558 HEARING. 



and back part, as a rudimentary concha ; and the feathers are so arranged around 

 as to serve the purpose of a concha : and this feature is very characteristic in owls. 

 In mammalia all the parts acquire their full development. The cochlea, the 

 size of which bears a pretty constant proportion to the acuteness of hearing, is 

 greatly developed, turbinated, and divided : the ampullae are often small : the 

 ossicula are first completely developed, are articulated, and supplied with 

 muscles : the Eustachian tube is lengthened; so likewise is the meatus externus, 

 and provided with hairs and a disagreeable secretion for defence ; and the 

 concha, the size and mobility of which indicate acute hearing, is developed in this 

 class only, though frequently small or absent in the inferior species and the 

 aquatic, as whales, beavers, and seals. In the timid, which are to be pursued, 

 both it and the meatus are directed backwards ; in the predaceous, which are to 

 pursue, forwards : in the former too it is large, and the brain is small ; in the 

 latter small, and the brain is large : in the quadrumana, and especially in the 

 oran-outang, it becomes short, round, and motionless as in us. In the aquatic 

 mammalia, in order to prevent the entrance of water, the meatus is narrow and 

 winding, and the orifice very small ; and in quadrupeds which dive or burrow, 

 a double membrane is provided, which can accurately close it. The hippopo- 

 tamus, which feeds at the bottom of rivers, has an apparatus for the same 

 purpose. (See Dr. Grant, Lancet, No. 569., and Outlines of Comparative Anatomy. 

 Dr. Roget, Bridgewater Treatise.} 



Many animals surpass us in acuteness of hearing. The common birds about 

 us hear the faintest sound. " Not only," says Gall, " are the vestibule and 

 semicircular canals proportionally larger in many brutes, but the acoustic 

 nerve and all its apparatus are more perfect. This nerve originates in a more 

 considerable mass of grey substance, and is consequently considerably larger, as 

 any one will find in the sheep, ox, horse, &c. The external concha is much 

 more developed in most brutes, and the great osseous cavities surrounding the 

 labyrinth in many produce a similar effect in augmenting the sound of the solid 

 and elastic vaults. These cavities, which must not be confounded with the mas- 

 toid processes, contain, in many brutes, for instance, in the calf, concentric canals 

 which unite into a common cavity, and must evidently increase the sound." 

 (1. c. 4to. vol. i. p. 161.) 



