OSSEOUS SYSTEM OF MAMMALIA. 



45 



process, even where present, but is often so degenerate as not to be dis- 

 tinguished ; while, to compensate for its loss, a large tympanic bulla, or 

 cavity, convex externally, thin and hollow, is superadded : this bulla 

 being, in fact, an enlargement of the tympanum, for the reception of a 

 greater volume of air to be acted upon by vibrations of the atmosphere, 

 and thus to fulfil more thoroughly the office of a drum. The tympanic bulla 

 is beautifully developed in the Rat, and numerous rodents, in the Cat, 

 Dog, and other Carnivora. Fig. 24 represents the mastoid process in 

 Man, and figs. 25 and 26 the tympanic bulla in the Cat. 



24. a, Mastoid process of the human skull . 25 a, Basal view of the tympanic bulla of the Cat. 26._<j, A nearly 



lateral view of the same. 



The form of the concha, or external ear, differs remarkably in different 

 Mammalia, and in all (where present) it is far more moveable than in Man, 

 or the Simise, which, in the form of the ear, resemble him the nearest. 

 The use of this part, in collecting and condensing the undulations of the 

 atmosphere, is evident ; hence we observe, that the Ox, or Horse, while 

 listening, turns the ears in different directions, the better to receive the 

 atmospheric vibrations flowing from various quarters. In the more timid 

 and defenceless of the Mammalia, the external ear is often large, and 

 the sense of hearing extremely acute : we find this to be the case with 

 the Hare, the Chinchilla, and many other rodents, and with the Deer 

 and Antelope. In aquatic Mammalia, as the Whale, Dolphin, &c., there 

 are no external ears, and the passage leading to the tympanum is a carti- 

 laginous tube, narrow and winding, with a minute external aperture, 

 capable of being firmly closed a structure evidently intended as a pre- 

 ventive to the entrance of water. In the Seal this passage makes a cir- 



