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 infterior 

 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 



