468 



CEPHALOPODA. 



retina of vertebrate animals. " In the eyes of different Sepice 

 which we had immersed in alcohol preparatory to dissection, we 

 have, however, invariably found between the pigment and the hya- 

 loid coat a distinct layer of opaque 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 connection 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 pree-pigmental retina." * 



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 intro- 

 ducing 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 opaque 

 matter. The lens of the Cuttle-fish is in like manner divided into 

 two parts of unequal size (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 eye-ball ; 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. 



(510.) The posterior portion of the orbital capsule is occupied 

 by a large cavity quite distinct from the globe of the eye, although 

 its walls are derivations from the sclerotic tunic, wherein is lodged 



* Cyclopaedia of Anatomy and Physiology, art. CEPHALOPODA. 



