CEPHALOPODA. 469 



the great ganglion of the optic nerve (&) imbedded in a mass of 

 soft white substance. This supplementary chamber is formed by 

 a separation of the sclerotic into two layers ; of which one, already 



Fig. 214. 



described ('), forms the posterior boundary of the eye-ball, while 

 the other (h) passing backwards circumscribes the cavity in ques- 

 tion. On entering the compartment thus formed the optic nerve 

 (/>) dilates into a large reniform ganglion, almost equal in size to the 

 brain itself ; and from the periphery of the optic ganglion arise the 



