538 THE HUMAN BODY. 



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 beloAv 

 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. 



The Internal Ear. The labyrinth consists primarily of 

 chambers and tubes hollowed out in the temporal bone and 

 inclosed by it on all sides, except for the oval and round 



FIG. 145. Casts of the bony labyrinth. A, left labyrinth seen from the outer 

 side; B, right labyrinth from the inner side; C, left labyrinth from above; Fc, 

 round foramen; Fv, oval foramen; h, horizontal semicircular canal; ha, its 

 ampulla; vaa, ampulla of anterior vertical semicircular canal; vpa, ampulla 

 of posterior vertical semicircular canal ; vc, conjoined portion of the two ver- 

 tical canals. 



foramens on its exterior, and certain apertures on its inner 

 side by which blood-vessels and branches of the auditory 

 nerve enter; during life all these are closed water-tight in one 

 way or another. Lying in the bony labyrinth thus consti- 

 tuted, are membranous parts, of the same general form but 

 smaller, so that between the two a space is left; this is filled 

 with a watery fluid, called the perilympli; and the mem* 

 Iranous internal ear is filled by a similar liquid, the endo* 

 lymph. 



The Bony Labyrinth. The bony labyrinth is described 

 in three portions, the msti&ule, the semicircular canals, 

 and the cochlea; casts of its interior are represented from 



