72 



THE SKELETON OF THE HORSE 



articular disc. The part below the condyle is ucually termed the neck of the 

 mandible (Collum mandibulse) ; 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 mandibulae. 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 Hyoid Bone 



The hyoid bone (Os hyoideum) is situated chiefly between the vertical parts 

 of the vaim 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- 

 roid cornu. The body, the lingual proc- 

 ess, and the thyroid cornua are fused 

 and may be compared to a spur or a fork 

 with a very short handle. 



The lingual process (Processus lingu- 



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 comua or thyrohyoids (Cornua thyreoidea)i 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 comua 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 

 ' These correspond to the great cornua of man. 



Fig. 46.- — Htoid Bone of Hoese, 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; /.great 

 cornu; /', muscular angle of great cornu; g, cartilage 

 of great comu. (Ellenberger-Baum, Anat. d. Haus- 

 tiere.) 



