STJPEA-HYOID MUSCLES. 



281 



of this muscle has been described by Macalister, as passing superficial to the 

 external pterygoid to be inserted into the tuberosity of the maxillary bone. 



Fig. 207. — View op the lower part Fig. 207. 



OP THE Skull ani> Face, from be- 

 hind, TO SHOW THE ATTACHMENTS OP 



THE Pterygoid and some other 

 IMuscLES (modified from Eonrgery). 

 (A. T.) i 



a, placed above the basi-splienoid 

 bone, below which are seen the posterior 

 nares and i^alate ; b, transverse section 

 through the temporal bone ; c, roof of 

 the mouth ; d, back of the head and 

 neck of the lower jaw, above which are 

 seen the synovial cavities of the joint 

 divided by the iuterarticular fibro- 

 cartilage ; e, i^laced below the symphysis 

 menti ; 1, on the left internal pterygoid 

 muscle ; 1', on the right side, the lower 

 ]»art of tlie same muscle, of which the 

 middle portion has been removed to 

 show the external pterygoid ; 2, the 



lower thick portion of the external pterj'goid ; 2', on the right side points to the upper 

 smaller portion of the muscle, attached in part to the interarticular plate ; 3, small 

 portions of the genio-hyoid and genio-hyo-glossus muscles cut short at their attachment 

 to the genial tubercles ; 4, the attachment of the mylo-hyoid muscle cut short ; 5, the 

 attachment at the side of the sjnnphysis of the anterior belly of the digastric muscle ; 

 6, the masseter muscle descending to the angle of the jaw. 



Nerves. — The foiu' muscles above described receive their motor nerves 

 from one source, viz., the external muscular brandies of the fifth pair, which 

 may be traced to the lesser or motor root of that nerve. These nei-ves are named 

 from the muscles they respectively supply. There are usually two branches to 

 the temporal, and one to each of the other muscles. 



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 jaterygoid muscle, having the great body of its 

 fibres nearly horizontal, di'aws forwards the condyle of the jaw, and, when the 

 muscles of both sides act together, the lower jaw is protracted 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 masseter 

 and internal pteiygoid muscles assist in protracting the jaw ; the temporal alone 

 is a retractor. The two pterygoid muscles of one side, in advancing one condyle 

 of the jaw, necessarily throw the teeth towards the opposite side. 



MUSCLES BETWEEN THE LOWER JAW AND THE HYOID BONE. 



The digastric muscle, extending from tlie temporal bone to the 

 lower jaw, consists of two elongated muscular bellies united by an 

 intervening rounded tendon. The posterior belly, longer than the 

 anterior, arises from the digastric groove of the temporal bone, and is 

 directed downwards, forwards, and inwards : the anterior is attached 

 to a rough depression situated on the lower border of the lower jaw, 

 close to the symphysis menti ; it is loss tapering than the posterior 

 belly, and is directed downwards and backwards. The intervening 



