THE SKULL THE CRANIUM 



221 



and on the inside by the bridge of bone extending to the petrous. On 

 the inside of the bridge is the slit for the stapedius tendon, and then 

 comes the petrous, forming the inner part of the posterior wall. The 

 foramen above, just median to the interbullar opening, is the fenestra 

 ovalis. 



THE EAR-OSSICLES. 



The ear-ossicles are three very small bones, known as the malleus. 



/ 



the incus, and the stapes, which form a chain across the upper part of 

 the tympanum, from the tympanic membrane, or drum of the ear, 

 which closes the external auditory meatus, to the membranous covering 

 of the fenestra ovalis of the petrous. The malleus is the anterior ossicle 

 (Fig. 150) ; its long handle projects downward, forward, and inward on 

 the inner surface of the tympanic membrane ; its rounded head is lodged 



FIG. 151. 



Inferior Process of Incus.- -- 



Auditory Meatus 

 Handle of Malleiis, 



Head of Stapes. 



Base of Stapes. 



Head of Incus. 

 Head of Malleus. 



Long Process of Malleus. 

 Ectotymiianic. 



LEFT EAR OSSICLES IN POSITION. INFERIOR MEDIAL ASPECT. 

 (Petrous and Lower Walls of Bulla removed. Enlarged Four Times.) 



in a fossa on the inferior surface of the squamous. The incus lies 

 almost directly behind the malleus, with the upper part of which it 

 articulates. The stapes is directed almost at right angles to the other 

 ossicles (Fig. 151), crossing the tympanum from without inward and 

 upward. Its narrow outer end is attached to the incus, and its broad 

 inner end, which is on a higher level, fits into the rim of the fenestra 

 ovalis. The vibrations of the tympanic membrane move the long lever 

 of the malleus and rock the heads of the malleus and incus on a longi- 

 tudinal axis, and thereby push the broad end of the stapes slightly 



