THE EUSTACHIAN TUBE. 805 



fenestra ovalis and keeps the stapes in its place becomes more tense. The activity 

 of this muscle, therefore, prevents too intense shocks, which may be communicated 

 from the incus to the stapes, from being conveyed to the perilymph. It is supplied 

 by the facial nerve ( 349, 3). 



The stapedius in many persons executes an associated movement, when the eyelids are forcibly 

 closed ( 349). Some persons can cause it to contract reflexly by scratching the skin in front 

 of the meatus, or by gently stroking the outer margin of the orbit (Henle). It seems to be 

 excited reflexly in many diseases of the ear when the tympanum is being syringed. 



Other Views. According to Lucae, when the stapes is displaced obliquely, its head forces the 

 long process of the incus, and also the membrana tympani, outwards, so that it is regarded as an 

 antagonist of the tensor tympani. Politzer observed that the pressure within the labyrinth 

 fell, when he stimulated the muscle. According to Toynbee, the stapedius acts as a lever and 

 moves the stapes slightly out of the fenestra ovalis, thus making it more free to move, so that 

 it is more capable of vibrating. Henle supposes that the stapedius is more concerned in fixing 

 than in moving the stapes, and that it comes into action when there is danger of too great move- 

 ment being communicated to the stapes from the incus. Landois agrees with this opinion, and 

 compares the stapedius with the orbicularis palpebrarum, both being protective muscles. 



Pathological. Immobility of the auditory ossicles, either by adhesions or anchyloses, causing 

 diminished vibrations, interferes with hearing ; while the same result occurs when the stapes is 

 firmly anchylosed into the fenestra ovalis. The tendon of the tensor tympani has been divided 

 in cases of contracture of the muscles. For paralysis of the tensor, see p. 598, and for the 

 stapedius, p. 603. 



411. EUSTACHIAN TUBE TYMPANUM. The Eustachian tube [4 centi- 

 metres in length, If in.] is the ventilating tube of the tympanic cavity. It keeps 

 the tension of the air within the tympanum the same as that within the pharynx 

 and outer air (figs. 577, $86). Only when the tension of the air is the same out- 

 side and inside the tympanum, is the normal vibration of the membrana tympani 

 possible. The tube is generally closed, as the surfaces of the mucous membrane 

 lining it come into apposition. During swallowing, however, the tube is opened, 

 owing to the traction of the fibres of the tensor veli palatini [spheno-salpingo- 

 staphylinus sive abductor tubae (v Trbltsch), sive dilator tubae (Riidinger)] inserted 

 into the membrano-cartilaginous part of the tube {Toynbee, Politzer). (Compare 

 139, 2.) When the tube is closed, the vibrations of the membrana tympani are 

 transferred in a more undiminished condition to the auditory ossicles than when it 

 is open, whereby part of the vibrating air is forced through the tube (Mack and. 

 Kessel). If, however, the tympanic cavity is closed permanently, the air within it 

 becomes so rarefied ( 139) that the membrana tympani, owing to the abnormally 

 low tension, becomes drawn inwards, thus causing difficulty of hearing. As the 

 tube is lined by ciliated epithelium it carries outwards to the pharynx the secre- 

 tions of the tympanum (p. 452). 



Noise in the Tube. A sharp hissing noise is heard in the tube during swallowing, when we 

 swallow slowly and at the same time contract the tensor tympani, due to the separation of the 

 adhesive surfaces of its lining membrane. Another person may hear this noise by using a 

 stethoscope or his ear. 



In Valsalva's experiment ( 60), as soon as the pressure of the air reaches 10 to 40 mm. 

 Hg, air enters the tube. The sound is heard first, and then we feel the increased tension of 

 the tympanic membrane, owing to the entrance of air into the tympanum. During forced 

 inspiration, when the nose and mouth are closed, air is sucked out, while the tympanum is 

 ultimately drawn inwards. 



The M. levator veli palatini, as it passes under the base of the opening of the tube into the 

 pharynx, forms the levator-eminence or cushion (fig. 354, W). Hence, when this muscle con- 

 tracts and its belly thickens, as at the commencement of the act of deglutition and during 

 phonation, the lower wall of the pharyngeal opening is raised, and the opening thereby 

 narrowed {Lucae). The contraction of the tensor, occurring during the later part of the act of 

 deglutition, dilates the tube. 



Other Views. According to Riidinger, the tube is always open, although only by a very 

 narrow passage in the upper part of the canal, while the canal is dilated during swallowing. 

 According to Cleland, the tube is generally open, and is closed during swallowing. 



[Practical Importance. The tympanic cavity forms an osseous box, and there- 

 fore a protective organ for the auditory ossicles and their muscles, while the 



