558 THE NERVOUS SYSTEM. 



only to a certain number of tones, separated by definite intervals;* 

 and that they will respond to a different set only after their tension has 

 been increased or diminished. In order, therefore, that a membrane 

 may be easily thrown into induced vibration, its tension must corre- 

 spond in a certain ratio with the tone of the sounding body. 



These considerations have induced a different view of the tensor 

 tympani as modifying the sensations of sound. With the membrane 

 in a state of moderate tension, a certain number of tones only are dis- 

 tinctly appreciated, the remainder being either inaudible or indistinct. 

 This is the state in which sounds are generally perceived, without 

 xact appreciation of their relative pitch. But when the ear follows 

 a succession of tones, or when it listens for a particular note, the 

 tension of the membrane is so increased or diminished as to transmit 

 the vibration with the greatest distinctness. With regard to modifica- 

 tions in the apparent intensity of sound, it is probable that Savart's 

 explanation holds good ; and that a diminished tension of the mem- 

 brane enables the ear to catch more readily sounds which are faint 

 or distant. This partial relaxation is accomplished by the stapedius 

 muscle, which is animated by a filament of the facial nerve, and is 

 therefore more directly under the control of the will ; while the tensor 

 tympani is supplied from the otic ganglion of the sympathetic, and is 

 involuntary in its action. 



The cavity of the tympanum communicates with the pharynx by 

 the Eustachian tube. The existence of this canal secures equality 

 of atmospheric pressure on both sides of the membrana tympani, 

 a condition essential to its free vibration under sonorous impulses. 

 The external barometric pressure varies from time to time ; and if the 

 middle ear were a closed cavity, this variation would of itself change 

 the tension of the membrana tympani and interfere with its function. 

 Although the walls of the Eustachian tube are habitually in contact 

 with each other, they readily yield to atmospheric pressure in either 

 direction, and thus reestablish the equilibrium between the outer air 

 and the cavity of the tympanum. 



Labyrinth. The internal ear, or labyrinth, so called from the com- 

 plicated extension of its cavities, is situated in the petrous portion of 

 the temporal bone. It may be divided into: 1. The vestibule and 

 semicircular canals, which constitute its most essential parts and are 

 present in all vertebrate animals ; and 2. The cochlea, which, in man 

 and the mammalia, is a more highly developed portion, but which is 

 absent in the fishes and naked reptiles, and only partially developed in 

 scaly reptiles and in birds. 



The vestibule (Fig. 150, x ) is so called because its cavity is that into 

 which the fenestra ovalis immediately opens, and which leads to the 

 semicircular canals and cochlea. It has an ovoid form, and presents, 

 on the side toward the tympanum, two openings, namely : 1. The 



* Daguin, Traite" elmentaire de Physique. Paris, 1867, tome i., p. 596. 



