The =hyoid bone= (Os hyoideum) is situated chiefly between the vertical
parts of the rami of the mandible, but its upper part extends somewhat
further back. It is attached to the petrous temporal bones by rods of
cartilage, and supports the root of the tongue, the pharynx, and larynx.
It consists of a body, a lingual process, and three pairs of cornua.
The =body= or =basihyoid= (Basis ossishyoidei) is a short transverse
bar, compressed dorso-ventrally. The upper surface is concave and smooth
in its middle, and presents at each end a convex facet or tubercle for
articulation with the small cornu. The lower surface is slightly
roughened for muscular attachment. The anterior border carries medially
the lingual process. The posterior border is concave and smooth in its
middle, and carries on either side the thyroid cornu. The body, the
lingual process, and the thyroid processes are fused together, and may
be compared to a spur or a fork with a very short handle.
The =lingual process= (Processus lingualis) projects forward medially
from the body, and is embedded in the root of the tongue during life. It
is compressed laterally and has a blunt-pointed free end. The lateral
surfaces are slightly concave. The upper border is narrow, the lower
thick and irregular.
The =thyroid cornua= or =thyrohyoids= (Cornua laryngea)[19] extend
backward and upward from the lateral parts of the body. They are
compressed laterally (except at their junction with the body), and their
posterior ends are connected with the anterior cornua of the thyroid
cartilage of the larynx.
The =small cornua= or =keratohyoids= (Cornua minora) are short rods
which are directed upward and forward from either end of the upper
surface of the body. Each is somewhat constricted in its middle part and
has slightly enlarged ends. The lower end has a small concave facet
which articulates with the body. The upper end articulates with the
great cornu, or with the middle cornu when present.
[Illustration:
FIG. 32.—HYOID BONE OF HORSE, VIEWED FROM THE SIDE AND SOMEWHAT FROM
IN FRONT.
_a_, Body; _b_, lingual process; _c_, thyroid cornu; _c′_, cartilage
of _c_; _d_, small cornu; _e_, middle cornu; _f_, great cornu; _f′_,
muscular angle of great cornu; _g_, cartilage of great cornu.
(Ellenberger-Baum, Anat. d. Haustiere.)
]
The =great cornua= or =stylohyoids= are much the largest parts of the
bone. They are directed upward and backward, and are connected above
with the base of the petrous temporal bones. Each is a thin plate, seven
or eight inches (ca. 18 to 20 cm.) long, which is slightly curved in its
length, so that the external surface is concave and smooth. The internal
surface is convex and smooth. The borders are thin. The upper extremity
is large and forms two angles; the upper angle is connected by a rod of
cartilage with the hyoid process of the petrous temporal bone; the lower
angle is somewhat thickened and rough for muscular attachment. The lower
extremity is small, and articulates with the small or the middle cornu.
The =middle cornua= or =epihyoids= are small wedge-shaped pieces or
nodules interposed between the small and great cornua. They are usually
transitory, and unite with the great cornua in the adult.
=Development.=—The hyoid ossifies in the cartilages of the second and
third visceral arches. Each part has a separate center, except the
lingual process, which ossifies by extension from the body. The latter
has primarily two centers (Martin).