444 



THE SENSORIAL FUNCTIONS. 



5,500th of an inch. The observation of another 

 optical phenomenon, of a still more delicate kind, 

 led Sir David Brewster to the further discovery 

 of the curious mode in which, (as is represented 

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



their edges by a series of teeth, resembling Ihose 

 of rack-work. He found the number of teeth in 

 each fibre to be 12,500 ; and as the whole lens con- 

 tains about 5,000,000 fibres, the total number of 

 these minute teeth must amount to 62,500,000,000.* 

 Some fishes, which frequent the depths of the 

 ocean, and are found at between three and four 

 hundred fathoms below the surface, to which it is 

 impossible that any sensible quantity of the light of 

 day can penetrate, have, like nocturnal quadrupeds, 

 very large eyes.t In a few species, which dwell 

 in the muddy banks of rivers, as the Ccecilia^ and 

 3Iiu'(cna ccEca, or blind eel, the eyes are quite rudi- 

 mental, and often nearly imperceptible; and in the 



* As far as his observations have extended, this denticulated 

 structure exists 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 Ornithorhynchus ; but he has not been able to tind it 

 in any of the Mammalia, not even in the Cefacea. (Phil. Trans, 

 for 1833, p. 323.) 



t See " Observations sur les Poissons recueillis dans un Voyage 

 aux lies Baleares et Pythiuses. Par M. Delaroche." 



