CEPHALOPODA. 



467 



Fig. 213. 



prominent surface of the crystalline lens (o) is found quite naked 

 beneath it ; neither an aqueous humour, nor an iris properly so called, 

 being present. The outer coat of the eye (g, g) represents the scle- 

 rotic tunic in man: 

 it is tough, fibrous, 

 and of a silvery 

 lustre ; perforated 

 anteriorly by a 

 large round aper- 

 ture representing 

 that which con- 

 tains the cornea 

 in the human eye, 

 and pierced pos- 

 teriorly by nu- 

 merous foramina, 

 through which 

 the multitudinous 

 branches derived 

 from the optic ganglion (k) enter. 



The second tunic is usually regarded as the retina, occupying 

 a singular situation and presenting a very anomalous structure. 

 No choroid intervenes between this retina and the sclerotic, as is 

 the case in the eye of man ; but numerous nervous branches given 

 from the optic ganglion (&), having penetrated into the interior of 

 the eye through the cribriform sclerotic, immediately expand into a 

 thick nervous membrane which lines the sclerotic tunic, and is 

 continued forward to a deep groove in the substance of the crystal- 

 line lens, wherein it is implanted so as to form a kind of ciliary zone 

 (m), which is slightly plicated, and obviously assists in keeping the 

 lens in situ. 



Between the retina and the vitreous humour is interposed a thick 

 layer of black pigment, which, being thus strangely situated, has 

 very naturally puzzled all physiological inquirers, inasmuch as it 

 would apparently form an insurmountable barrier between the rays 

 of light and the retinal membrane. The researches of Professor 

 Owen would seem, however, to have removed the difficulty pre- 

 sented by this hitherto incomprehensible and anomalous arrange- 

 ment ; as he has succeeded in discovering, in addition to the thick 

 post-pigmental nervous expansion, a delicate lamella in front of the 

 pigmentum nigrum, correspondent in position at least with the 



