HEARING 



conducts the vibrations of the tympanic membrane to the fluid of 

 the internal ear, which in its turn excites the receptor mechanism. In 

 the frog we find only a single cartilaginous rod the columella. A 

 chain of bones, however, has considerable me- 

 chanical advantages. The handle of the hammer 

 is inserted into the tympanic membrane, and 

 leads upwards to the head, which is placed above 

 the level of the tympanic membrane, and is fixed 

 in position by ligaments which pass to a fissure in 

 the bone, one from a delicate forward pointing 

 process the processus gracilis another from the 

 head of the malleus to the roof. This is one of 

 the fixed points about which the bones rotate. p IG . 374. VIEW 

 The anvil is in such a position that its conical TYMPANIC 

 process points downwards. This process ends by BRAXK - 

 bending inwards to a flattened knob the 

 lenticular process. From the base of the anvil, ligaments pass to 

 the posterior wall of the cavity, while the head of the- anvil articulates 

 with the malleus. The stapes is fixed by the head of the stirrup to 

 the lenticular process of the anvil, and passes horizontally to be 

 inserted by the foot of the stirrup into the oval window. 



FIG. 375. METHOD OF EXAMINATION OF THE EAR DRUM BY REFLECTED LIGHT. 



The malleus and incus rotate as one bone round a horizontal axis. 

 When the handle of the malleus is pushed inwards, the head of the 

 bone moves outwards, carrying with it the body of the incus, excessive 

 movement being prevented by the ligaments of the malleus. The 

 descending process of the incus is thereby moved inwards, and pushes 

 the stapes against the fenestra ovalis. The chain acts as a bent lever, 

 so that, when the malleus moves a certain distance, the stapes moves 

 but two-thirds of that distance; the resulting impact, owing to the 

 order of the lever, is increased by half, and since the area of the 

 tympanic membrane is about twenty times as great as that of the 

 base of the stapes, the force falling upon the oval window at the base 

 of the stapes is about thirty times as great as that falling on the tym- 

 panic membrane at the umbo. For this reason, it is easy to understand 

 that hearing is seriously interfered with when the action of the ossicles 

 is deranged by middle-ear disease. 



