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 processes, 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 tymparmm. The long process curves downwards and inwards 

 to terminate in a rounded point — the orbictdar p)rocess, which articulates 

 with the head of the stapes. 



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

 a head, a nech, a base, and two crura. The head is' depressed for articula- 

 tion with the orbicular process, and is succeeded by the slightly 

 constricted nech. The bcise 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 



