THE VERTEBRATED ANIMALS. 67 



which indeed constitutes the marrow, and which, 

 after having crossed its filaments, spreads out in 

 order to form the different tubercles which compose 

 the brain, and to terminate in two medullary ca- 

 vities, called hemispheres, the volume of which is 

 usually proportioned to the extent of intellectual 

 capacity. 



There are always two jaws: the principal mo- 

 tion exists in the lower, which has the power of 

 elevation or depression. The upper jaw is some- 

 times completely fixed and motionless. Both are 

 generally provided with teeth, excrescences of a 

 peculiar nature, very similar to the character of 

 the bones, as far as the chemical composition is 

 concerned, but which grow from certain beds, and 

 by means of the process of transudation. The jaws 

 of one entire class, however, (that of birds) are in- 

 vested with a horny substance, and the genus of 

 the tortoise in the class of reptiles, is in a similar 

 situation. 



The intestinal canal extends from the mouth to 

 the anus in various degrees of expansion or con- 

 traction, possessing certain appendices, and re- 

 ceiving liquids of a solvent nature, some of which, 

 being poured into the mouth, are-' 'called saliva; 

 others, which enter the intestines only, have several 

 names. The two principal are the secretion of the 

 gland denominated pancreas ; and the bile, which is 

 produced by another considerable gland called the 

 liver. 



During the passage of the food through the ali- 



F 2 



