64 THE SKELETON OF THE HORSE 



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 mandibulae). 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 mandi- 

 bulse); this part is thick and has two roughened hps, internal and external, sep- 

 arated l)y 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 mandibulse), 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 

 mandibulse) 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 mandibula?); 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 connec- 

 tive tissue which overhes 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 th(> ramus is also 

 thick. Later, as the teeth are extruded from the bone, the body becomes flattened and narrower, 

 and the horizontal i)art of the ramus is thinner, especially in its lower part; the angle and the 

 impression in front of it are more pronounced. 



The Hyoid 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 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. Tiu* upper surface is concave and smooth in its middle, and pre- 

 sents 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 ]:)order is concave and 

 smooth in its middle, and carries on either side the thyroid cornu. The body, the 



