The =mouth=[77] (Cavum oris) is the first part of the alimentary canal.
In the horse it is a long cylindrical cavity, and when closed, it is
almost entirely filled up by the contained structures. The entrance to
it (Rima oris) is closed by the lips. Laterally it is bounded by the
cheeks; above, by the hard palate; below, by the body of the mandible
and the mylo-hyoid muscles; behind, by the soft palate.
The =cavity= of the mouth is subdivided into two parts by the teeth and
alveolar processes. The space external to these and inclosed externally
by the lips and cheeks is termed the vestibule of the mouth (Vestibulum
oris). In the resting state of the parts the walls of this cavity are in
contact, and the space is practically obliterated. Its existence becomes
very evident in facial paralysis, when the food tends to collect in it
laterally, pouching out the cheeks. The space within the teeth and
alveolar processes is termed the mouth cavity proper (Cavum oris
proprium). When the teeth are in contact, it communicates with the
vestibule only by the interdental spaces and the intervals behind the
last molar teeth. Posteriorly it communicates with the pharynx through
the isthmus of the fauces.
The =mucous membrane= lining the mouth (Tunica mucosa oris) is
continuous at the margin of the lips with the common integument, and
behind with the mucous lining of the pharynx. During life it is chiefly
of a pink color.
The =lips= (Labia oris) are two musculo-membranous folds which surround
the orifice of the mouth. Their angles of union (Anguli oris s.
commissuræ labiorum) are situated near the first cheek tooth and are
rounded. Each lip presents two surfaces and two borders. The outer
surface is covered by the skin, which presents long tactile hairs in
addition to the ordinary fine hair. The upper lip shows a shallow median
furrow (Philtrum), the lower a rounded prominence, the chin (Mentum).
The inner or oral surface is covered with mucous membrane which is
commonly more or less pigmented. The small papillæ on this surface show
on their summits the openings of the ducts of the labial glands. Small
folds of mucous membrane which pass from the lip to the gum represent
the frænula labii (superioris, inferioris). The free border of the lip
is dense and presents short, very stiff hairs. The attached border is
continuous with the surrounding structures, and is adherent to the
alveolar borders of the bones of the jaws.
[Illustration:
FIG. 238.—SAGITTAL SECTION OF HEAD OF HORSE. THE SECTION IS ABOUT 1
CM. TO THE LEFT OF THE MEDIAN PLANE.
_1_, Hard palate; _2_, soft palate; _3_, posterior pillar of soft
palate; _4_, Eustachian opening; _5_, great cornu of hyoid bone;
_6_, stylo-pharyngeus muscle; _9_, glosso-pharyngeal nerve; _10_,
hypoglossal nerve; _11_, perpendicular plate of ethmoid; _12_,
presphenoid; _13_, sphenoidal sinus; _14_, occipital bone; _15_,
median surface of cerebral hemisphere; _16_, corpus callosum; _17_,
septum lucidum; _18_, fornix; _19_, thalamus opticus; _20_, corpora
quadrigemina; _21_, pons; _22_, medulla oblongata; _23_, spinal
cord; _24_, cerebellum; _25_, pituitary body; _26_, hyo-epiglottic
muscle; _27_, superior longitudinal muscle of tongue; _28_,
omo-hyoideus; _29_, hyo-glossus; _30_, hyoideus transversus; _31_,
thyroid cartilage (body); _S. f._, frontal sinus; _L. n._, lig.
nuchæ; _C I_, atlas; _C II_, axis; _R. c. m._, rectus cap. ant.
major and minor; _E p._, epiglottis; _A r._, arytenoid cartilage; _P
m._, premaxilla; _M._, mandible. The leader line for the guttural
pouch points to the external carotid artery.
]
=Structure.=—The lips are covered externally by the skin, and are lined
by mucous membrane; between these are muscular tissue and glands. The
skin lies directly on the muscles, many fibers of which are inserted
into the former. The muscles have been described (page 214). The =labial
glands= (Glandulæ labiales) form a compact mass near the angles; they
are numerous in the upper lip, fewer in the lower. The mucous membrane
is often pigmented, and is reflected upon the bones of the jaws to form
the gums.
=Blood-vessels and Nerves.=—The arteries are derived from the superior
and inferior labial and palato-labial arteries. The sensory nerves come
from the trigeminus, and the motor from the facial nerve.
