MYOLOGY. 51 



situated at the lower part of the face ; it arises from the external oblique 

 line on the outer side of the lower jaw, which extends from the anterior 

 edge of the masseter muscle to the mental foramen ; its insertion is into the 

 commissure of the lips ; its name denotes its use. 



Depressor labii inferioris vel quadratus menti is broad and somewhat 

 square ; arises from the side and front of the lower maxilla, and is inserted 

 into half the lower lip, and into the orbicularis oris ; it conceals the follow- 

 ing muscle. 



Levator labii inferioris vel menti arises from the alveoli of the lower incisors, 

 by the side of the symphysis ; it is inserted into the integument of the chin ; 

 its use is to elevate the chin and lower lip. 



Orbicularis vel sphincter oris surrounds the opening of the mouth, and 

 consists of two fleshy fasciculi, one for each lip, whose fibres decussate at the 

 commissures, and intermix with all the dilating muscles inserted there ; its 

 use is to approximate the lips, and regulate their motion in the acts of 

 speaking and breathing. This muscle has no bony attachment. 



Buccinator. This is broad, thin, and somewhat square ; situated between 

 the two alveolar arches, it forms the inner side of the cheek and the lateral 

 boundary of the mouth, lying close to the mucous membrane of the latter. 

 It arises from the two posterior alveoli of the superior maxilla, from the 

 external surface of the posterior alveoli of the lower maxilla, and from a 

 strong aponeurosis (the pterygo-, or inter-maxillary ligament). It is inserted 

 into the commissure of the lips ; its uses are to press the cheek against the 

 teeth, and to assist in other operations of mastication and articulation. 



The first of the deep-seated muscles of the face is the masseter. This 

 strong muscle covers the ramus and angle of the jaw, and consists of two 

 parts, one anterior, the other posterior. The anterior arises from the supe- 

 rior maxilla where it joins the malar bone, and from the inferior edge of 

 the latter, and is inserted into the outer surface of the angle of the lower 

 maxilla. The posterior portion arises from the edge of the malar bone, and 

 is inserted into the external side of the angle and ramus of the jaw. When 

 both portions of both muscles act together, they elevate the lower jaw ; if 

 the anterior portions only of opposite sides act, they carry the face forwards 

 and upwards ; if the posterior alone act, they carry it backwards and 

 upwards ; if the superficial layer of one side act alone, it can rotate the chin 

 to the opposite side ; and if the deep layer only act, it can rotate the chin to 

 its own side. 



The temporalis is concealed by the temporal aponeurosis, the zygoma, 

 and the masseter; it fills the temporal fossa, and arises from all sides of the 

 cranium beneath the semicircular ridge on the parietal bone, and from all 

 the temporal fossa and fascia; it is inserted by a strong tendon into the 

 coronoid process of the inferior maxilla ; it nearly surrounds that process, 

 except on its outer side, and is continued along its fore part as far as the last 

 molar tooth ; its principal use is to raise the lower jaw, but it may assist to 

 move the jaw forwards, backwards, and laterally. 



Pterygoideus iniemtis is situated on the inner side of the ramus of the 

 jaw : it arises from the inner side of the external pterygoid plate and ptery- 



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