174 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



SECTION VII. 

 Sense of Hearing and its Organ, the Ear. 



Sight and hearing are the two higher senses. In 

 these alone the impression of the sensible body is not on 

 the specialized nerve directly, but indirectly through the 

 vibrations of a medium. In these alone, therefore, in 

 addition to the specialized nerve and in front of it, there 

 is a mechanical instrument for making the impression 

 stronger and more definite. 



The eye is undoubtedly the most refined mechanism 

 in the animal body, and yet its structure is more easily 

 explained than that of the ear. The structure of the ear 

 is not only very complex, but it is lodged in intricately 

 winding passages in the interior of the hardest bone in 

 the body. In these passages the branches of the eighth 

 pair of nerves are distributed and specialized to respond 

 to vibrations of the air. 



Structure of the Human Ear.— The ear consists 

 of three general parts — the exterior, the middle, and the 

 interior ear. The first two are air-filled, the third is en- 

 tirely cut off from the air and is water-filled. The first 

 two are instrumental; the third alone contains the spe- 

 cialized nerve (Fig. 109). 



The exterior ear includes all that is visible from the 

 outside — i. e., as far as the membrane of the drum. It 

 consists of the conch and the meatus. The conch 

 collects the aerial vibrations, and the meatus carries 

 them to the membrane of the drum. The meatus se- 

 cretes a kind of wax — ear wax — which by accumu- 

 lation may cause partial deafness, but is easily re- 

 moved. 



The mid-ear is a cavity just beyond the membrane 

 of the drum. It is about one third of an inch in diame- 



