VISION. 443 



mersed, requires no provision of a secreted fluid 

 for that purpose ; and there are consequently 

 neither lacrymal apparatus, nor proper eye-lids ; 

 the integuments supplying only a thin transparent 

 membrane, which passes over and protects the 

 cornea, serving the office of a conjunctiva. The 

 eye retains its form by the support it receives from 

 the sclerotic coat, which is of extraordinary thick- 

 ness and density. In the Shark and the Skate the 

 eye is supported from the bottom of the orbit, by a 

 cartilaginous pedicle, which enables it to turn as on 

 a pivot, or lever. 



Sir David Brewster has recently made an in- 

 teresting analysis of the structure of the crystalline 

 lens of the Cod, to which he was led by noticing 

 some remarkable optical appearances presented 

 by thin layers of this substance when transmitting 

 polarised light. He found that the hard central 

 portion is composed of a succession of concentric, 

 and perfectly transparent, spheroidal laminae, the 

 surfaces of which, though apparently smooth, have 

 the same kind of iridescence as mother-of-pearl, 

 and arising from the same cause ; namely, the 

 occurrence of regularly arranged lines, or stricB* 

 These lines, which mark the edges of the separate 

 fibres composing each lamina, converge like meri- 

 dians from the equator to the two poles of the 

 spheroid, as is shown in Fig. 431. The fibres 

 themselves are not cylindrical, but flat ; and tliey 

 taper at each end, as they approach the points of 

 convergence. The breadth of the fibres in the 

 most external layer, at the equator, is about the 



* See vol. i. p. 208. 



