DISSECTION OF THE HEAD AND NECK. 185 



The External Pterygoid Muscle (Plate 30) is conical in form, with 

 the apex below. Its fibres arise from the outer surface of the sphenoid 

 bone above the entrance to the subsphenoidal canal, and from the entire 

 outer surface of the subsphenoidal jDrocess. They pass upwards and 

 backwards to be inserted into the inner aspect of the neck, or constric- 

 tion, below the condyle of the lower jaw. 



Action. — When the right and left muscles act simultaneously, the 

 lower incisors are made to protrude in front of the upper. When the 

 muscles of one side act singly, the entire inferior maxilla is thrown to 

 the opposite side ; and by the alternate contraction of the two muscles, 

 the triturating action of the jaws and teeth is produced. 



The Internal Pterygoid (Plate 30) is a much more powerful muscle 

 than the preceding, from which it is separated by the inferior dental 

 nerve and vessels. It occupies a position on the inside of the vertical 

 ramus analagous to that of the masseter on the outside, the ramus 

 being included between the two muscles. Its fibres arise from the 

 bony crest formed by the subsphenoidal process and the palatine bone, 

 and they pass backwards to be inserted into the depressed inner surface 

 of the vertical ramus. 



Action. — The principal action of the muscle is to elevate the lower 

 jaw directly, the two muscles acting together ; but acting singly and 

 alternately, they assist in producing lateral movement. 



The Temporal Muscle occupies the fossa of the same name. Its 

 fibres arise from the parietal, frontal, and squamous temporal bones 

 where they bound this fossa, and from the sphenoid bone above the 

 orbital hiatus. They become inserted into the coronoid process and 

 adjacent part of the anterior border of the vertical ramus. 



Action. — Chiefly to elevate the lower jaw, acting in conjunction with 

 the masseter and internal pterygoid. The temporals also oppose the 

 action by which the external pterygoids protrude the lower incisors in 

 front of the upper. 



Directions. — Emerging at the line of apposition of the two pterygoid 

 muscles are the inferior dental vessels and nerve, the mylo-hyoid nerve, 

 the internal maxillary vein, and the lingual branch of the 5th nerve. 

 These are now to be examined. 



The Inferior Dental Artery (Plate 30) is a branch of the internal 

 maxillary, detached before that vessel enters the subsphenoidal canal. 

 It passes first between the two pterygoid muscles, and then between the 

 inner muscle and the bone, and enters the inferior dental canal of the 

 inferior maxilla. On examining the inner aspect of the vertical ramus, 

 the truncated end of the vessel will be seen at the upper orifice of the 

 canal. It is continued in the bone beneath the molar, canine, and 

 incisor teeth, supplying these, and detaching at the mental foramen a 

 mental branch. 



