654: THE SENSES. 



These considerations have induced a different view of the action of 

 the tensor tympani in modifying the sensations of sound. With the 

 membrane in a state of moderate tension, a certain proportion of tones 

 only are distinctly appreciated, while the remainder are either inaudible 

 or imperfectly transmitted to the internal ear. This is the state in 

 which sounds generally are received by the organ of hearing, without 

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

 distinctly successive tones of varying pitch, or when it listens intently 

 for a particular note, the tension of the membrana tympani is increased 

 or diminished to such a degree as will enable the vibration to be trans- 

 mitted with the most complete distinctness by the chain of bones to the 

 fluid of the labyrinth. With regard to the modifications induced in the 

 apparent intensity of sound, it is probable that Savart's explanation 

 holds good ; and that a diminished tension of the membrane enables 

 the ear to catch more readily sounds which are faint or distant. This 

 partial relaxation is accomplished by the action of the stapedius muscle, 

 which is animated directly by a filament of the facial nerve ; while the 

 tensor tympani is supplied only from the otic ganglion of the sympa- 

 thetic. 



The cavity of the tympanum is not hermetically closed, but commu- 

 nicates with the pharynx by means of the Eustachian tube. The exist- 

 ence of this opening secures the equality of atmospheric pressure within 

 and without the membrana tympani, a condition which is essential to its 

 proper vibration under the influence of sonorous impulses. The ex- 

 ternal barometric pressure varies from day to day, and even for dif- 

 ferent periods of the same day ; and if the middle ear were a closed 

 cavity, this variation would of itself change the tension of the mem- 

 brana tympani, independently of the action of the muscles. Although 

 the mucous surfaces of the Eustachian tube are habitually in contact 

 with each other, the}^ readily yield to a preponderance of atmospheric 

 pressure in either direction, and thus the equilibrium is maintained 

 between the air inside and outside the cavity of the tympanum. 



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

 cated extension and windings of its various cavities and passages, is 

 situated in the petrous portion of the temporal bone. Its external wall 

 consists of a thin lamina of compact osseous tissue, which is readily 

 isolated in the foetus and in newly born infants, owing to its being im- 

 mediately surrounded by spongy tissue; while in the adult it is more or 

 less completely consolidated with the adjacent bony parts. It may be 

 divided physiologically into: 1. The vestibule and semicircular canals, 

 which constitute its most essential parts and are present in all verte- 

 brate animals ; and 2. The cochlea, which, in man and the mammalia, is 

 a more highly developed portion, of complicated structure, but which 

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

 in the scaly reptiles and in birds. 



The vestibule (Fig. 211,i) is so called because its cavity is that into 

 which the fenestra ovalis immediately opens, and from which those of 



