304 ANATOMY OF THE RABBIT 



mandible. A better understanding of the attachments and re- 

 lations of the muscles of mastication may be obtained, however, if 

 this is done in steps, as follows. 



Divide the mandibular symphysis and free one half of the 

 mandible from its attachments to the lips and to the lining of the 

 mouth. Pass a scalpel along the ventral part of the medial surface 

 of the bone so as to detach the digastricus and mylohyoideus 

 muscles and also the pterygoideus internus (see below). The 

 insertion of the pterygoideus externus, which is more dorsally 

 situated, should remain intact. The tip of the knife should be 

 kept close to the bone so that the underlying soft parts, except in 

 that they are divided at their attachments, will be kept uninjured, 

 the medial surface of the mandible being clean when removed. 

 The ventral edge of the mandible should then be turned laterad 

 so as to rotate the bone towards a horizontal position, thereby 

 exposing the following structures without further injury to any. 

 (In order that this may be done, the zygomatic process of the 

 squamosal bone must have been cut through its most dorsal part 

 when the zygomatic arch was removed, as directed on the previous 

 page.) 



The structures appearing on the cut surface include the in- 

 sertion of the digastricus and the margin of the mylohyoideus; also 

 the following parts are exposed. 



(c) The pterygoideus internus muscle. Origin: Pterygoid 

 process of the skull. Insertion: Ventral portion of the 

 medial surface of the angle. 



Still intact and attached to the mandible are : 



(d) The pterygoideus externus. The muscle comprises two 

 portions. Superior head. Origin: Infratemporal surface 

 of the alisphenoid. Insertion: Medial surface of the ramus 

 of the mandible and sulcus ascendens. 



This portion has also been described as a division of the temporalis. 

 Inferior head. Origin: Posterior edge and both surfaces 

 of the lateral plate of the pterygoid process. Insertion: 

 The depression in front of the neck of the mandible, the 

 interarticular cartilage of the temporo-mandibular joint, 

 and the whole medial edge of the head of the mandible. 



