814 MAMMALIA. 



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 perfect quadrupeds. 



(2395.) 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 ( 2089) are scarcely 

 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 into various sinuosities, so disposed, no 

 doubt, as to concentrate sonorous impressions. In Man, as the ana- 

 tomist is aware, numerous small muscles act upon the auricular carti- 

 lages ; but in quadrupeds possessed of moveable ears the number and 

 size of these muscles are prodigiously increased, and the ears are thus 

 directed with facility in any required direction. 



(2396.) More minutely to describe the structure of the auditory 

 apparatus in the Mammiferous class would be foreign to our present 

 purpose ; nevertheless we must not omit to notice one most remarkable 

 provision whereby the Whales, strangely circumstanced as those crea- 

 tures are, are permitted to hear either through the medium of the air 

 they breathe, or of the water in which they pass their lives. The reader 

 will at once appreciate the difficulties of the case : the ear of a fish, 

 without any external communication, although best adapted to receive 

 the stunning concussions conveyed through the denser element, could 

 never appreciate the more delicate vibrations of the air ; and the ordi- 

 nary Mammiferous ear would be perpetually deafened by the thundering 

 of the water. How is the Whale to hear what is going on in either the 

 sea or the atmosphere ? 



(2397.) The plan adopted is simple and efficacious : The external 

 meatus of the ear is reduced to the smallest possible diameter, the canal 

 being barely wide enough to admit a small probe : this is the hydro- 

 phonic apparatus, and is all that is exposed for the reception of aquatic 

 sounds. The Eustachian tube, on the contrary, is very large, and opens 

 into the blow-hole through which the Whale respires atmospheric air : 

 if, therefore, the Cetacean comes to the top of the water to breathe, it 

 is the Eustachian tube that conveys aerial sounds to the ear. And thus 

 it hears sufficiently under both conditions. 



(2398.) So far, as the student will have perceived, the different 

 portions of the encephalon to which we have adverted correspond most 



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

 for 1805. 



