Phonoreception 473 



energy as that which can just barely be detected. This difference may be 

 expressed as 120 db and is just about the ratio of the energy of the sound 

 of very loud thunder to that of a sound just at the threshold of hearing. On 

 the decibel scale the intensities (i.e., energies) of some common sounds are: 



Since the decibel scale is one of ratios, it is necessary to have some standard 

 for comparison. The threshold of hearing may be used for this purpose, 

 but it is known to vary from person to person and also to vary greatly with 

 frequency. Therefore, an arbitrary standard of 1 microwatt (a watt is a 

 measure of the rate of flow of energy) per square centimeter is adopted for 

 reference, and this approximates the average threshold for a 1000 cycle tone. 

 Problems of Phonoreception. In the process of hearing the physiological 

 problem of the animal is how to detect sound waves, how to distinguish one 

 frequency from another, how to determine the relative intensity of each 

 frequency, how to determine the direction from which the sound is coming, 

 and, moreover, how to do all of these things simultaneously when the on- 

 coming sound waves are of a high degree of complexity, consisting of many 

 frequencies and intensities combined together in a complex disturbance of 

 the medium. In some animals, e.g., in fishes, this problem is only partly 

 solved in that there is a mechanism for detection of sound but^ almost none 4/e, 

 for analysis of the sound in terms of frequency. In man the mechanism for 

 analysis is very good. In bats it may be better, in that the frequency range 

 is much wider. 



PHONORECEPTION BY MAN 



Morphology: External Ear. The structure of the hearing mechanism of 

 man is shown diagramatically in Fig. 154. The external ear, or pinna, leads 

 into an auditory canal (external auditory meatus), which conducts sound 

 waves to the ear drum or tympanic membrane. The external ear of man is 

 too small to act as a concentrating reflector of sound waves and its principal 

 function is that of scattering the waves so that some of the sound passes into 

 the auditory canal. In donkeys and other large-eared animals, concentration 

 by reflection may be important. The tympanic membrane, which separates 

 the auditory canal from the middle ear, is slightly conical or funnel-like in 

 shape, is about 0.1 mm. thick, and is composed of an outside layer of skin, 

 a layer of connective tissue, and an inner layer of mucous membrane. 



Middle Ear. The middle ear is filled with air and contains three bones: 

 the malleus, the incus, and the stapes, which are attached to each other. 

 Furthermore, one arm of the malleus is attached close to the center of the 

 tympanic membrane and another projection is attached close to its periphery. 

 Vibration of the membrane causes a rocking of the malleus about the per- 

 ipherally attached projection. The incus is rather firmly fastened to the 

 malleus so that, at least for motions of small amplitude, the two bones tend 



