HEARING. 3.97 



Other Cetacea, are inclosed in a bone of extraordi- 

 nary hardness, which, instead of forming a con- 

 tinuous portion of the skidl, is connected to it only 

 by ligaments, and suspended in a kind of osseous 

 cavity, formed by the adjacent bones. The cochlea 

 is less developed than in quadrupeds ; for it only 

 takes one turn and a half, instead of two and a 

 half. The existence of the semicircular canals in 

 the Cetacea was denied by Camper ; but they have 

 since been discovered by Cuvier. 



Several quadrupeds, which are in the habit of 

 burrowing, or of diving, as the Sorex fodieus, or 

 water-shrew, are furnished with a valve, composed 

 of a double membrane, capable of accurately 

 closing the external opening of the meatus, and 

 protecting it from the introduction of water, earth, 

 or other extraneous bodies.* In like manner the 

 external ear of the Hippopotamus, which feeds at 

 the bottom of rivers, is guarded by an apparatus 

 which has the effect of a valve. 



We find, indeed, the same provident care dis- 

 played in this, as in every other department of the 

 animal economy : every part, however minute, of 

 the organ of this important sense, being expressly 

 adapted, in every species, to the particular circum- 

 stances of their situation, and to that degree of 

 acuteness of perception, which is best suited to 

 their respective wants and powers of gratification.! 



* GeofFroy St. Hilaire ; Memoires du Museum, i. 305. 



t The Comparative Physiology of the Voice, a function of which 

 the object, in animals as well as in man, is to produce sounds, ad- 

 dressed to the ear, and expressive of their ideas, feelings, desires and 

 passions, forms a natural sequel to that of Hearing ; but Sir Charles 

 Bell having announced his intention of introducing it in his Treatise 

 on the Hand, 1 have abstained from entering into this extensive sub- 

 ject. 



