268 THE ANATOMY OF THE HORSE. 



The rim of the membrane is fixed in a groove of the bone. The mem- 

 brane is slightly 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 exten- 

 sive. 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 inciis, and the stapes. 



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

 largest bone. It possesses a head, a handle, and two j)r<^cesses. The 

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

 fixed on the inner surface of the membrana tympani. The lony jjrocess 

 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 2^rocesses, or crura. The body has a saddle-shaped articular facet for 

 the malleus. The short p7-ocess is directed backwards to be fixed to the 

 wall of the tympanum. The ^o?iy^j);-oef«5 curves downwards and inwards 

 to terminate in a rounded point — the orbicular jyrocess, which articulates 

 with the head of the stapes. 



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

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

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

 constricted necl: 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 pyrcnnid — a small process of bone at 



