chap, xii.] EYES OF CHORD ATA. 451 



is to be found in the dibranchiate Cephalopoda, 



and it is remarkable for being protected by a cartila- 

 ginous orbit ; the sides of the eye are protected by a 

 hard layer which has been called the sclerotic; 

 this, which, in front, passes into the transparent cor- 

 nea, is either entire, or perforated in its centre by 

 a more or less large aperture. Beneath this is a 

 chamber which is not so small as in the vertebrate 

 eye, and which sends down a narrow process on either 

 side. The hinder part of this chamber is, in the long 

 axis of the eye, occupied by the lens, which is bounded 

 on either side by the iris; the hinder part of the 

 lens projects into the hinder or inner optic chamber, 

 the posterior wall of which is formed by the retina. 

 In this retina, as in those of nearly all Invertebrates, 

 the sensitive portion or layer of rods is turned 

 towards the impinging rays of light, and the con- 

 nective elements are posterior to it. We shall shortly 

 see that the reverse of this arrangement obtains among 

 Vertebrates. 



The eyes of the Vertebrata are constantly paired, 

 and lie, as an ordinary rule, on either side of the more 

 anterior portion of the head ; they are always divisible 

 into two portions, an anterior and a posterior cham- 

 ber, and the hind wall of the latter, far away as it 

 lies from the surface of the body, is the percipient 

 portion of the optic organ, and has an epiblastic 

 origin. In comparison with any other fact as to the 

 vertebrate eye, this one fact stands out pre-eminently, 

 and first deserves our attention. 



We have already learnt that in the Chordata the 

 central nervous system arises as an infolding of the 

 epiblasfc, which gradually becomes separated from the 

 surface of the body ; as we know, the result of this 

 infolding is to reverse the relations of the outer and 

 inner strata of the epiblast, or, so to speak, to turn 

 them inside out (Fig. 192; A, B). The nervous tube 



