HEARING 367 



nostrils ; and thus must tend to draw a little air out of 

 that chamber from behind. At the same time, or imme- 

 diately afterwards, the air sucked in at the nostrils 

 entering with a sudden vertical rush, part of it must tend 

 to flow directly into the olfactory chamber, and replace 

 that thus drawn out. 



The loss of smell which takes place in the course of a 

 severe cold may, in part, be due to the swollen state of 

 the mucous membrane which covers the inferior turbinal 

 bones, impeding the passage of odoriferous air to the 

 olfactory chamber. 



Very little is known of the physiology of smell, and 

 smells have not so far been classified except as agreeable 

 or the reverse ; but recent observations seem to show that 

 a much more detailed classification is possible. Everyday 

 experience shows that the sense is extremely delicate, 

 the most minute amount of odoriferous matter, such as 

 musk, serving to excite it. 



9. The Ear and the Sense of Hearing.— The ear, or 

 organ of the sense of hearing, is very much more complex 

 than either of the sensory organs yet described ; and in it 

 both the essential and the accessory parts are much more 

 highly developed. 



In our discussion of cutaneous sensation we saw that 

 the skin could not be regarded as a single sense organ 

 but that it could appreciate sensations of very diflerent 

 characters for which there were different types of sense 

 organs. The same is true of the ear. It has two quite 

 different functions, one of which is to hear sounds, the 

 other is to acquaint the mind with the position, or 

 alterations of the position, in which the head is placed. 

 These different functions are located in two different parts 

 of the ear, which, taken together, are called the inner ear. 

 The inner ear is situated in the skull at some distance 

 from the surface ; between it and the outer, or visible 

 portion of the ear, is the middle ear. 



(i) The Esternal Ear. — The outer extremity of the 



