252 MUSCLES AND FASCIAE. 



its fibres pass downwards, outwards, and backwards, to be inserted, by strong 

 tendinous laminae, into the lower and back part of the inner side of the ramus and 

 angle of the lower jaw, as high as the dental foramen. 



Relations. By its external surface, with the ramus of the lower jaw, from which 

 it is separated, at its upper part, by the External Pterygoid, the internal lateral 

 ligament, the internal maxillary artery, and the dental vessels and nerves. By its 

 internal surface, with the Tensor palati, being separated from the Superior con- 

 strictor of the pharynx by a cellular interval. 



The External Pterygoid is a short thick muscle, somewhat conical in form, and 

 extends almost horizontally between the zygomatic fossa and the condyle of the 

 jaw. It arises from the pterygoid ridge on the great wing of the sphenoid, and 

 the portion of bone included between it and the base of the pterygoid process ; 

 from the outer surface of the external pterygoid plate ; and from the tuberosity of 

 the palate and superior maxillary bones. Its fibres pass horizontally backwards 

 and outwards, to be inserted into a depression in front of the neck of the condyle 

 of the lower jaw, and into the corresponding part of the interarticular fibro- 

 cartilage. This muscle, at its origin, appears to consist of two portions separated 

 by a slight interval ; hence the terms upper and lower head sometimes used in the 

 description of the muscle. 



Relations. By its external surf ace, with the ramus of the lower jaw, the internal 

 maxillary artery which crosses it, the tendon of the Temporal muscle, and the 

 Masseter. By its internal surface, it rests against the upper part of the Internal 

 pterygoid, the internal lateral ligament, the middle meningeal artery, and inferior 

 maxillary nerve; by its upper border it is in relation with the temporal and 

 masseteric branches of the inferior maxillary nerve. 



Nerves. These muscles are supplied'by the inferior maxillary nerve. 



Actions. The Temporal, Masseter, and Internal pterygoid raise the lower ja 

 against the upper with great force. The two latter muscles, from the obliquity 

 in the direction of their fibres, assist the External pterygoid in drawing the 

 lower jaw forwards upon the upper, the jaw being drawn back again by the deep 

 fibres of the Masseter, and posterior fibres of the Temporal. The External 

 pterygoid muscles are the direct agents in the trituration of the food, drawing the 

 lower jaw directly forwards, so as to make the lower teeth project beyond the 

 upper. If the muscle of one side acts, the corresponding side of the jaw is drawn 

 forwards, and the other condyle remaining fixed, the symphysis deviates to the 

 opposite side. The alternation of these movements on the two sides produces 

 trituration. 



MUSCLES AND FASCIA OF THE NECK. 



The Muscles of the Neck may be arranged into groups, corresponding with the 

 region in which they are situated. 



These groups are nine in number : 



1. Superficial Eegion. 5. Muscles of the Pharynx. 



2. Infra-hyoid Eegion. 6. Muscles of the Soft Palate. 



Depressors of the Os Hyoides 



and Larynx. 7. Muscles of the Anterior Verte- 



bral Eegion. 



3. Supra-hyoid Eegion. 



Elevators of the Os Hyoides 8. Muscles of the Lateral Vertebral 

 and Larynx. Eegion. 



4. Lingual Eegion. 9. Muscles of the Larynx. 



Muscles of the Tongue. 



id 



: 



