878 THE SENSE ORGANS AND SKIN OF THE HORSE 



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

 (Capitulum stapedis) is directed outward and articulates Avith the os lenticulare. 

 The crura, anterior and posterior (Crus anterius, posterius), are directed inward 

 from the head, and join the ends of the base. The base or foot-plate (Basis stapedis) 

 occupies the fenestra vestibuli, to which it is attached. The space between the 

 crura and the base is closed by a membrane. 



The articulations and ligaments of the auditory ossicles comprise: (1) A 

 diarthrodial joint between the head of the malleus and the body of the incus (Articu- 

 latio incudomalleolaris), enclosed by a capsule. (2) An enarthrosis between the os 

 lenticulare and the head of the stapes (Articulatio ancudostapedia) , also surrounded 

 by a capsule. (3) The base of the stapes is attached to the margin of the fenestra 

 vestibuli by a ring of elastic fibers (Lig. annulare baseos stapedis). (4) Small 

 ligaments attach the head of the malleus and the short crus of the incus to the roof 

 of the epitympanic recess. (5) The axial ligament (of Helmholtz) attaches the 

 neck of the malleus to a small projection (Spina tympanica anterior) above and in 

 front of the annulus tympanicus. 



The muscles of the auditory ossicles are two in number, viz., the tensor tym- 

 pani and the stapedius. The tensor tympani arises from the upper wall of the 

 osseous Eustachian tube, and ends in a delicate tendon which bends outward and 

 is inserted into the handle of the malleus near its upper end. When it contracts, 

 it draws the handle of the malleus inward and tenses the membrana tympani; it 

 probably also rotates the malleus around its long axis. It is innervated by the 

 motor part of the trigeminus through the otic ganglion. The stapedius arises from 

 a small prominence (Eminentia pyramidalis) of the posterior wall of the tympanum, 

 runs forward on the facial nerve, and is inserted into the neck of the stapes. Its 

 action is to draw the head of the stapes backward and rotate the anterior end of the 

 base outward, thus tensing the annular ligament. It is innervated by the facial 

 nerve. 



The tympanic mucous membrane (Tunica mucosa tympanica) is continuous 

 with that of the pharynx and the guttural pouch through the auditive or Eustachian 

 tube. It is thin, closely imited with the underlying periosteum, and is reflected 

 over the ossicles, ligaments, and muscles, the chorda tympani, and the facial nerve 

 in the open part of the facial canal. It contains minute lymph nodules and small 

 mucous glands (Glandulse tympanicse). The epithelium is in general columnar 

 ciliated, but over the membrana tympani, ossicles, and promontory it is flattened. 



The artery of the tympanum is the stylo-mastoid, a small vessel which arises 

 from the posterior auricular branch of the internal maxillary artery. It enters the 

 tympanum through the stylo-mastoid foramen, and forms a circle around the tym- 

 panic membrane. The nerves of the mucous membrane come from the tympanic 

 plexus. 



The Auditive or Eustachian Tube 

 The auditive or Eustachian tube (Tuba auditiva) extends from the tympanic 

 cavity to the pharynx; it transmits air to the former and equalizes the pressure 

 on the two surfaces of the membrana tympani. It is directed forward, downward, 

 and slightly inward, and is four to five inches (ca. 10 to 12 cm.) in length. Its 

 posterior extremity lies at the medial side of the root of the muscular process of 

 the petrous temporal, and communicates with the anterior part of the tympanic 

 cavity by the small, slit-like t3rmpamc opening (Ostium tympanicum tubs auditivse). 

 For a distance of about a quarter of an inch (ca. 6-7 mm.) in front of this opening 

 it is a complete tube, with a curved lumen which is little more than a capillary space. 

 Further forward it has the form of a plate which widens anteriorly and is curved to 

 enclose a narrow groove that opens ventrally into an extensive diverticulum termed 

 the guttural pouch. The pharyngeal opening (Ostium pharyngeum tubse audit- 



