72 THE SKELETON OF THE HORSE 
articular dise. The part below the condyle is usually termed the neck of the 
mandible (Collum mandibule); on its antero-medial part is a depression, the fovea 
pterygoidea, in which the lateral pterygoid muscle is attached. 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 connec- 
tive tissue which overlies the paired Meckel’s cartilages. At birth it consists of 
two symmetrical halves which meet at the median symphysis mandibula. 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 long 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. In the old subject 
the angle, the vascular impression in front of it, and the lines for the attachment of tendinous 
layers of the masseter and pterygoid muscles are more pronounced. 
THE Hyoip BONE 
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 the 
larynx. It consists of a body, a lingual 
process, and three pairs of cornua. 
The body or basihyoid (Corpus ossis 
hyoidei) is a short transverse bar, com- 
pressed dorso-ventrally. The dorsal sur- 
face is concave and smooth in its middle, 
and presents at each end a convex facet 
or tubercle for articulation with the small 
cornu. The ventral surface is flattened 
and is slightly roughened for muscular 
attachment. The anterior border carries 
medially the lingual process. The pos- 
terior border is concave and smooth in its 
middle, and carries on either side the thy- 
Fic. 46.—Hyom Boxe of Horst, Virwen rrow tue = TOI cornu. The body, the lingual proc- 
SIDE AND SOMEWHAT FROM IN FRONT. ess, and the thyroid cornua are fused 
a, Body; 6, lingual process; ¢, thyroid cornu; ¢’, and may be compared to a spur or a fork 
cartilage of c; d, small cornu; e, middle cornu; J, great : 
Commi: J aadeedlut ingle of @xcateormur gicartlage “WeORe wery short handle, 
of great cornu. (Ellenberger-Baum, Anat. d. Haus- The lingual process (Processus lingu- 
ee) alis) 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 dorsal border is thin and irregular, 
and the ventral border is thick and rough. 
The thyroid cornua or thyrohyoids (Cornua thyreoidea)! extend backward and 
upward from the lateral parts of the body. They are compressed laterally (except 
at their junction with the body), and the posterior end has a short cartilaginous 
prolongation which is connected with the anterior cornu 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 body. Each is somewhat 
constricted in its middle part and has slightly enlarged ends. The ventral end has 
1 These correspond to the great cornua of man. 
