 



which become denser towards the centre, where is probably required in this species in order to 



the nucleus resists further unravelling of its protect the cornea against the spiculae of ice 



structure. It is of a brown colour, and pre- with which its native seas abound, especially 



serves its transparency in alcohol. The laminae in the summer or thawing season. In the 



are composed of denticulated fibres; but the Calamary, on the other hand, there is no tegu- 



minute description of their texture and arrange- mentary fold. Upon carefully inspecting the 



ment will be given in another place. cornea of the Cuttle-fish, a minute foramen 



The white substance (h) which surrounds will be seen near the inner or anterior margin 



the optic ganglion is divided into lobes, but of the cornea, covered by the upper extremity 



of the fold of integument. The aperture leads ob- 

 Un ne cor^oit pas comment elle n est pas un 



obstacle insurmomable a la vision." Mem. sur le * See Appendix to Sir John Ross's Voyage, 4to. 



Poulpe, p. 39. p. xii. pi. B. c. 



VOL. r. 2 o 



