ORGANS OF THE SENSES. 263 



magnitude of the optic ganglion (Fig. 93. C. c), which is 

 greater than the brain (93. C. a), and from the glandu- 

 lar masses (93. C. e, e,) contained within the outer layer 

 of the choroid and the thickened posterior portion of 

 the sclerotic (93. C. b), behind the retina and the inner layer 

 of the choroid as in the eyes of fishes. But a small part 

 therefore of these large organs is occupied with the trans- 

 parent optical apparatus, and their movements are nearly as 

 limited as those of a murex or a buccinum. There is a rudi- 

 ment of the membrana nectitans, or third eye-lid so gene- 

 rally developed in the vertebrated classes^ and the eye-lids, 

 like the iris, are still almost motionless. The ciliary pro- 

 cesses, as in the higher cartilaginous fishes, are remarkable 

 for their great development, and the crystalline lens for its 

 double structure and its great posterior convexity; it occu- 

 pies two thirds of the axal diameter of the eye in most of the 

 naked cephalopods, and consists of two plano-convex hemi- 

 spheres of unequal size applied to each other by their flat 

 surfaces. The vitreous humour, of a thin fluid consistence, 

 is enclosed in a distinct cellular hyaloid membrane, and the 

 soft loose pigment of the inner layer of the choroid is gene- 

 rally of a deep purple colour. In the loligo there is a small 

 aperture on the surface leading to the glandular cavity of the 

 eye like a lachrymal pore. The retina of the cephalopods is 

 placed, as in other animals, within or anterior to the pigment 

 and the inner layer of the choroid, as shown long since by 

 Chiaje, and the crystalline lens is composed of concentric 

 layers of minute transparent fibres, like that of vertebrated 

 animals. Thus in the isolated organs of vision of the mollus- 

 cous classes we constantly observe a crystalline lens and 

 transparent parts anterior to the optic nerves as in the 

 higher articulata; and in the general plan of their formation 

 and the higher complexity of their structure they form a 

 nearer approach to the ordinary condition of these organs in 

 fishes and higher vertebrated classes. 



The eyes are two in number and symmetrically disposed 

 on the sides of the head in all the vertebrated classes and 

 nearly in all the species, and the differences which they pre- 

 sent relate chiefly to the density of the media through which 

 the various animals receive the rays of light, and the extent of 

 development of the external protecting parts of these delicate 



