158 TOPOGRAPHICAL ANATOMY OF THE 



the facial canal, the bony wall of which is here usually more or less 

 defective ; while the floor, formed by the osseous bulla, is concave and 

 thin, and crossed by curved plates of bone radiating from the bon}'^ ring 

 to which the membrana tympani is attached. The posterior wall 

 (paries mastoidea) of the horse's tympanum presents no noteworthy 

 features. 



Dissection. — Snip all round the outer wall of tympanum, so that it 

 may be removed. In the process of the operation an irregular space, the 

 epitympanic recess (recessus epitympanicus), containing the incus and the 

 head of the malleus, will be revealed in connection with the roof of 

 the cavity. 



The membrana tympani is the thin membrane, composed of fibrous 

 tissue covered laterally with skin, and medially with mucous membrane, 

 that separates the external acoustic meatus from the cavity of the 

 tympanum. It is oval in outline, and is placed obliquely, so that it 



Incus. 



- Malleus. 



Stapes. 



Membrana tympani. — ^Yrt" 



Fig. 71. — The auditory ossicles and the 

 membrana tympani. 



forms an angle of about 30° with the inferior wall of the meatus. The 

 circumference of the membrane is slightly thickened and attached to a 

 ring of bone, the annulus tympanicus. The handle of the malleus is 

 attached to the membrane, and draws it inwards, so that the lateral 

 surface is concave and the medial correspondingly convex. 



The auditory ossicles (ossicula auditus), three in number, namely 

 the malleus, the incus and the stapes, cross the cavity of the tympanum 

 from the membrana tympani to the fenestra vestibuli. The malleus'^ 

 is the outermost and the largest of the three bones, and consists of a 

 large upper part or head (capitulum mallei), below which is a neck 

 (collum mallei), and a manubrium (manubrium mallei)- and two 

 processes. The head carries a diarthrodial articular surface for contact 

 with the incus. The neck is crossed medially by the chorda tympani 

 nerve. The long manubrium is attached to the membrana tympani, as 

 is also the short lateral process (processus lateralis). The anterior 



1 Malleus [L.], a hammer. 



2 Manubrium [L.], a handle. 



