306 THE SPECIAL SENSES 



If the ear be pressed against one end of a long beam, the 

 scratching of a pin at the other extremity may be distinctly 

 heard, which will not be at all audible when the ear is removed 

 from the beam. Although air is not the best medium for con- 

 veying sound, it is necessary for its production. Sound can- 

 not be produced in a vacuum, as is shown by ringing a bell in 

 the exhausted receiver of an air-pump, for it is then entirely 

 inaudible. But let the air be re-admitted gradually, then the 

 tones become more and more distinct, and when the receiver is 

 again full of air, they will be as clear as usual. 



89. All sonorous bodies do not vibrate with the same degree 

 of rapidity, and upon this fact depends the pitch of the sounds 

 that they respectively produce. The more frequent the num- 

 ber of vibrations within a given time, the higher will be the 

 pitch ; and the fewer their number, the lower or graver will it 

 be. Now, the rate of the successive vibrations of different 

 notes has been measured, and it has thus been found that if 

 they are less than sixteen in a second, no sound is audible ; 

 while, if they exceed 60,000 per second, the sound is very 

 faint, and is painful to the ear. The extreme limit of the 

 capacity of the human ear may be considered as included 

 between these points, but the sounds which we ordinarily hear 

 are embraced between 100 and 3000 vibrations per second. 



90. The ear, which is the proper organ of hearing, is the 

 most complicated of all the structures that are employed in 

 the reception of external impressions. The parts of which it 

 is composed are numerous, and some of them are extremely 

 small and delicate. Nearly all these parts are located in an 

 irregularly shaped cavity hollowed out in the temporal, or 

 "temple" bone of each side of the head. That part of the 

 bone in which the auditory cavity is placed has the densest 

 structure of all bones .of the body, and has, therefore, been 

 called the " petrous," or rocky part of the temporal bone. In 

 studying the ear, it is necessary to consider it. as divided into 

 three portions, which are called, from their relative positions, 



89. Pitch? To what due? Capacity of ear ? 



90. The ear. Its divisions. 



