800 



HEARING, SMELL, AND TASTE. 



into contraction at the commencement of a sound or noise, but to return to 

 rest during the continuance of a musical note. Efferent impulses reach it 

 through fibres of the fifth nerve, and its activity is regulated by a reflex 

 action. In some persons the muscle seems to be partly under the dominion 

 of the will, since a peculiar crackling noise which these persons can produce 

 at pleasure appears to be caused by a contraction of the tensor tympani. 



The so-called laxator tympani is considered to be not a muscle at all, but 

 a part of the ligamentous supports of the malleus. 



710. The stapedius muscle by pulling upon the head of the bone (Fig. 

 196) is supposed to regulate the movements of the stapes, and especially to 

 prevent its base being driven too far into the fenestra ovalis during large or 

 sudden movements of the membrana tympani. It is governed by fibres 

 from the facial nerve. 



711. The Eustachian tube. This serves to maintain an equilibrium of 

 pressure betwen the external air and that within the tympanum, and to 

 serve as an exit for the secretions of that cavity. Were the tympanum per- 

 manently closed the vibrations of the membrana tympani would be inju- 

 riously affected by variations of pressure occurring either inside or outside. 

 The Eustachian tube is undoubtedly open during swallowing, but it is still 

 disputed whether it remains permanently open or is opened only at inter- 

 vals ; probably it is, at most times, neither widely open nor closely shut. 



Auditory Sensations. 



712. Each vibration communicated by the stapes to the perilymph 

 travels as a wave over the vestibule, the semicircular canals, and other parts 

 of the labyrinth ; and from the perilymph is transmitted through the mem- 



FIG. 197. 



i.v 



n.aud. 



t.spn. 



tg.sp. 



Diagram of the Organ of Corti. (After Retzius.) i.r, inner rod of Corti ; o.r, outer rod of Corti. 

 i.h.c, inner hair-cells ; n.c, the group of nuclei beneath it ; o.h.c, outer hair-cell, or cell of Corti, 

 of the first row ; c.D, its twin cell of Deiters four rows of these twin cells are shown. 



n.aud, the auditory nerve perforating the tympanic lip, l.t, and lost to view among the nuclei 

 beneath the inner hair-cell ; i.sp.n, the inner spiral strand of nerve-fibrillse ; t.sp.n, the spiral 

 strand of the tunnel; o.sp.n, the outer spiral strand belonging to the first row of outer hair-cells; 

 the three succeeding spiral strands belonging to the three other rows are also shown. Nerve- 

 fibrillse are shown stretching radially across the tunnel. 



H.c, Hensen's cells ; Cl.c, Claudius's cells; m.b, basilar membrane; t.l, lymphatic epithelioid 

 lining of the basilar membrane on the side toward the scala tympani ; Ig.sp, spiral ligament ; c' t 

 cells lining the spiral groove, overhung by I.v, the vestibular lip ; m.t, the tectorial membrane a 

 fragment of it is seen torn from the rest and adherent to the organ of Corti just outside the outer- 

 most row of outer hair-cells. 



branous wall to the endolymph. From the vestibule it passes on into the 

 scala vestibuli of the cochlea, and descending the scala tympani, ends as an 

 impulse against the membrane of the fenestra rotunda. In the regions of 

 the maculae and cristse the vibrations of the endolymph are suppose^ to throw 



