HEAD AND NECK OF THE HORSE 159 



process (processus anterior) springs from the neck, is very slender, and is 

 attached by ligament to the wall of the tympanic cavity. 



The incus^ bears some resemblance to a human bicuspid tooth with 

 widely divergent roots, 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 tympanum ; while the long crus (crus longum) ends in 

 a nodule of bone (lenticular process), frequently separable, that 

 articulates with the head of the stapes by a diarthrodial joint. 



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

 stapedis), two crura (crus anterius et crus posterius), and a base. The 

 base (basis stapedis) is connected with the ends of the crura and is in 

 the form of an oval plate that is lodged in the fenestra vestibuli, and 

 connected with the margin of the opening by an annular ligament in 

 such a manner that some amount of movement is possible. 

 Head. Articular surface. 



. . - V ^ ■■* . ' Neck. Body. Articular surface. 



Anterior process. - 



Muscular process. — 



Manubrium. _ 



Short crus. 



Long crus. Process. 



Fig. 72. — The auditory ossicles ( x 3). A = malleus; B = incus; C = stapes. 



Connected with the auditory ossicles are two small muscles (musculi 

 ossiculorum auditus). The tensor of the tyinpanu')n (m. tensor tym- 

 pani) is short and conical, with an origin from the bone just above the 

 opening into the auditory tube. A slender tendon connects the muscle 

 with the manubrium of the malleus close to its root. The stapedius 

 muscle (m. stapedius) arises from an eminence on the posterior wall of 

 the tympanic cavity, and is inserted into the head of the stapes. 



The whole of the tympanic cavity, including its recesses, is lined by 

 mucous membrane (tunica mucosa tympanica), which is continuous with 

 that of the pharynx through the auditory tube. It is closely adherent 

 to the periosteum of the bone, covers the auditory ossicles and their 

 muscles and the chorda tympani nerve, and closes the defect in the 

 floor of the facial canal. 



The dissector should open up the whole length of the facial canal 



(canalis facialis), beginning at the stylomastoid foramen and workino- 



towards the internal acoustic meatus. He will then find that the canal 



curves round the base of the cochlea and the posterior part of the 



1 Incus [L.], an anvil. 2 (j^^g ["l ]^ g, leg, a limb, 3 stapes [L.], a stirrup. 



