THE HEAD AND NECK 303 



pterygoid (c). They are inserted along the ventral edge 

 almost to its posterior extremity. 



The masseter muscle consists of external and internal divisions 

 which are readily separable posteriorly but not anteriorly. The ex- 

 ternal originates from the lateral surface of the zygomatic arch along 

 slightly less than the anterior half of its length, tendinous from its 

 anterior angle and fleshy behind that. Its insertion is near the ventral 

 edge of the mandible. The main part of the internal division takes 

 origin from the internal surface of the same part of the arch and is 

 inserted dorsal to the external division. In the rabbit a readily separable 

 portion, probably to be included with the internal division, originates 

 from the remainder of the inner surface of the arch and is inserted on 

 the lateral surface of the ramus of the mandible. 



The orbital structures should be freed from the zygo- 

 matic arch by passing a knife along its dorsal margin. The 

 zygomatic processes of the maxilla and of the squamosal 

 bone should then be divided and the zygomatic arch should 

 be removed, together with the whole of the masseter 

 muscle, which should be cleanly cut from its attachment 

 to the mandible. Care should be taken not to injure the 

 insertion tendon of the temporalis muscle just internal 

 to the arch. 

 (b) The temporalis is a slender muscle, being much smaller in 

 the rabbit than in many mammals. It originates in the 

 reduced temporal fossa (sulcus temporalis) of the skull 

 back to about the posterior margins of the squamosal and 

 parietal bones and is inserted by a long stout tendon on the 

 edge and adjacent part of the lateral surface of the reduced 

 coronoid process. Fibres from the masseter and the su- 

 perior portion of the external pterygoid muscles are at- 

 tached along the side of the tendon. The muscle may be 

 exposed by dividing the temporal portion of the posterior 

 supraorbital ligament which holds its tendon in place, and 

 the muscles in front of the external ear if these are still in 

 position. The temporal muscle itself may then be divided. 

 On account of the narrowness^ of the space between the two 

 limbs of the mandible and the great depth of its angle, it is neces- 

 sary, in order to expose the surface for the deep dissection of the 

 ventral portion of the head and neck, to remove one-half of the 



