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 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 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 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 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 united 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 (Glandule tympanic). 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 cirele 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 em.) 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 tympanic opening (Ostium tympanicum tube auditive). 
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 tubs audit- 
