316 THE SENSES, 



hearing apparatus may, in general terms, be divided into 

 external and internal: the former to collect and direct the 

 tremors or vibrations, which beget sensation ; the latter to 

 receive those tremors, and bring them into relation with the 

 auditory or special hearing nerve. 



The first thing to strike an observer, taking cognisance 

 generally of external hearing apparatus in the higher animals, 

 is the variety as to shape and direction of the external ear. 

 Predacious animals have their ears directed forwards ; timid 

 animals, such as hares and rabbits, backward. Oxen, horses, 

 donkeys, and mules can point their ear-trumpets in whatever 

 direction they like. I accept Mr. Darwin of the Zoonomia as 

 my authority for the statement that, when horses or mules 

 are travelling in single file, the leading animal pricks his ears 

 forward, the rear-guard as one may call him backwards. 

 The cause of this difference is obvious when reflected upon. 

 The foremost animal is most concerned to hear sounds in 

 advance ; the rear-guard, sounds behind. 



Amongst men, savages usually have projecting ears ; the 

 habit of listening to distant sounds having promoted, it should 

 seem, the development. When a civilised individual wants 

 to hear very distinctly, he holds his hand up in such a way 

 as to improvise a large external ear. Deaf people increase 

 the effective length of their external ears by devices of metal 

 hearing-trumpets ; the reason obvious. 



Internally the ear is subject to much variety in different 

 animals. Assuming the internal auditory apparatus of man 

 as typical, we find it consist of a labyrinthine maze of bony 

 material, constituting an extensive passage, in which auditory 

 vibrations must linger before arriving at a membrane called 

 the tympanum, or drum of the ear. Behind, or internal to 

 the tympanum, is a passage" which leads to the throat. It is 

 called the JEustachian tube : the chief use of it is imagined by 

 physiologists to be equalisation of atmospheric pressure on 

 both sides of the drum-membrane. 



