766 



THE SENSE OF HEARING 



It need scarcely be emphasized that the external auditory meatus 

 plays the part of a tube tending to conserve the character of the sound 

 waves. They are deflected from its walls into the pit of the funnel- 

 shaped tympanic membrane, but since the sides of the latter are 

 convex, their amplitude must be diminished, while their striking force 

 is increased. In this way, this membrane is set into vibration in 

 complete harmony with the undulations in the air. Moreover, since it 

 is small in size, it is able to move as one whole and with a definite perio- 

 dicity. The latter peculiarity is of 

 particular value, because it prevents 

 the magnification of certain overtones 

 to the exclusion of others. In addi- 

 tion, its structure and position are 

 such that it is able to offer a certain 

 resistance to the oscillations of this 

 system which causes the latter to 

 cease almost as soon as the sound 

 has been completed. The dampen- 

 ing effect is of great functional im- 

 portance, because it keeps these parts 

 in a state of readiness to receive new 

 vibrations. 



The Ear Bones or Ossicles. The 

 connection between the eardrum and 

 the membrane of the fenestra ovalis 

 is formed by three bones which are 

 known as the malleus, incus, and 

 stapes. 



Fia. 384. VIEW OF THE MEM- 

 BRANA TYMPANI AND AUDITORY OS- 

 SICLES FROM THE INNER SIDE. 

 (Schdfer.) 



m, Malleus; i, incus; st, stapes; py, 

 pyramid, from which the tendon of the 

 stapedius muscle is seen emerging; it, 

 tendon of the tensor tympani cut 

 short near its insertion; la, anterior 

 ligament of the malleus; the anterior 

 process (processus gracilis) is concealed 

 by the lower fibers of this ligament; Is, 

 superior ligament of the 'malleus; li, 

 ligament of the incus; ch, chorda tym- 

 pani nerve passing across the outer 

 wall of the tympanum. 



The malleus or hammer bone, is 8-9 

 mm. in length and possesses an average 

 weight of 23 mgrs. It consists of a rounded 

 head, grooved on one side for its articulation 

 with the incus, a short massive neck and a 

 long handle, or manubrium. The latter is 

 securely fastened in the tissue of the eardrum 

 and presents two processes, one of which is 

 known as the processus brevis and the other 

 as the processus gracilis. The former presses 

 against the eardrum above the umbo, while the latter extends into the Gasserian 

 fissure in the wall of the tympanum. The malleus is held hi place by three liga- 

 ments, a superior, anterior and posterior. The first of these holds the head of 

 this bone against the roof of the tympanic cavity, while the second and third 

 secure its neck in a position near the anterior wall of this space. Besides serving 

 as supports, these ligaments also force this bone to rotate around a perfectly definite 

 axis. This is true especially of the anterior and posterior ligaments which tend 

 to fix its neck portion as if it were placed in a sling. Consequently, the inward 

 movement of the eardrum and manubrium must cause the head of this bone to 

 move outward, while their outward movement must force the latter inward. 



The incus or anvil bone, weighs about 25 mgrs. and possesses a shape somewhat 

 similar to that of a bicuspid tooth, its heavier upper portion being hollowed out for 