The =cheeks= (Buccæ) form the sides of the mouth, and are continuous in
front with the lips. They are attached above and below to the alveolar
borders of the bones of the jaws.
=Structure.=—This comprises: (1) The skin; (2) the muscular and
glandular layer; (3) the mucous membrane. The skin offers nothing in
particular to notice. The muscular tissue is formed mainly by the
buccinator, but also by parts of the panniculus, zygomaticus, dilatator
naris lateralis, levator nasolabialis, and depressor labii inferioris.
The =buccal glands= (Glandulæ buccales) are arranged in two rows. The
upper row (superior buccal or molar glands) is found on the outer
surface of the buccinator muscle, near its upper border. The anterior
part of the row consists of scattered lobules; the posterior part, which
lies under cover of the masseter muscle, is more developed and compact.
The lower row (inferior buccal or molar glands), less voluminous than
the upper, is situated in the submucous tissue at the lower border of
the buccinator muscle. The mucous membrane is reflected above and below
upon the gums, and is continuous behind with that of the soft palate. It
is reddish in color and frequently shows pigmented areas. Opposite the
third upper cheek tooth is the opening of the parotid (Stenson’s) duct,
surrounded by a circular fold of the mucous membrane. A linear series of
small papillæ above and below indicates the orifices of the small ducts
from the buccal glands.
=Blood-vessels and Nerves.=—The blood-supply is derived from the facial
and buccinator arteries. The sensory nerves come from the trigeminus and
the motor from the facial nerve.
The =gums= (Gingivæ) are composed of a dense fibrous tissue intimately
united with the periosteum of the alveolar processes, and blending at
the edges of the alveoli with the alveolar periosteum, which fixes the
teeth in their cavities. They are covered by a smooth mucous membrane,
destitute of glands, and of a low degree of sensibility.
The =hard palate= (Palatum durum) is bounded in front and on the sides
by the alveolar arches; behind it is continuous with the soft palate.
Its osseous basis is formed by the premaxilla, maxilla, and palate
bones. The mucous membrane is smooth, and is attached to the bones by a
submucosa which contains in its anterior part an exceedingly rich venous
plexus, constituting an erectile tissue. A central raphé (Raphé palati)
divides the surface into two equal portions. Each of these presents
about eighteen transverse curved ridges (Rugæ palatini) which have their
concavity and their free edges directed backward. They are further apart
and more prominent anteriorly. There are no glands in the submucosa.
=Vessels and Nerves.=—The blood-supply is derived chiefly from the
palatine arteries and the nerves from the trigeminus.
The =soft palate= (Palatum molle) is a musculo-membranous curtain which
separates the cavity of the mouth from that of the pharynx. It slopes
downward and backward from its junction with the hard palate. The =oral
surface= faces downward and forward, and is covered with a mucous
membrane continuous with that of the hard palate. It is corrugated and
presents numerous small orifices (of gland-ducts) and two sagittal
ridges. On each side a short, thick fold passes to join the base of the
tongue; this is the =anterior pillar= of the soft palate (Arcus
glossopalatinus). The =pharyngeal surface= looks upward and backward and
is covered by a mucous membrane continuous with that of the nasal
cavity. The =free border= (Arcus palatinus) is concave and thin; it is
in contact (except during deglutition) with the epiglottis. It is
continuous with a fold of the mucous membrane, which passes on each side
along the lateral wall of the pharynx and unites with its fellow over
the beginning of the œsophagus; this fold is termed the =posterior
pillar= of the soft palate (Arcus pharyngopalatinus). The space between
the diverging anterior and posterior pillars (Sinus tonsillaris) is
occupied by the faucial =tonsil=. In the horse, however, there is not a
compact tonsil, as in man, dog, etc., but a somewhat extended group of
mucous glands and masses of lymphoid tissue. These cause elevations of
the surface, on which are seen numerous depressions (crypts) in which
the gland-ducts open. The soft palate is greatly developed in equidæ,
its length being five to six inches (12 to 15 cm.). Its length and
contact with the epiglottis account for the fact that in these animals
mouth-breathing does not occur under normal conditions, and that in
vomiting the ejected matter escapes usually through the nasal
cavity.[78]
[Illustration: