CEPHALOPODA. 



553 



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 anterior 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 (</). Cuvier, who re- 

 gards the preceding tunic as the only part 

 analogous to the retina in the eye of the Ce- 

 ^halopods, 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 Sepia 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 white 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 

 prae-pigmental retina. 



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, and, 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 laminae, 

 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 laminae 

 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 



" On ne con9oit pas comment elle n'est pas un 

 obstacle insurmontable a la vision." Mem. sur le 

 Poulpe, p. 39. 

 VOL. I. 



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 inferior rcctus 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 margin 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 spiculae 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 

 of the fold of integument. The aperture leads ob- 



* See Appendix to Sir John Ross's Voyage, 4to. 

 p. xii. pi. B. C. 



2 o 



