THE EUSTACHIAN TUBE 755 



(Capitulum stapedis) is directed outward and articulates with 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 be- 

 tween 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), inclosed by a capsule. (2) An enarthrosis between the os 

 lenticulare and the head of the stapes (Articulatio incudostapedia), 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 tAvo 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 in.serted 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 tjnnpanic mucous membrane (Tunica mucosa tympanica) is continuous 

 with that of the pharynx and the guttural pouch through the Eustachian tube. 

 It is thin, closely united with the underlying periosteum, and is reflected over the 

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

 open ixirt 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 stylomastoid foramen, and forms a circle around the tym- 

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

 plexus. 



The Eustachian Tube 

 The Eustachian or auditory tube (Tuba auditiva [Eustachii]) extends from the 

 t}Tnpanic 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 inner side of the root of the muscular process of 

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

 cavity by the small slit-like tympanic opening (Ostium tympanicum tuba^ auditiva?). 

 For a distance of about a quarter of an inch (ca. 6 to 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 

 inclose a narrow groove, that opens ventrally into an extensive diverticulum termed 

 the guttural pouch. The pharyngeal opening (Ostium pharyngeum tuba? audi- 

 tivse) is situated on the postero-superior part of the lateral wall of the pharynx just 



