THE MIDDLE EAR 877 
like tympanic opening of the auditive tube. Above this and incompletely separated 
from it by a thin 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 im the fenestra ves- 
tibuli. 
The malleus ov hammer, the largest of the ossicles, consists of a head, neck, 
handle, and two processes. The bead (Capitulum mallei) is situated in the epi- 
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Fic. 704.—Avtprrory OsstcLes AND MEMBRANA Tic. 705.—RicuHr Avprrory Ossicitres, ENLARGED as IN- 
TymPaNI; MepraLt View, ENLARGED. pIcaTED BY Lines GIVING THEIR AcTUAL LENGTH. 
1, Annulus tympanicus; 2, membrana tym- a, Malleus: 1, head; 2, neck; 3, handle; 4, long proc- 
pani; 3, malleus; 4, incus; 5, stapes. (After ess; 5, muscular process. b, Incus: 1, body; 2, short 
Ellenberger, in Leisering's Atlas.) branch; 3, long branch; c, os lenticulare. c, Stapes; 1, 
head; 2, crus; 3, base; 4, attachment of stapedius. (After 
Ellenberger, 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. 
