894 EUSTACHIAN TUBE. TYMPANIC CAVITY. 



in this way, more firmly fixed, as through the oblique position mentioned 

 the ligamentous mass inserted around the edge of the base must be 

 more strongly stretched. Hence, the action of the muscle prevents 

 unduly strong impulses communicated by the incus from being trans- 

 mitted in their full strength to the endolymph. The nerve is derived 

 from the facial. 



In some- persons the stapedius nerve is innervated through associated move- 

 ment by forcible closure of the eyelids, a rumbling noise being heard at the same 

 time. Landois was able to excite such innervation through reflex influences 

 by scratching with the fingernail directly in front of the auditory meatus; Henle 

 accomplished the same thing by gently stroking the outer margin of the orbit. 

 The nerve seems to be susceptible to reflex irritation also in many ear-patients 

 by syringing the tympanic cavity. Under fhese circumstances Voltolini and 

 Politzer observed contractions of the auricle as associated movements and Ziem, 

 blepharospasm . 



Opinions are still much divided as to the action of the stapedius. In the 

 oblique position of the stapes, the head of the bone forces the long process of 

 the incus, and with it the malleus and tympanic membrane, outward. Conse- 

 quently the stapedius has been designated also the antagonist of the tensor t}'m- 

 pani. Politzer observed a decrease of the pressure in the labyrinth on irritation 

 of the muscle. According to Toynbee the stapedius is supposed to raise the stapes 

 out of the fenestra ovalis, and make it more movable, so that 

 it will vibrate more readily. The stapedius would, there- 

 fore, be the true listening muscle of the ear. Henle believed 

 that the stapedius is more concerned in fixing the stapes 

 than in making it movable, and that it acts only when 

 there is danger of a violent motion being transmitted to 

 the stapes from the malleus through the incus. Landois 

 agreed with this view, and considered the orbicularis 

 palpebrarum and the stapedius as muscles for the protec- 

 tion of important sense-apparattis. Both are innervated 

 FIG. 322. Stapedius by the facial nerve, and both can be stimulated reflexly by 



Muscle (Right). irritation of the sensory nerves in the vicinity of the 



sense-organ. Strong contraction of the orbicularis induces 

 associated movement of the stapedius. Lucae, who demon- 

 strated an associated movement of the stapedius with powerful movements of the 

 facial muscles, for example with closure of the lids (in association with which a 

 deep entotic sound is heard) , believes that the muscle effects an accommodation 

 of the tympanic membrane for the. highest, nonmusical tones (just as the tensor 

 does for musical tones). These highest tones sound louder, therefore, in this 

 experiment. 



Pathological. Immobility of the ossicles in consequence of cicatricial adhe- 

 sions of their joints or of ankyloses causes impairment of hearing in accordance 

 with the degree of immobility. Firm adhesions of the stapes within the fenestra 

 ovalis have the same effect. In the presence of contractures of the tensor 

 tympani, the tendon of this muscle has occasionally been cut. Paralysis of the 

 tensor is discussed in connection with the otic ganglion (p. 691) that of the sta- 

 pedius on p. 698. 



EUSTACHIAN TUBE. TYMPANIC CAVITY. 



The Eustachian tube, which is 4 cm. long, is the ventilating tube for 

 the tympanic cavity. It keeps the air in the interior of the cavity of the 

 same density as the outer air by means of the communication that is 

 established between the two in the pharynx (Figs. 314, 321). The 

 normal vibration of the tympanic membrane is possible only under this 

 condition. The tube is ordinarily closed. In swallowing, however, 

 the canal is dilated by the traction of the fibers of the tensor of the 

 veil of the palate (sphenosalpingostaphylinus or abductor tubse, or dila- 

 tator tubse) upon its membranocartilaginous portion, into which they 

 are inserted (Fig. 323). As the tube is closed, the vibrations of the 



