86 INTRODUCTION. 



it is in the development of the cerebral hemispheres, 

 the complexity and volume of the apparatus (espe- 

 cially of the corpus callosum), by which its several 

 parts are brought into communication, and the in- 

 creased extent given to the surface of the hemis- 

 pheres, by means of the convolutions, that the brain 

 of man rises above that of other mammalia."* The 

 size of the nerves, relatively to the brain, is very 

 various ; those animals with large jaws, and power- 

 ful muscles, having them considerably larger than 

 others, but only in proportion to the supply of ner- 

 vous energy required for their mode of life. 



The eyes of mammalia are invariably placed in 

 their orbits, and are preserved by two eyelids, and 

 in some with a kind of third, or nictitating mem- 

 brane. In the ear a cavity is always found called 

 the tympanum or ear-drum, which communicates 

 with the hinder part of the mouth by a canal, called 

 the Eustachian tube : this cavity is closed exter- 

 nally by a membrane, termed the membrana tympani, 

 and contains within it a chain of four bones, called 

 the malleus or hammer bone, the incus or anvil, the 

 os orbiculare or lenticular bone, and the stapes or 

 stirrup : the stapes rests upon a vestibulum or cen- 

 tral porch, which communicates with three semicir- 

 cular canals ; and, on the other side, to a cochlea or 

 spiral canal, communicating by one extremity with 



* Martin's Introduction to the Nat. Hist, of Quadrupeds, 

 p. 14, a work which, it is greatly to be regretted, that the 

 public has not supported adequately to its high merit. 



