594 CEPHALOPODA. 



different Sepice which we had immersed in alcohol preparatory to dis- 

 section, we have, however, invariably found, between the pigment and 

 the hyaloid coat, a distinct layer of opake white pulpy matter, of suffi- 

 cient consistence to be detached in large flakes and easily preserved 

 and demonstrated in preparations. We confess, however, that we can 

 discover no connexion between this layer and the thick nervous expan- 

 sion behind the pigment ; but nevertheless we cannot but regard it as 

 being composed of the fine pulpy matter of the optic nerve, and as con- 

 stituting a true pra3-pigmental retina*." 



(1592.) 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 Yertebrata, is of short 

 focus and great power, being, in fact, not merely, as it is generally de- 

 scribed, a double convex lens, which is the usual shape of this im- 

 portant 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 opake matter. The 

 lens of the Cuttle-fish is in like manner divided into two parts of un- 

 equal size (fig. 292, 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 pene- 

 trates deeply into the vitreous humour : the latter, enclosed in a deli- 

 cate hyaloid membrane, fills up, as in Man, the posterior part of the 

 eyeball ; while the small space that intervenes between the posterior 

 surface of the crystalline and the back of the ocular chamber suffi- 

 ciently attests the shortness of the focus of so powerful a lens. 



(1593.) 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 the great 

 ganglion of the optic nerve (fc), imbedded in a mass of soft white sub- 

 stance. This supplementary chamber is formed by a separation of the 

 sclerotic into two layers, of which one, already described (i), forms the 

 posterior boundary of the eyeball, while the other (h), passing back- 

 wards, circumscribes the cavity in question. On entering the compart- 

 ment thus formed, the optic nerve (q) dilates into a large reniform 

 ganglion (k\ almost equal in size to the brain itself ; and from the peri- 

 * Cyclopaedia of Anatomy and Physiology, art. CEPHALOPODA. 



