754 



THE SENSE ORGANS AND SKIN OF THE HOKSE 



tympanic opening of the Eustachian tube. Al)ove this and incompletely separated 

 from it by a thin plate of bone is the semicanal for the tensor tympani muscle. 



The superior wall or roof (Paries tegmentalis) is crossed in its inner 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 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 inferior 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 

 vestibuli. 



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

 handle, and two processes. The head (Ciipitulum mallei) is situated in the 

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



Fig. 567. — Auditory Ossicles and Membrani 

 Tympani, Inner View, Enlarged. 

 1, .\imulus tympanicus; 2, membiana tym- 

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

 berger, in Leisering's .-Vtlas.) 



Fig. 568. — Right Auditory Ossicles, Enlarged as In- 

 dicated BY Lines Giving their Actual Length. 

 a, Malleus; 1, head; £, neck; S, handle; 4, long pro- 

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

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

 2, cru.s; 3, base; 4, attachment of stapedius. (After Ellen- 

 berger, in Leisering's Atlas.) 



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

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

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

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

 its entire length to the meml)rana tympani. On its inner surface, near the upper 

 end, there is a slight ])r()j('cti()n 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 outer side of the neck, and is attached 

 to the u]ii>er part of the membrana tympani. 



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

 said to reseml)le a 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 (Cms 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 bre\-e) projects chiefly backward, and is attached to the wall of the recess by 

 a small ligament. 



The stapes or stirrup consists of a head, two crura, and a base. The head 



