I : 4/ Sound and the Ear 19 



location of the organs responsible for the effects observed. These 

 hearing tests are simple in that they do not necessitate surgery or putting 

 electrodes into people. By contrast, the studies described in the next 

 section and in Chapter 6 allow one to determine whether the effects are 

 mechanical or nervous and to gain insight into the mechanism of 

 hearing. 



4. Anatomy and Action of the Ear 



The ear is the organ of hearing. Sound waves impinge on the ear 

 which couples them to the endings of the sensory nerve associated with 

 hearing. It is customary to divide the mammalian ear into three major 

 divisions: the outer ear, the middle ear, and the inner ear. The outer 

 and the middle ear are filled with air; their primary purpose seems to be 

 to conduct sound to the inner ear. The inner ear consists of several 

 parts, some of which are concerned with balance, and one of which is 

 part of the hearing apparatus. Although anatomically the inner ear 

 is one organ and is served by one cranial nerve, only the cochlear portion 

 of the inner ear is associated with hearing. 



The incident sound waves in the air surrounding the head enter the 

 outer ear first. This consists of three parts, an external auricle (or pinna), 

 a narrow tube called the external auditory meatus, and the tympanic membrane 

 (or eardrum). These are illustrated in Figure 8. The auricles are 

 almost vestigial in humans and play a very minor role in the phenomenon 



of hearing. In most mam- 

 mals, the pinnae are large 

 and can be raised, lowered, 

 and rotated. In this way 

 they can be used to help 

 locate the origin of a given 

 sound. In rodents, and 

 some other mammals also, 

 the auricle is at times laid 

 down across the opening 

 to the meatus to give some 

 protection against very 

 loud sounds. 



In humans, the external 

 auditory meatus (or ear 

 canal) is somewhat cir- 

 cular in cross section and 

 more or less a straight tube. In an average adult, it is about 1 .04 ml 

 in volume and about 2.7 cm long. As in many other biological 



Pinna or 

 Auricle 



Part of Middle Ear 

 r^y showing Malleus 



Tympanic Membrane 

 or Ear Drum 



External Auditory Meatus 

 or Ear Canal 



Figure 8. The outer ear. After A. J. Carlson and 

 V. Johnson, The Machinery of the Body (Chicago: 

 The University of Chicago Press, 1941). 



