are the same as those of muscular fibre. As its 

 operation also resembles that of these organs, it 

 seems more than probable, that it ought to be con- 

 sidered as one of the muscles of the body, and in 

 man, as one of those, which.are not under the con* 

 trol of the will; although, on the other hand> its 

 motion in several birds (for instance, the genera 

 strix and psitacus,) seems to depend on the will. 



The humours which fill the globe of the eye 

 are three, viz* the aqueous humour, the crystaline 

 lens, and the vitreous humour. The first and 

 the last of these humours are very thin, and agree 

 in their composition altogether with the fluid of 

 the serous membranes ; but they are distinguish- 

 ed from them in this, that they are completely 

 void of colour, and the aqueous humour seems to 

 contain a smaller quantity of albumen. The 

 crystaline lens, on the contrary, is very remark- 

 able, both as to its texture and its composition* 

 Its density is least of all at the surface, but con* 

 tinues to increase towards the interior ; and CHE- 

 NEVIX found, by removing the coats or layers 

 successively, that its specific gravity increased, in 

 proportion as he approached the centre, When 

 the process had been performed on a lens weigh- 

 ing 30 grains, with a specific gravity of 1,076, 



G 4 



