342 THE MUSCULATURE 



dible — the more anterior directly, the posterior by means of an aponeurosis. The deep layer 

 arises from the lower border and internal surface of the zygomatic arch. The fibre-bundles 

 pass nearly verticallj^ downward, and are inserted upon the upper half of the external surface 

 of ,the ramus. The origin and insertion are by tendinous bands, to which the fibre-bundles are 

 attached in a multipenniform manner. The two layers are fused near the origin and insertion 

 and in front. From the temporal surface of the zygomatic bone and the neighbouring part of 

 the deep layer of the temporal fascia there arises a fasciculus which is separated by a pad of 

 fat from the main body of the temporal muscle, and is inserted into the lateral surface of the 

 lower extremity of the tendon of the temporal muscle and into the ventro-lateral surface of the 

 tip of the coronoid process. This fasciculus, sometimes described as a part of the temporal 

 muscle, is innervated by the masseteric nerve. 



Nerve-supply. — The branch arises in common with the posterior nerve to the temporal 

 muscle from the motor root of the trigeminal (the masticator nerve). It passes above the 

 external pterygoid, through the mandibular (sigmoid) notch, and enters the deep surface of 

 the muscle near the dorsal margin. 



Relations. — It is covered by the masseteric fascia (see above). It lies upon the ramus of 

 the jaw and ventrally is separated by a pad of fat from the buccinator muscle. At the mandibu- 

 lar (sigmoid) notch the sigmoid septum separates it from the external pterygoid muscle. 

 The parotid gland partly overlaps its posterior border. 



The pterygoideus externus (figs. 343-346) consists of two fasciculi. Each is thick and tri- 

 angular. The superior is flattened in a horizontal, the inferior in a vertical, plane. At their 

 iy origin they are separated by a narrow cleft. Near the insertion they become more or less fused. 



\, The superior fasciculus arises by short tendinous processes from the infratemporal (pterygoid) 



crest and from the neighbouring portion of the under surface of the great wing of the sphenoid. 

 ^/X Its fibre-bundles converge toward the insertion, which takes place by short tendinous processes 

 Y ^, into — (1) the capsular ligament in front of the articular disc and (2) the upper third of the front 

 . / ~ of the neck of the condyle. The inferior fasciculus is the larger. It arises by short tendinous 



Y processes from the lateral surface of the lateral lamina of the pterygoid process, from the pyrami- 



dal process of the palate bone, and from the adjacent portions of the maxillary tuberosity. 

 The fibre-bundles converge toward their insertion into a depression on the front of the neck of 

 the condyle. 



Nerve-supply. — A branch from the masticator nerve (motor root of the trigeminus) ap- 

 proaches the muscle near the upper border of the medial surface of the superior fasciculus and 

 gives branches to both portions. 



Relations. — It is partly covered by the maxillary fasciculus of the internal pterygoid and by 

 the temporal and masseter muscles. Medial to it lies the chief fasciculus of the internal ptery- 

 goid muscle. The masseteric and the posterior and middle temporal nerves usually pass above 

 the muscle, the anterior temporal and the buccinator nerves and frequently the internal maxil- 

 lary artery between the two fasciculi. The internal maxillary vessels usually pass below the 

 lower border of the muscle and across its external surface; and the auriculo-temporal, lingual, 

 and inferior alveolar (dental) nerves cross the deep surface of the muscle. 



The pterygoideus internus (fig. Z^^) .—Origin. — From (1) the pterygoid fossa, and (2) from 

 the maxillary tuberosity and the pyramidal process of the palatine, where these adjoin. 



Structure and Insertion. — From the medial and lateral laminae of the pterygoid process 

 there arise aponeuroses and from the palatine bone at the lower margin of the fossa, and from 

 the maxillary tuberosity and palatine bone in front of the external pterygoid, there arise short 

 tendons. From these aponeuroses and tendons and directly from the fossa the fibre-bundles 

 take a nearly parallel course downward, backward, and outward, and are inserted in part in a 

 multi-penniform manner into the lower half of the internal surface of the ramus of the mandible. 

 The insertion extends to the mylo-hyoid ridge. The muscle is divided at its origin into two 

 fasciculi by the distal margin of the external pterygoid. 



Nerve-supply. — The internal pterygoid nerve arises from the back of the mandibular nerve 

 near the foramen ovale. It passes near or through the otic ganglion, and thence to the medial 

 surface of the muscle near the dorsal edge. Both the buccinator and lingual nerves are also 

 described as sending filaments to this muscle. 



Relations. — Laterally the muscle is covered by the interpterygoid fascia and the spheno- 

 mandibular ligament, the external pterygoid, temporal, and masseter muscles, and the ramus 

 of the mandible. The inferior alveolar (dental) and lingual nerves and the corresponding vessels 

 run across this surface. Medial to the muscle lie the lateral pharyngeal fascia, the tensor veli 

 palatini muscle, and the superior constrictor of the pharynx. 



Action. — The muscles of this group adduct the lower jaw and serve to carry it forward and 

 backward and from side to side. The elevation is produced by the masseter, temporal, and 

 internal pterygoid muscles. The suprahyoid muscles and the external pterygoid are the feeble 

 antagonists. The forward movement of the jaw is produced by the simultaneous action of the 

 two external pterygoids (slightly by the superficial layer of the masseter, and the anterior 

 fibres of the temporal) while the inferior posterior portions of the temporal muscles carry the 

 jaw at the tcniporo-discoidal joint somewhat backward. OI)li(iuc lateral rotatory movements 

 are produced (;hicfly by the action of one of the external pterygoids. The alternate action of 

 these two nisucIcH associated with the elevating action of the other muscles of the group, 

 gives rise to the grinding movement of the molar teeth. Purely lateral movements of the jaw 



(may be produced by the internal pterygoids, acting alternately. Lord (Anat. Rec, vol. 7, p. I 

 35.5, 1913) states that in ordinary opening of the mouth the external pterygoids pull the! 

 articular discs and condyles forward while the jaw rotates about an axis passing through! 

 the insertions of the styio-mandibular ligaments. ' 



Variations. — The temporal muscle may have a more extensive cranial origin than usual. 

 It may be formed of two superimposed layers. It may be more or less fused with the external 

 pterygoid, or send a fasciculus to the coronoid process. The masseter may be completely 



