THE MUSCLES OF MASTICATION. 295 



both heads pass backwards and outwards, and converge to be inserted into the 

 depression on the front of the neck of the lower jaw, and into the interarticular 

 fibro-cartilage of the temporo-maxillary articulation. 



Relations. The internal maxillary artery is usually placed on the outer surface of this 

 muscle, passing- thence between the heads of origin ; while the buccal nerve issues from 

 between those heads. The deep surface rests against the upper part of the internal pterygoid 

 muscle, the direction of which it crosses, also the internal lateral ligament of the lower jaw, 

 the inferior maxillary nerve, and the middle meningeal vessels. The upper border is crossed 

 by the deep temporal and masseteric nerves. 



Varieties. The ptcrygoideus pvoprius is a nearly vertical band of muscular and tendinous 

 fibres, sometimes entirely tendinous, passing from the infratemporal crest of the great wing of 

 the sphenoid, over the surface of the external pterygoid muscle, to the lower part of the outer 

 pterygoid plate, or to the tuberosity of the palate or superior maxillary bone. It has been 

 seen sending a slip to the pterygo-maxillary ligament, or even to the lower jaw. 



Pterygo-spinosus. This is a muscular slip occasionally seen, springing from the spine 

 of the sphenoid, or the adjoining part of the vaginal process, and inserted into the 

 hinder margin of the outer pterygoid plate, between the external and internal pterygoid 

 muscles. A fibrous band connecting these parts (pterygo-splnous ligament') is frequently 



Fig. 275. VIEW OF THE LOWER PART OP THE SKULL 



AND FACE, FROM BEHIND, TO SHOW THE ATTACH- 

 MENTS OF THE PTERYGOID AND SOME OTHER MUSCLES. 



(Allen Thomson, after Bourgery. ) 



a, body of the sphenoid, below which are seen the 

 posterior nares ; 6, section through the temporal bone ; 

 c, hard palate ; d, back of the head and neck of the 

 lower jaw, above which are seen the synovial cavities 

 of the joint separated by the interarticular fibro- 

 cartilage ; e, placed below the symphysis menti ; 1, 

 on the left internal pterygoid muscle ; 1', on the right 

 side, the lower part of the same muscle, the middle 

 portion of which has been removed to show the 

 external pterygoid ; 2, the lower head of the external 

 pterygoid ; 2', on the right side, points to the upper 

 head of the muscle, attached in part to the inter- 

 articular disc ; 3, origins of the genio-hyoid and genio-glossus muscles from the mental spines ; 

 4, origin of the mylo-hyoid ; 5, attachment of the anterior belly of the digastric ; 6, 6, masseter 

 muscles. 



present, and is sometimes converted into bone (cf. p. 47). (J. Poland, " Variations of the 

 External Pterygoid Muscle," Journ. Anat., xxiv.) 



Nerves. The four muscles above described receive their nerves from the muscular 

 branches of the inferior maxillary, which may be traced to the small or motor root of the 

 fifth nerve. 



Actions. The masseter, temporal and internal pterygoid muscles are elevators of the 

 lower jaw, and generally act in concert, bringing the lower teeth forcibly into contact with 

 the upper. The opposite movement of depressing the jaw, not being opposed by any resisting 

 obstacle, requires less force, and is effected by muscles of much smaller size, the principal of 

 which is the digastric muscle hereafter described. The external pterygoid muscle draws 

 forwards the condyle of the jaw, and, when the muscles of both sides act together, the lower 

 jaw is protruded so as to make the lower incisor teeth project beyond the upper ; but their 

 more usual mode of action is alternately on the two sides, as in the grinding movement of 

 the molar teeth, in which the horizontal movements of the external pterygoids are associated 

 with the elevating actions of the other muscles. The external pterygoids also act in opening 

 the mouth, when the condyles are carried forwards onto the articular eminences. The hinder 

 portion of the temporal muscle retracts the jaw, and is thus the antagonist of the external 

 pterygoid : in this action the depressor muscles also assist. 



SUBCUTANEOUS MUSCLE OP THE NECK. 



The platysma myoides (fig. 266, 14) is a pale-coloured thin sheet of muscular 

 fibres, superficial to the deep cervical fascia, and extending over the front and side 

 of the neck and lower portion of the side of the face. Its fibres arise by thin bands 



