230 PHYSIOLOGY. 



The handle of the mallet is attached to the membrane of the 

 drum, and its other extremity rests on the anvil, which is 

 connected with the orbicular bone, the smallest in the body, 

 and not much larger than a grain of sand. This is inter- 

 posed between the round bone and the stapes, whose base 

 rests upon the membrane of the oval fenestra. Now the 

 contraction of the muscles which connect these bones, puts 

 the drum-head on the stretch, and thus adapts it for the ready 

 transmission of sound.? 



9. Mastoid Cells. These are seated in the hard portion 

 of the temporal bone, and are also filled with air. The ear 

 is thus surrounded with an atmosphere of its own. In the 

 elephant, the two tables of the skull are separated from each 

 other by a bony cellular structure to the extent of upwards 

 of a foot, and these cells are filled with air which communi- 

 cate with the drum of the ear. We find in the cat tribe, in 

 dogs, and in gnawing animals, that there is a hollow sphere 

 of very hard bone attached to the drum, shaped something 

 like a conch, and well adapted for reflecting the vibrations 

 of sound, and rendering them more intense, like the sound- 

 ing board of a piano-forte. A similar hard conch is found 

 in the whale tribe. The two tables of the skull in birds are 

 also widely separated, thus rendering them lighter, and as 

 the cells communicate with the drum, doubtless for the pur- 

 pose of increasing the volume of air for the reception of 

 vibration. 



10. Petrous Portion. The internal ear is situated in a 

 portion of the temporal bone, called petrous, from petra, a 

 rock, because of its solid structure. The object of this is 

 evident from the fact, that hard elastic substances transmit 

 vibrations of sound far better than matter of a softer texture. 

 This portion of the temporal possesses indeed almost an 

 ivory hardness, being the densest structure in the animal 

 body, next to the enamel of the teeth ; and in this solid body 

 the labyrinth of the ear is situated. Now in whales, the 

 skeleton is formed of loose spongy bones, with a considerable 



