350 



THE SENSORIAL FUNCTIONS. 



or striae.* These lines, which mark the edges of the sepa- 

 rate fibres, composing each lamina, converge like meridians 

 from the equator to the two poles of the spheroid, as is 

 shown in Fig. 431. The fibres themselves are not cylindri- 



431 



cal, but flat; and they 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 5,500th part 

 of an inch. The observation of another optical phenomenon, 

 of a still more delicate kind, led Sir David Brewster to the 

 farther discovery of the curious mode in which, (as is re- 

 presented in Fig. 432,) the fibres are locked together at 

 their edges by a series of teeth, resembling those of rack- 

 work. He found the number of teeth in each fibre to be 

 12,500; and, as the whole lens contains about 5,000,000 

 fibres, the total number of these minute teeth amounts to 

 62,500,000,000. t 



Some fishes, which frequent the depths of the ocean, be- 

 ing found at between three and four hundred fathoms below 

 the surface, to which it is impossible that any sensible quan- 

 tity of the light of day can penetrate, have, like nocturnal 

 quadrupeds, very large eyes.J In a few species, which 



* See vol. i. p. 169. * 



f As far as his observations have extended, this denticulated structure ex- 

 ists in the lenses of all kinds of fishes, and likewise in those of birds. He has 

 also met with it in two species of Lizards, and in the Ornithorhyncus; but 

 he has not been able to find it in any of the Mammalia^ not even in the Ce- 

 tacea. (Phil. Trans, for 1833, p. 323.) 



t See " Observations sur les Poissons recueillis dans un Voyage aux lies 

 Baleares et Pythiuses. Par M. Delaroche." 



