CHAP, in.] HEARING, SMELL, AND TASTE. 557 



The Acoustic Apparatus. 



Waves of sound can and do reach the endolymph of the 

 labyrinth by direct conduction through the skull. Since however 

 sonorous vibrations are transmitted with great difficulty from the 

 air to solids and liquids, and most sounds come to us through the 

 air, some special apparatus is required to transfer the aerial vibra- 

 tions to the liquids of the internal ear. This apparatus is supplied 

 by the tympanum and its appendages. 



The concha. The use of this, as far as hearing is concerned, is 

 to collect the waves of sound coming in various directions, and to 

 direct them on to the membrana tympani. In ourselves of moderate 

 service only, in many animals it is of great importance. 



The membrana tympani. It is a characteristic property of 

 stretched membranes that they are readily thrown into vibration by 

 aerial waves of sound. The membrana tympani, from its peculiar 

 conformation, being funnel-shaped with a depressed centre sur- 

 rounded by sides gently convex outwards, is peculiarly susceptible 

 to sonorous vibrations, and is most readily thrown into correspond- 

 ing movements when waves of sound reach it by the meatus. It 

 has moreover this useful feature, that unlike other stretched 

 membranes, it has no marked note of its own. It is not thrown into 

 vibrations by waves of a particular length more readily than 

 by others. It answers equally well within a considerable range, to 

 vibrations of very different wave-lengths. Had it a fundamental 

 tone of its own, we should be distracted by the prominence of this 

 note in most of the sounds we hear. When sounds impinge on the 

 solids of the head, as when a watch is held between the teeth, the 

 membrana tympani is still functional. Vibrations are conveyed 

 from the temporal bone to it and hence pass in the usual way, 

 in addition to those transmitted directly from the bone to the peri- 

 lymph. 



The auditory ossicles. The malleus, the handle of which de- 

 scending forwards and inwards, is attached to the membrana tym- 

 pani, and the incus, whose long process is connected by means of its 

 os orbiculare or lenticular process and the stapes to the fenestra 

 ovalis, form together a body which rotates round an axis, passing 

 through the short process of the incus, the bodies of the incus and 

 malleus, and the processus gracilis of the malleus. When the mal- 

 leus is carried inwards, the incus moves inwards too, and when the 

 malleus returns to its position, the incus returns with it, the peculiar 

 saddle-shaped joint with its catch teeth permitting this movement 

 readily, but preventing the stapes being pulled back when the mem- 

 brana tympani with the malleus is, for any reason, pushed outwards 

 more than usual ; the joint then gapes, so as to permit the malleus 

 to be moved alone. Various ligaments, the superior or suspensory, 



