890 THE AUDITORY OSSICLES AND THEIR MUSCLES. 



vibration on the sounding of its fundamental tone is diminished. There- 

 fore, the membrane is better adapted to respond to the vibrations of 

 different wave-lengths, although to a lessened degree. The damping 

 also prevents effectually disturbing after- vibrations. (2) The sym- 

 pathetic vibrations of the membrane must be small, in accordance with 

 its diminutive size. Further, these slight elongations are quite sufficient 

 to transfer the sound-movement to the extremely delicate end-organs of 

 the auditory nerves. In fact in the description of the auditory ossicles 

 it will be seen that there exist other arrangements that still further 

 diminish the oscillations of the tympanic membrane. 



As v. Helmholtz has pointed out, the increased associated vibration of the 

 tympanic membrane when its own note is sounded is not completely equalized 

 by the damping arrangement described. He calls attention to the fact that to most 

 men the tones of the sixth octave e and g are especially piercing and shrill (for 

 example the shrill tones of the cricket), and he supposes, therefore, that the 

 individual note of the auditory apparatus, including the tympanic membrane, 

 lies in this region, so that the membrane vibrates strongly in unison when these 

 tones are sounded. In general the sounds that are designated as piercing seem 

 especially to cause the fundamental vibrations of the auditory apparatus. 



According to Kessel, the individual portions of the tympanic membrane 

 have an independent relation to sounds: the shortest radial fibers in the upper 

 portion of the anterior segment and in the upper segment vibrate with the highest 

 tones, while the longest fibers on the posterior segment vibrate with the deepest 

 tones. Noises are supposed to be transmitted by the upper portion of the posterior 

 segment; therefore, deep tones are readily disturbed and extinguished by noises. 



According to Fick the tympanic membrane possesses, in addition to the prop- 

 erty of taking up all vibrations almost equally well, also that of a resonance- 

 apparatus, that is, it admits of an accumulation of the energy of successive vibra- 

 tions. It owes this property to its funnel-shaped retracted form, as well as to 

 the radially placed, rigid handle of the malleus, as artificially constructed models 

 have shown. 



Pathological. Thickening and inflexibility of the tympanic membrane 

 diminish the acuity of hearing, in consequence of the lessened vibrating ability 

 of the membrane. Perforations and loss of substance have the same effect. In 

 cases of extensive destruction, artificial eardrums have been inserted into the 

 canal, the vibrations of which replace to a certain extent those of the lost mem- 

 brane. 



THE AUDITORY OSSICLES AND THEIR MUSCLES. 



The auditory ossicles have a double function: (i) They transmit 

 the vibrations of the tympanic membrane to the endolymph of the 

 labyrinth by means of the chain that they form. (2) They afford 

 points of attachment for the muscles of the middle ear, which through 

 the bones alter the tension of the tympanic membrane and the pressure 

 on the fluid of the labyrinth. 



Figs. 319 and 320 show the form and position of the ossicles, which constitute 

 an articulated chain connecting the tympanic membrane (M) with the labyrin- 

 thine fluid through the malleus (h) , the incus (a) , and the stapes (S) . The manner 

 in which the ossicles move deserves especial attention. The handle of the malleus 

 Fig. 320, ) is firmly attached to the fibers of the tympanic membrane. In 

 addition, the malleus is fixed by ligaments that regulate the direction of its move- 

 ments. Two of them, the anterior ligament of the malleus, arising from the 

 processus Fohanus, and the posterior ligament, arising from a small crest on the 

 neck of the malleus, form together a common axial band, which crosses the tym- 

 panic cavity from behind forward, consequently parallel to the surface of the 

 tympanic membrane. The neck of the malleus lies between the insertions of 

 the two ligaments. The united ligament determines the axis of rotation for the 

 movement of the malleus. When the handle of the malleus is drawn inward, 

 its head must naturally make the opposite outward movement. The incus 



