CHAP, iv.] HEARING. 1319 



Waves of sound may be and to a certain extent are conducted 

 in a direct manner to the perilymph, through the tissues, especially 

 the harder bony tissues, of the head, reaching the perilymph across 

 its bony envelope. The vast majority however of the waves of 

 sound which fall upon the head travel through the medium of the 

 air, and in order to reach the perilymph have to pass from a 

 gaseous medium, the air, into the solid and liquid media of the 

 head. Now the vibrations of particles constituting waves of sound 

 are not readily communicated from a gaseous to a liquid or solid 

 medium ; special conditions are required to effect this. The 

 transference of sound from the air to the perilymph is attended 

 with considerable difficulty ; and the parts of the ear which we have 

 spoken of above as constituting the middle and outer ear, serve as 

 an acoustic apparatus for facilitating this transference and thus 

 bringing the aerial waves to act on the auditory epithelium, the 

 action of the apparatus being somewhat as follows. 



Waves of sound falling on the side of the head reach the 

 tympanic membrane by the external meatus, and throw that 

 membrane into vibrations. These vibrations are transmitted 

 through the chain of ossicles to the membrane of the fenestra 

 ovalis and so to the perilymph lying on its far side; sweeping 

 over the perilymph in its continuous cavity the waves eventually 

 break upon the fenestra rotunda, having on their way affected the 

 auditory epithelium. We have first to inquire how this subsidiary 

 acoustic apparatus performs its work. 



The Conducting Apparatus of the Tympanum. 



811. The auditory ossicles. The malleus, or hammer bone 

 (Fig. 164 A and D), has a rounded head (cp.) bearing a peculiar 

 saddle-shaped surface for articulation with the incus, and ends 

 below in a tapering process, the manubrium, or handle (mbr.) by 

 which it is attached, in a manner to be described presently, to the 

 inner surface of the tympanic membrane. To the handle at its 

 upper part is attached on the inner side the tendon of the tensor 

 tympani muscle (Figs. 167, 170, 173 T.T.) ; and at the top of the 

 handle, on the outer side, is a short blunt process, processus brews, 

 (Fig. 164 A p.b.) which as we shall see abuts on a particular part 

 of the tympanic membrane. Still higher up is a thinner and 

 generally much longer process (Fig. 164 A p.f. and Fig. 173 p.f.), 

 processus gracilis or Folianus, the base of which with part of the 

 thick neck of the malleus above serves for the attachment of liga- 

 ments, and the end of which is inserted into a fissure in the bony 

 wall of the cavity. 



The incus, ambos or anvil bone (Fig. 164 B and D) has a 

 less well defined head, bearing a surface for the articulation with 

 the malleus, and a short body which immediately divides into two 



