TENSOR TYMPANI 269 



the one attached to the stapes pull in opposition to keep the articulating 

 surfaces between malleus and incus in variable coupled contact. The ef- 

 fective magnitude of their tensions determines the characteristics of the 

 transmitted energy from the tympanum-malleus to the incus-stapes 

 transmission units. 



The tensor tympani (Fig. VII-8) is a long, narrow muscle lying in a 

 bony canal acoustically insulated until it almost reaches the tympanic 

 cavity. Here it becomes converted into a round tendon (T) which passes 

 through an eyelet to bend at right angles and terminates in the neck of 

 the malleus opposite the lateral process. 



If the tympanic tensor is stimulated to contract, it pulls inward, 

 forward, and upward, thus applying tension to the lower segments of 

 the tympanic membrane and to the ligaments attached to the malleus. 

 The ligaments supporting the malleus are the anterior, the external, the 

 superior, and the capsular ligaments bridging the malleus-incus articula- 

 tion. If the tympanic tensor is under tension, the capsular ligament is 

 under tension, a condition which pulls the malleus-incus articulating 

 surfaces apart. This produces a loose coupling at this joint. As the ten- 

 sion on the tympanic tensor is reduced, the coupling becomes greater. 

 If the tensor is cut, the coupling is maximal. Ossification of this joint 

 produces rigid coupling. 



It has been shown by Kohler [1910] and others that in man the aural 

 muscles exhibit a reflex response to acoustic excitation of the eardrum. 

 The pressure variation on the eardrum stimulates these muscles to 

 contract. The effect of this contraction on the transmitted wave form 

 will be discussed after the discussion of the part played by the stapedius 

 tensor on the overall response of the mechanism. As a preliminary state- 

 ment it may be said that the contraction of the interaural muscles 

 reduces the overall sensitivity of the receptor mechanism for low fre- 

 quencies and increases the ability to pass high frequencies. 



Protection against rupture due to a sudden large increase in pressure, 

 such as would be caused by a sharp blow over the ear by means of an 

 open hand, is obtained from the articulation between the malleus and 

 the incus, which can dislocate so that the excessive pressure is not com- 

 municated to the stapes. Under these circumstances the articulation 

 acts as a protective device and the tensor as a snubber to prevent the 

 drum from being pulled out too far as the result of the sudden external 

 decompression. 



The Incus 



The incus is the auditory ossicle inserted between the malleus and the 

 stapes. It has almost the shape of a molar tooth (Fig. VII-9). The 



