142 



ESSENTIALS OF PHYSIOLOGY. 



shaped joint of such a nature that, when the head of the malleus moves 

 outwards, a cog-like process upon it is locked in a corresponding depres- 

 sion on the incus. If, however, the head of the malleus moves inwards, 

 the joint surfaces separate, and in this way traction on the membrane 

 enclosing the internal ear is avoided. The incus, or anvil bone, consists 

 of a body and two processes (crura). The body articulates with the 

 malleus, and the longer of the two processes with the stapes. The 

 stapes, or stirrup bone, articulates by its head with the long crus of 

 the incus, and its base is attached to the membrane which closes the 



Semicircular canals 



opening into 



utricle. 



Cochlea. ~j. 



FIG. 45. Scheme of ear. (After Landois.) 



fenestra ovalis. The head of the malleus is attached by an anterior, 

 a superior, and a lateral ligament to the wall of the tympanic cavity, 

 and the short process of the incus is attached by a ligament to the 

 posterior wall of the cavity. As the result of these attachments the 

 malleus and incus can be rotated only around an axis which passes 

 through the processus gracilis of the malleus and the short process of 

 the incus. This movement of rotation takes place when the handle 

 of the malleus moves inwards with the membrana tympani, the head of 

 the malleus and body of the incus moving outwards, and the long 

 process of the incus moving inwards and exerting pressure through the 

 stapes on the membrane in the fenestra ovalis. The movements of the 



