THE EAR. 513 



sounded by itself sounds but feebly, but placed on a reso- 

 nance box of the proper kind it is increased many-fold 

 in strength. So, too, in the middle ear the air serves such 

 resonance purposes. Even the bone surrounding the mid- 

 dle ear is very porous, indeed, and it is not impossible that 

 this porosity may help to increase still further this reso- 

 nance function. 



It will be noticed that both external and middle ears 

 were purely physical arrangements to transmit the sound 

 in a definite way to the internal ear where the real act of 

 perception occurs. 



THE INTERNAL EAR. 



The real sensory internal ear, that part to which the 

 auditory nerve is distributed, is a series of membranous 

 canals and sacs which lie in corresponding openings in 

 the petrous portion of the temporal bone. We have there- 

 fore to do first with the bony internal ear, by which is 

 meant nothing more than that system of spaces in the 

 bone in which the membranous or sensory ear is located. 

 The membranous ear does not, however, occupy all of the 

 space of the bony ear, but floats in the perilymph, a liquid 

 which fills this space. 



The bony ear shows three well-marked divisions which 

 appear also in the membranous ear. First, the vestibule; 

 second, the semi-circular canals; third, the cochlea. These 

 three portions are continuous so that the perilymph in the 

 vestibule might, if it could be set in motion, pass through 

 any one of the semicircular canals or flow up the windings 

 of the cochlea. 



1. Vestibule. The vestibule has a large opening com- 

 municating with the middle ear. This is the foramen 

 ovale into which the end of the stapes fits. It is evident, 

 therefore, that the vibrations communicated to the internal 

 ear by the stapes reach the perilymph of the vestibule. 



Connected with the vestibule towards the back and 

 slightly above are three semicircular canals arranged like 

 33 



