188 EXPERIMENTAL PHYSIOLOGY. 



malleus inward and through it exert a pull on the membrane. What the useful- 

 ness is of this extra tension on the membrane, and the slight increase in pressure 

 of the middle chamber which it causes, is not clearly understood. It has been 

 suggested that the action is protective, damping the response of the membrane 

 to excessive sounds. The suggestion has also been made that the muscle con- 

 tracts reflexly when the attention is directed to sounds of high pitch, or rapid 

 vibrations, to increase the tension of the membrane and thus to make its natural 

 vibration period more rapid. The second muscle in the middle chamber, the 

 STAPEDIUS, arises from a projection on the inner wall and is inserted into the neck 

 of the stapes, on which, when it contracts, it exerts a lateral pull. The conjectures 

 as to the use of the tensor tympani apply also to this muscle. 



If the tympanic membrane is to swing freely in response to sound waves 

 in the air the pressure on both its surfaces must be kept equal. This is en- 

 sured by the opening of the EUSTACHIAN TUBE which occurs during swallow- 

 ing and gives a communication between the middle ear and the pharynx. When 



^^ 



Frc. 56. Cross section of the bony labyrinth showing the spiral of the cochlea. 



the mucous membrane of the tube becomes swollen and the passage closed, 

 as in severe colds, the gradual absorption which goes on of the air contained in 

 the chamber of the middle ear lowers the pressure in it, the membrane no longer 

 vibrates freely, and a partial deafness results. 



PHYSIOLOGY OF THE ESSENTIAL ORGAN OF HEARING. 



THE INTERNAL EAR. The cavity of the skull which contains this part of the 

 labyrinth is spiral in shape, gradually tapering off to a point (Fig. 56). It is 

 lined with membrane and is filled with fluid. From the part of the bone which 

 forms the central pillar of the spiral a ledge projects into the cavity, reaching 

 about half way through its width and running almost the entire length of the spiral. 

 This partial partition of the chamber is completed by two membranes attached 

 at one side to the ridge of bone and at the other to two lines a little distance apart 

 on the membrane of the outer wall (Fig. 57). As a result the fluid of the spiral 



