THE BONES OF THE SKULL. 157 
horizontal portion forms an acute angle with that of the bone of the 
opposite side, except anteriorly, where it bears a roughened area 
for articulation with the latter. Running backward from the 
symphysis there is a broad horizontal ridge, representing the 
mylohyoid line (linea mylohyoidea), the line of attachment of the 
mylohyoid muscle. The mandibular foramen, through which, 
in the natural condition, the inferior alveolar nerve and artery 
gain access to the interior of the bone, lies on this surface at the 
junction of the horizontal portion with the ramus. The corres- 
ponding mental foramen (foramen mentale), through which 
branches of these structures leave the mandible, is situated on the 
lateral surface in front of the first premolar. The mandibular 
foramen is closely connected with a second aperture lying at the 
ventral end of the sulcus ascendens, directly behind the last molar, 
and serving for the transmission of a vein connecting the inferior 
alveolar and inferior orbital veins. 
The mandibular ramus forms in general an obtuse angle 
with the horizontal portion. As in other herbivores, the ventral 
part, distinguished as the angle, is greatly increased in size at the 
expense of the condyloid process and to a still greater extent of the 
coronoid process, the latter being vestigial. In addition to a low 
pterygoid tuberosity (tuberositas pterygoidea), situated at the 
posterior projecting point of the angle, the posterior and ventral 
margins of the angle are excavated on the medial side of the bone, 
so that they form the boundary of a pronounced, though shallow, 
inferior pterygoid depression for the insertion of the ptery- 
goideus internus muscle. The area occupied by the pterygoideus 
internus is separated by a low ridge from a more dorsally placed 
superior depression for the pterygoideus externus muscle. <A 
somewhat similar depression, termed the masseteric fossa, 
occupies the lateral surface of the angle, its raised ventral margin 
terminating posteriorly in the masseteric tuberosity (tuberositas 
masseterica). The articular portion, or head of the mandible is 
greatly elongated in the anteroposterior direction in accordance 
with the anteroposterior action of the lower jaw, this feature being 
one which is of general occurrence in the rodent order, and more 
fully expressed in the great extension forward and backward of the 
attachment areas of the muscles of mastication. The connection 
