384 



THE SPECIAL SENSES. 



of the membrane are transmitted directly to these bones. The 

 pecuhar form of the membrane, its funnel shape, its arched sides, 

 and its unsymmetrical division by the umbo are supposed to con- 

 tribute to its value as a transmitter of the sound vibrations of the 

 air. In the first place, the membrane shows little tendency to 

 after-vibrations, —that is, when set in motion by an air wave it 

 shows little or no tendenc}^ to continue vibrating after the acting 



2 1 



Fig. 169. — Tympanum of right side wath ossicles in place, viewed from within (after 

 Sif orris): 1, Body of incus; 2, suspensory ligament of malleus; 3, ligament of incus; 4, 

 head of malleus; 5, epitympanic cavity; 6, chorda tympani nerve; 7, tendon of tensor 

 tj-mpani muscle; 8, foot-piece of stirrup; 9, os orbiculare; 10, manubrium; 11. tensor 

 tympani muscle; 12, membrana tvmpani: 13, Eustachian tube. 



force has ceased. It is obvious that such a property is valuable in 

 rendering hearing more distinct, and the peculiarity of the mem- 

 brane in this respect is attributed partly to its special form and 

 partly to the damping action of the bones attached to it. In the 

 second place, the arched sides of the funnel act as a lever, so that 

 the movements at these parts are transmitted to the umbo with a 

 diminution in amplitude, but an intensification in force. It is at 

 the umbo that the movement is communicated to the ear bones. 



The Ear Bones. — ^The three ear bones — the malleus, the incus, 

 and the stapes — taken together form a chain connecting the tym- 

 panic membrane with the membrane of the fenestra ovalis. By 

 this means the vibrations of the tympanic membrane are com- 

 municated to the membrane of the fenestra ovalis and thus to the 

 perilymph filling the cavity of the internal ear. The bones consist 



