230 THE HUMAN BODY 



cavity, forms an anteroposterior axial ligament, on which the 

 malleus can rotate slightly, so that the handle can be pushed in 

 and the head out and vice versa. If a pin be driven through Fig. 70 

 just below the neck of the malleus and perpendicular to the paper 

 it will very fairly represent this axis of rotation. Connected with 

 the malleus is a tiny muscle, called the tensor tympani; it is inserted 

 on the handle of the bone below the axis of rotation, and when it 

 contracts pulls the handle in and tightens the drum membrane. 

 Another muscle (the stapedius) is inserted into the outer end of the 

 stapes, and when it contracts fixes the bone so as to limit its range 

 of movement in and out of the fenestra ovalis. 



Functions of the Auditory Ossicles. When the air in the ex- 

 ternal auditory meatus is condensed it pushes in the tympanic 

 membrane which carries with it the handle of the malleus. This 

 bone then slightly rotates on the axial ligament and, locking 

 into the incus where the two bones articulate, causes the long 

 process (Jl, Fig. 70) of the latter to move inwards. The incus 

 thus pushes in the stapes; the reverse occurs when air in the au- 

 ditory passage is rarefied. Aerial vibrations thus set the chain of 

 bones swinging, and push in and pull out the base of the stapes, 

 which sets up waves in the fluid of the labyrinth. This fluid being 

 chiefly water, and practically incompressible, the end of the stapes 

 could not work in and out at the oval foramen, were the labyrinth 

 elsewhere completely surrounded by bone: but the membrane 

 covering the round foramen bulges out when the base of the stapes 

 is pushed in, and vice versa; and so allows of waves being set up in 

 the labyrinthic fluid. These correspond in period and form to 

 those in the auditory meatus; their amplitude is determined by the 

 extent of the vibrations of the drum membrane. 



The form of the tympanic membrane causes it to transmit to its 

 center, where the malleus is attached, vibrations of its lateral 

 parts in diminished amplitude but increased power; so that the 

 tympanic bones are pushed only a little way but with considerable 

 force. Its area, too, is about twenty times as great as that of the 

 oval foramen, so that force collected on the large area is, by push- 

 ing the tympanic bones, all concentrated on the smaller. The 

 ossicles also form a bent lever (Fig. 70) of which the fulcrum is at 

 the axial ligament and the effective outer arm of this lever, is about 

 half as Jong again as the inner, and so the movements transmitted 



