276 THE ANATOMY OF THE HORSE. 



cupped towards the outer ear ; while its inner surface is convex, and 

 has the handle of the malleus (the outermost ossicle) attached to it. 

 The surfaces of the membrane are inclined so that the outer surface 

 looks somewhat downwards, and the inner upwards. In structure the 

 membrane comprises (1) a middle fibrous stratum, the fibres being 

 arranged both radially and circularly, with (2) an outer and (3) an 

 inner epithelial covering. The roof and the floor of the tympanum 

 present nothing of interest. The former is the more extensive. The 

 anterior extremity of the chamber shows a fissure by which air is 

 admitted from the Eustachian tube. Through this opening also the 

 mucous lining of the cavity is continuous with that of the Eustachian 

 tube. The posterior extremity and part of the floor and outer wall 

 communicate with the cellular spaces of the mastoid protuberance. 



The Auditory Ossicles. — There are three of these, viz., the malleus, 

 the incus, and the stapes. 



The Malleus, named from its resemblance to a hammer, is the 

 largest bone. It possesses a head, a handle, and two processes. The 

 head is articulated by a synovial joint to the stapes. The handle is 

 fixed on the inner surface of the membrana tympani. The long process 

 is slender, and projects forwards to be fixed in a slit of the petrous 

 temporal. The short process is a mere projection of the root of the 

 handle, and is fixed to the membrana tympani. 



The Incus is named from its supposed resemblance to an anvil, but 

 it has more likeness to a human bicuspid tooth. It presents a body, 

 and two ywocesses, or crura. The body has a saddle shaped articular 

 facet for the malleus. The short process is directed backwards to be 

 fixed to the wall of the tympanum. The long jwocess curves downwards 

 and inwards to terminate in a rounded point — the orbicular process, 

 which articulates with the head of the stapes. 



The Stapes is stirrup-shaped. It is the smallest bone, and pos- 

 sesses a head, a neck, a base, and two crura. The head is depressed 

 for articulation with the orbicular process, and is succeeded by the 

 slightly constricted neck. The base is a thin plate which closes the 

 fenestra ovalis. The crura are slender rods of bone connecting the 

 base and the neck. 



Muscles of the Ossicles. — These are two — the tensor tympani and 

 the stapedius. (The so-called laxator tympani is now believed to 

 be a ligament.) 



The Tensor Tympani arises from the petrous temporal bone near 

 the Eustachian orifice, and it is inserted by a slender tendon into 

 the handle of the malleus near its root. 



Action. — To tense the membrana tympani. 



The Stapedius arises within the pyramid — a small process of bone at 

 the back of the tympanum. Issuing from the pyramid, it is inserted 



