The =mandible= or =inferior maxilla= (Mandibula) is the largest and the
only movable bone of the face. The two halves of which it consists at
birth unite during the second or third month, and it is usually
described as a single bone. It carries the lower teeth, and articulates
by its condyles with the squamous temporal on either side. It consists
of a body and two rami.[18]
The =body= (Corpus mandibulæ) is the thick anterior part which bears the
incisor teeth. It presents two surfaces and a border. The =lingual= or
=superior surface= (Facies lingualis) is smooth and slightly concave;
during life it is covered by mucous membrane, and the tip of the tongue
overlies it. The =labial= or =mental surface= (Facies mentalis) is
convex and is related to the lower lip. It is marked by a median furrow
which indicates the position of the primitive symphysis mandibulæ. The
curved =alveolar border= (Limbus alveolaris) presents six alveoli for
the incisor teeth, and a little further back two alveoli for the canine
teeth in the male; in the mare the latter are usually absent or small.
The =rami= (Rami mandibulæ) extend backward from the body and diverge to
inclose the =submaxillary space= (Spatium mandibulare). Each ramus is
bent so as to consist of a horizontal part (Pars molaris) which bears
the lower cheek teeth, and a vertical part (Ramus mandibulæ) which is
expanded and furnishes attachment to powerful muscles; the term =angle=
is applied to the most prominent part of the curve. The ramus presents
two surfaces, two borders, and two extremities. The =external surface=
is smooth and slightly convex from edge to edge on the horizontal part;
at the junction with the body it presents the =mental foramen= (Foramen
mentale), which is the external opening of the mandibular or inferior
dental canal. On the vertical part it is somewhat concave and presents a
number of rough lines for the attachment of the masseter muscle. The
=internal surface= of the horizontal part is smooth, and presents a
shallow longitudinal depression in its middle; above this there is often
a faint mylo-hyoid line for the attachment of the muscle of like name.
At the lower part of the junction with the body there is a small fossa
for the attachment of the genio-hyoid and genio-glossus muscles. On the
vertical part the surface is concave, and is marked in its lower and
posterior part by rough lines for the attachment of the internal
pterygoid muscle. In front of its middle is the =mandibular= or
=inferior maxillary foramen= (Foramen mandibulare), which is the
posterior orifice of the =mandibular= or =inferior dental canal=
(Canalis mandibulæ). The canal curves downward and passes forward below
the cheek teeth, opening externally at the mental foramen; it is
continued into the body of the bone as a small canal (Canalis alveolaris
incisivus), which carries the vessels and nerves to the incisor teeth.
The =superior= or =alveolar border= forms anteriorly part of the
interalveolar space; here it is thin. Behind this it is thick and is
excavated by six alveoli for the lower cheek teeth. Behind the last
alveolus it curves sharply upward and is narrow and rough. In the young
foal there is commonly a small alveolus for the vestige of the first
premolar (“wolf tooth”) close to the first large one. The =inferior
border= of the horizontal part is nearly straight; it is thick and
rounded in the young horse, becoming narrower and sharp in old subjects.
At its posterior part there is a smooth impression (Incisura vasorum)
where the facial vessels and parotid duct turn round the bone. Behind
this point the border curves sharply upward, forming the =angle=
(Angulus mandibulæ); this part is thick and has two roughened lips,
internal and external, separated by a considerable intermediate space;
near the condyle it becomes narrower. The =anterior extremity= joins the
body. The =superior extremity= comprises the =coronoid process= in front
and the =condyle= behind, the two being separated by the =sigmoid notch=
(Incisura mandibulæ), through which the nerve to the masseter muscle
passes. The =coronoid process= (Processus coronoideus) is thin
transversely and curved slightly inward and backward. It projects upward
in the temporal fossa, and furnishes insertion to the temporal muscle.
The =condyle= (Capitulum mandibulæ) lies at a much lower level than the
end of the coronoid process. It is elongated transversely and
articulates with the squamous temporal through the medium of an
articular disc. The part below the condyle is usually termed the =neck=
(Collum mandibulæ); on its antero-internal part is a depression (Fovea
pterygoidea) for the attachment of the external pterygoid muscle. The
middle of the vertical part of the ramus consists to a large extent of a
single plate of compact substance which may be so thin in places as to
be translucent.
=Development.=—The mandible develops from two chief centers in the
connective tissue which overlies the paired Meckel’s cartilages. At
birth it consists of two symmetrical halves which meet at a median
symphysis. Fusion usually occurs in the second or third month.
=Age changes.=—These are associated largely with the growth and later
with the reduction of the teeth. In the young horse, in which the
teeth are large and are in great part embedded in the bone, the body
is thick and strongly curved, and the horizontal part of the ramus is
also thick. Later, as the teeth are extruded from the bone, the body
becomes flattened and narrower, and the horizontal part of the ramus
is thinner, especially in its lower part; the angle and the impression
in front of it are more pronounced.