Mr. J. Lockhart Clarke on the Optic Lobes of the Cuttle-fish, 59 

 PROCEEDINGS OF LEARNED SOCIETIES. 



ROYAL SOCIETY. 



June 21, 1866. — Lieut- General Sabine, President, in the Chair. 



" On the Structure of the Optic Lobes of the Cuttle-fish." By 

 J. Lockhart Clarke, F.R.S. 



The brain of the Cuttle-fish consists of several ganglia closely ag- 

 gregated around the upper part of the oesophagus. The foremost 

 or pharyngeal ganglion, which is much the smallest, is bilobed and 

 somewhat quadrangular. The next is a large bilobed ganglion which 

 forms the roof of the canal for the oesophagus. Beneath the oeso- 

 phagus is another large and broad mass, which is connected on each 

 side with the supra- oesophageal masses by bands that complete the 

 oesophageal ring. 



From each side of the cephalic masses springs a thick optic pe- 

 duncle which ends in the optic lobe. Each optic lobe is larger than 

 all the other cerebral masses taken together, and has a striking re- 

 semblance in shape to the human kidney. It is completely enve- 

 loped in a thick layer of optic nerves disposed in flattened bands 

 which issue from all parts of its substance and proceed to the back 

 of the eye in a fan-Hke expansion, the upper and lower bands 

 crossing each other in their course. The substance of each lobe 

 consists of two distinct portions, which differ from each other 

 entirely in appearance. The outer portion resembles a very thin 

 rind or shell, is extremely delicate, and very easily torn from the 

 central substance which it encloses. It consists of three concentric 

 layers — an external dark layer, an internal dark layer, and a middle 

 pale and broader layer containing thin and concentric bands of 

 fibres. 



The first or outer layer consists of a multitude of nuclei and a few 

 small nucleated cells, with which filaments of the optic nerves are 

 connected. The second or middle layer is composed entirely of fine 

 nerve-fibres which form two sets — one vertical, and the other hori- 

 zontal. The vertical fibres issue at the under surface of the first 

 layer from the network which its nuclei form with the fibres of the 

 optic nerves. Some are continuous with the horizontal fibres, but 

 the majority continue downward across them to the third or inner 

 layer. At the junction of these two layers is a row of nucleated 

 cells which send thin processes in difl^erent directions, and with 

 which some of the nerve-fibres are connected. The third or inner 

 layer is composed entirely of closely aggregated nuclei, which are 

 joined together in a network by the fibres which issue from the under 

 surface of the middle layer. 



The cortical substance, consisting of these three layers, forms only 

 a very small portion of the optic lobe. Out of the nuclear network 

 of the inner layer fine nerve-fibres descend into the body of the lobe 

 which it encloses. At first these fibres are vertical, parallel, and 

 arranged in uniform series, with scattered nuclei between them ; but 



