CHAP, iv.] HEARING. 987 



certain parts of the brain along the auditory nerve ; and these 

 auditory impulses are generated through vibrations, or rhyth- 

 mically repeated variations of pressure which we call, 'waves of 

 sound,' in some way or other acting upon the terminations of 

 the auditory fibres in the auditory epithelium. The waves 

 of sound gain access to the epithelium by means of the peri- 



FIG. 173. THE OSSICLES IN POSITION. Magnified four times. (After Hensen.) 



The figure represents a section through tympanum in the line of the long axis 

 of the malleus and incus ; the short process of the incus p'b' has been cut through. 



T.C. The tympanic cavity, mbr. handle of malleus, u. umbo. p. b. short 

 process of the malleus shewn in dotted outline as pushing outwards the rnem- 

 brana flaccida. T. T. the attachment of the tendon of the tensor tympani. Ig. 

 the attachment of the external ligament of the malleus. Ig.s. the superior liga- 

 ment of the malleus, t.t. the teeth of the incus, p'l'. the long process, shaft, of 

 the incus. St. the stapes. 



lymph, passing probably in some parts directly through the 

 dermis of the membranous sac to the overlying epithelium, 

 and being in other parts transmitted to the endolymph from 

 the perilymph across the membranous walls, and acting on the 

 epithelium through the endolymph. 



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

 in a direct manner to the perilymph, through the tissues, espe- 

 cially 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 



