AUDITORY OSSICLES 555 



diamete^of about one-third of an inch. It is placed posterior 



to the tympanum, and communicates by a relatively large 



opening, the aditus, with the upper part of that cavity. It 



lies at a depth of about half an inch from the surface of the 



skull in the adult, but in the child it is placed much more 



superficially. It is lined with mucous membrane, which is 



continuous with the lining membrane of the tympanum. The 



mastoid portion of the temporal bone also is occupied by air- 



spaces, the air-cells^ which may 



extend downwards into the mas- f 



toid process. They are continu- 



ous with one another and with B^lil 



the tympanic antrum, and are 



lined by a continuation of the 



same mucous membrane. 



Tympanic Mucous Membrane. 

 The tympanum is lined 

 throughout with a thin mucous 

 membrane which is continuous 

 with the mucous membrane of 

 the pharynx. As already men- 

 tioned, it forms the innermost 

 layer of the membrana tympani, FIG. 239. Left Malleus and Incus. 

 and it is prolonged posteriorly 



intO the tympanic antrum and - Tendon oftensorjympani. 



mastoid air-cells. It covers the 3- Long cms of the incus. 



. , , , . . , 4. Short crus of the incus. 



OSSlCleS alSO, and it invests the 6. Anterior process of the malleus. The 



tendons of the stapedius and 



tenSOr tympani mUSCleS. incus with the extremity of the 



. J L manubnum of the malleus. 



Ossicula Auditus. The audi- 

 tory ossicles are the malleus, the incus, and the stapes. 



The malleus presents a head, a neck, a manubrium, and 

 two processes termed the processus lateralis and the pro- 

 cessus anterior. The head is large and rounded. It is 

 directed upwards, and lies above the level of the mem- 

 brana tympani, in the recessus epitympanicus, close to the 

 roof of the tympanum. On its posterior aspect there is 

 a notch-like articular surface, for articulation with the body 

 of the incus. The manubrium is attached to the fibrous 

 layer of the membrana tympani. The processus lateralis (O.T. 

 brevis] is a stunted projection which springs from the root of 

 the manubrium. It is directed laterally, and abuts against the 



