THE MIDDLE EAR 



877 



like tympanic opening of the auditive tube. Above this and incompletely separated 

 trom it by a thm plate of bone is the semicanal for the tensor tympani muscle. 



The tegmental wall or roof (Paries tegmentalis) is crossed in its medial part by 

 the facial nerve; here the facial canal is more or less deficient ventrally, and the 

 nerve is covered by the mucous membrane of the tympanum. 



The posterior, mastoid wall (Paries mastoidea) presents nothing of importance; 

 a tympanic antrum and mastoid cells, such as are found behind the tympanic 

 cavity proper in man and many animals, are not present in the horse. 



The tympanic wall or floor (Paries tympanica) is concave and thin. It is 

 crossed by delicate curved ridges, which radiate from the greater part of the annulus 

 tympanicus. 



The auditory ossicles (Ossicula auditus) form a chain which extends from the 

 outer to the inner wall of the cavity. They are named, from without inward, the 

 malleus, the incus, the os lenticulare, and the stapes. The first is attached to the 

 inner surface of the tympanic membrane and the last is fixed in the fenestra ves- 

 tibuli. 



The malleus or hammer, the largest of the ossicles, consists of a head, neck, 

 handle, and two processes. The head (Capitulum mallei) is situated in the epi- 



C"-^^' 



Fig. 704. — Auditory Ossicles and Membbana 

 Tympani; Medial View, Enlarged. 

 1, Annulus tympanicus; 2, membraua tym- 

 pani; S, malleus; 4-, incus; 5, stapes. (After 

 EUenberger, in Leisering's Atlas.) 



Fig. 705. — Right Auditory Ossicles, E.vlarged as In- 

 dicated BY Lines Giving their Actual Length. 

 a, Malleus: 1, head; 2, neck; 3, handle; 4, long proc- 

 ess; .5, muscular process, b. Incus: 1, body; 2, short 

 branch; 3, long branch; c, os lenticulare. c, Stapes; 1, 

 head; ;S, cms; 3, base; .f, attachment of stapedius. (After 

 EUenberger, in Leisering's Atlas.) 



tympanic recess. It is smooth and convex above and in front, and presents on 

 its postero-medial aspect a concave facet for articulation with the body of the 

 incus. The neck (Collum mallei) is the constricted part below the head; its medial 

 surface is crossed by the chorda tympani nerve. The handle (Manubrium mallei) 

 is directed downward, inward, and a little forward from the neck, and is attached 

 along its entire length to the membrana tympani. On its medial surface, near the 

 upper end, there is a slight projection to which the tendon of the tensor tympani 

 muscle is attached! The long process (Processus longus) is a pointed spicule which 

 projects forward from the neck toward the petro-tympanic fissure. The short 

 process (Processus brevis) is a slight projection of the lateral side of the neck, and 

 is attached to the upper part of the membrana tympani. 



The incus or anvil is situated chiefly in the epitympanic recess. It may be 

 said to resemble in miniature a human bicuspid tooth with two divergent roots, and 

 consists of a body and two processes. The body (Corpus incudis) articulates with 

 the head of the hammer. The long process (Crus longum) projects downward 

 from the body and then curves inward; its extremity has attached to it a small 

 nodule of bone, the os lenticulare, which articulates with the head of the stapes. 

 The short process (Crus breve) projects chiefly backward, and is attached to the 

 wall of the recess by a small ligament. 



