TRANSMISSION OF SOUND IN THE EAR 



495 



The membrane is set in vibration by the oscillations of the atmosphere 

 and is so arranged that it does not favor any particular tone. 



Fick has worked out the following conception of the mechanics of the 

 drum. The radii from the tip of the long process of the malleus to the 

 periphery of the drum being of different lengths, the different sectors of 

 the drum may be looked upon as to a certain extent independent of each 

 other. If they were entirely independent strands, each would vibrate in re- 

 sponse to its own particular tone. Being joined together into a membrane 

 they do not thus select individual tones, although separate parts can be thrown 

 into action without moving distant parts very much. Consequently, because 

 of the summation of successive 

 vibrations, regular periodic move- 

 ments are more favorably received 

 by the drum than single vibra- 

 tions. And yet vibrations of any 

 form or frequency are faithfully 

 transmitted to the handle of the 

 malleus, for among the sectors 



and segments of the membrane FlG - 197 - Nearly horizontal section through the 



tympanic cavity, CT, of the right ear, enlarged 

 four times, after Hensen. The section is taken 

 just above the notch of Rivinus and vertical to 

 the plane of Fig. 196. H, medial edge of the 

 head of the hammer. A. anvil. The ligamen- 

 tum anterius, Lig. a., is seen springing from the 

 larger process, Sp. m, of the wall of the tympanic 

 cavity and passing to the hammer where it be- 

 comes continuous with the ligamentum laterale. 

 LI of the anvil-hammer joint. The ligamentum 

 externum, Lig. ext. springs the notch of Rivinus 

 and passes to the hammer. 



there are always some which are 

 suited to the component vibra- 

 tions. Since, however, the mal- 

 leus is a rigid body and can only 

 vibrate as a whole, all the com- 

 ponents will be represented in the 

 form of its movements. 



The peculiar form of the tym- 

 panic membrane is of special sig- 



nificance in another respect also; 



the sound waves converging toward the middle point are damped i. e., dimin- 

 ished in amplitude, but are increased in intensity. This aids greatly in the 

 transmission of vibrations to the perilymph (Helmholtz). 



2. The Auditory Ossicles. For the anatomical details of the auditory 

 ossicles the reader is referred to text-books of anatomy. We give here briefly 

 only the facts with reference to their mode of attachments, as made out by 

 Hensen and Schwalbe, which are of most importance for an understanding 

 of their physiological purpose (Figs. 196 and 197). 



The malleus, or hammer, is attached to the wall of the tympanic cavity by 

 three ligaments (Fig. 197). The first of these, the anterior, passes from the 

 processus longus and around its base partly to the larger spine of the tym- 

 panum (Sp. m.), partly through the Glaserian fissure to the angular spine of 

 the sphenoid bond. The second or external ligament (Lig. ext.) is a short tense 

 band which springs from the whole posterior half of the notch of Rivinus as 

 far as the smaller spine of the tympanum opposite the hammer, and from this 

 relatively long line of insertion its fibers converge to the crista mallei. The 

 third or superior ligament (Fig. 196, LS) limits the movability of the ossicles 

 downward. 



The tip of the short leg of the incus is fixed by means of a strong ligament 

 (ligamentum incudis posterius) to the opposite wall of the tympanic cavity. 



