700 MAMMALIA. 



ferous car and that of the Bird are still more striking and de- 

 cided. 



The cavity of the tympanum in the class before us is very 

 extensive, and not unfrequently its extent is considerably enlarged 

 by the addition of capacious mastoid cells. By means of the 

 Eustachian tube it communicates freely with the throat. Upon 

 its inner wall it offers the fenestra ovalis and the fenestra rotunda, 

 closed by their respective membranes ; and externally is the mem- 

 brana tympani, the vibrations of which are to be conveyed to the 

 labyrinth. 



In Reptiles and Birds the communication between the drum of 

 the ear and the membrane of the fenestra ovalis was effected by 

 the interposition of a single ossicle, called the " columnella ;" but 

 in Mammals a chain of four ossicles, named respectively the mal- 

 leus, the incus, the os orbiculare, and the stapes, intervenes be- 

 tween the labyrinth and the membrana tympani : these ossicles, 

 both in their disposition and connections, are precisely similar to 

 those of Man, and, moreover, are acted upon by little muscles in 

 every respect comparable to those of the human subject. 



However remote the structure of the tympanic chain of ossicles 

 in the Mammal may appear to be from that of the simple co- 

 lumnella of the Bird, it is interesting to see how gradually the 

 transition is effected from one class to another even in this par- 

 ticular of their economy ; for in the Ornithorynchus, the Echidna, 

 and the Kangaroo, so bird-like is the form of the stapes, that it 

 might easily be mistaken for the ossicle" of one of the feathered 

 tribes,* and every intermediate shape is met with as we advance 

 from this point towards the stirrup-shaped bone of the most per- 

 fect quadrupeds. 



It is in the class under consideration, that for the first time an 

 external ear properly so called makes its appearance, for the fea- 

 thered appendages of the Owl or of the Bustard ( 684) are scarce- 

 ly entitled to such an appellation. In the Mammifera, however, 

 with a very few exceptions, such as the CETACEA, Moles, and the 

 Seal tribe, a moveable cartilaginous concha is appended to the 

 exterior of the head, adapted by its form and mobility to collect 

 the pulses of sound and convey them inwards towards the drum of 

 the ear. The basis of this external auricle is composed of fibro- 

 cartilage covered with a delicate skin, and its cavity is moulded 



* Vide Sir Anthony Carlisle, " on the Physiology of the Stapes" Phil. Trans, for 

 1805. 



