DISSECTION OF THE DOG 231 



membrane (membrana tympani). On the inner wall, which separates the 

 cavity from the labyrinth of the internal ear, is a well-marked projection, 

 the promontory (promontorium) , above and slightly in front of which is the 

 fenestra of the vestibule. Below the promontory is the fenestra of the cochlea. 

 Leading from the anterior part of the tympanic cavity is the narrow opening 

 of the auditory tube of Eustachius. 



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

 may be removed. In the process 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 tympanic 

 cavity. 



Ossicula auditus. — The auditory ossicles are three in number, namely, 

 the malleus, the incus, and the stapes. The malleus is the outermost and the 

 largest of the three bones. It 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 an articular surface for contact 

 with the incus. The manubrium is attached to the tympanic membrane, as 

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

 anterior) is extremely slender, and is attached by ligament to the wall of the 

 tympanic cavity. 



The incus bears some resemblance to a human bicuspid tooth, and possesses 

 a body and two crura. The body (corpus incudis) carries a saddle-shaped 

 surface for articulation with the head of the malleus. The short crus (crus 

 breve) is movably connected with the wall of the tympanum ; while the 

 long crus (crus longum) articulates with the head of the stapes. 



The stapes, a stirrup-shaped bone, consists of a head (capitulum stapedis), 

 two crura (crus anterius etcrusposterius), and a base. The base (basis stapedis) 

 is in the form of an oval plate lodged in, and so connected with the margin of 

 the fenestra vestibuli that some amount of movement can take place. 



Mxjsculi ossiculobum AUDiTus. — Connected with the auditory ossicles 

 are two small muscles. The tensor muscle of the tympanum (m. tensor tympani) 

 is short and conical, with an origin from a depression just anterior to the 

 epitympanic recess. A slender tendon connects the muscle with the manubrium 

 of the malleus close to its root. The stapedius muscle (m. stapedius) is inserted 

 into the head of the stapes. 



Canalis facialis. — The dissector should open up the whole length of 

 the facial canal, beginning at the stylomastoid foramen and working toward 

 the internal acoustic meatus. He will then find that the canal curves round 

 the base of the cochlea and the posterior part of the internal wall of the 

 tympanum. 



