THE HEAD AND NECK 305 



It will be observed that the more powerful of the jaw-muscles 

 are those which raise and protract the mandible, the combination 

 of movements in these two directions being particularly important 

 in animals with rodent habits. Raising is accomplished by the 

 masseter, particularly its internal division, and the internal 

 pterygoid, aided by the superior head of the external pterygoid and 

 the temporalis. Protraction is the work of the masseter, particularly 

 its external division, and of the inferior head of the external 

 pterygoid. Retraction is a weaker movement accomplished mainly 

 by the posterior part of the internal division of the masseter 

 with some aid from the superior head of the external pterygoid 

 and a little from the temporalis (all three pulling both upwards and 

 backwards.) Lowering of the jaw is the work of the digastricus, 

 aided by a simultaneous forward pull of the inferior portion of the 

 external pterygoid on the head of the mandible. 



(e) The inferior alveolar artery (a. alveolaris inferior) lies be- 

 tween the two pterygoidei, and enters the mandible through 

 the mandibular foramen. The corresponding inferior 

 alveolar vein leaves the mandible at this point. 



(/) The inferior alveolar nerve (n. alveolaris inferior) accom- 

 panies the inferior alveolar artery to the mandible. The 

 continuation of the nerve is the mental nerve. It appears 

 at the mental foramen, and passes to the lower lip. 



The displaced half of the mandible should now be freed from 

 the foregoing structures and removed entirely. 



The origin of the inferior alveolar nerve may be traced. It 

 arises from the mandibular nerve (n. mandibularis), the third 

 division of the fifth cranial or trigeminal nerve (n. trigeminus). 

 The mandibular nerve also gives off anteriorly the stout lingual 

 nerve to the tongue and posteriorly the slender mylohyoid nerve 

 to the digastric and mylohyoid muscles. These structures, together 

 with the inferior alveolar artery, may be freed from their loose 

 connections with the pterygoidei, so that they may be left in place 

 for further study. The two pterygoidei may then be detached at 

 their points of origin from the skull and removed. 



