508 STUDIES IN ADVANCED PHYSIOLOGY. 



ties and so may be looked upon, physiologically at least, as 

 an inherited remnant of a once more serviceable structure. 



Fig. 156. A SEMI-DIAGRAMMATIC SECTION THROUGH THE RIGHT EAR. 



A, auditory nerve; S, semicircular canal; <?, meatus; M, concha; T, tympanic mem- 

 brane; P, middle ear; R, Eustachian tube; S, cochlea; Vt, scala vestibuli; Pt, scala tym- 

 pani; r, round foramen; o, oval foramen. (The membranous cochlea is not shown.) 



2. Meatus. Through the auditory meatus the sound 

 waves are led to the tympanic membrane. The meatus is 

 lined with skin, in which are imbedded numerous wax 

 glands, the secretion of which serves to keep the wall of the 

 passage, as well as the tympanic membrane at its end, in a 

 pliable condition. The presence of relatively large hairs in 

 it serves mechanical purposes only, in preventing access to 

 foreign bodies. It was held by some physiologists that the 

 external' auditory meatus serves as a resonance cavity for the 

 sounds, but owing to its small size this physical effect is very 

 trifling, and what resonance there is in connection with the 

 ear belongs probably entirely to the middle ear. 



As the efficiency of the eye may be materially increased 

 by the addition of optical instruments, so it is possible to 

 magnify the perceptive capacity of the auditory meatus by 

 special instruments, such as ear- trumpets and the profes- 

 sional stethoscope. 



