THE TRIGEMINAL MUSCLES. 477 



passing between the muscle and the bone. Above its larger head is covered by the 

 external pterygoid. Its inner surface is in contact above with the tensor palati, the 

 superior constrictor of the pharynx, and the ascending palatine artery, while towards 

 its insertion it is in relation with the stylo-hyoid and posterior belly of the digastric 

 and with the submaxillary gland. 



Variations of the Muscles of Mastication. — The muscles of mastication are all derivatives 

 of a single muscular mass represented by the adductor tna?idibi(lce of fishes, and indications 

 of their common origin are not infrequently to be seen in partial unions of the various muscles. 

 Thus, fibres from the posterior portion of the deeper head of the masseter may join the tem- 

 poral, fibres from both the temporal and masseter sometimes pass to the anterior border of the 

 fibro-cartilage of the mandibular articulation, and connections have also been observed between 

 the temporal and the external pterygoid. 



Additional independent muscles apparently belonging to this group sometimes occur in 

 the pterygoideus proprius, which extends from the infratemporal crest of the sphenoid to the 

 posterior edge of the external pterygoid plate, and in the pterygo-spinosus, which has for its 

 attachments the spine of the sphenoid and the posterior border of the external pterygoid plate. 

 The significance of these muscles passing between points which are immovable is somewhat 

 obscure. The close relationship which the pterygo-spinosus bears to the spheno-mandibular 

 ligament seems to indicate that it represents the musculature of that portion of the mandibular 

 arch which has become transformed into the ligament, and that usually it is represented by the 

 connective tissue enclosing the ligament. 



{b) THE SUBMENTAL MUSCLES. 



I. Mylo-hyoideus. 2. Digastricus (Anterior Belly), 



This group of trigeminal muscles contains but two representatives, the mylo- 

 hyoid and the anterior belly of the digastric. This latter muscle, as ordinarily 

 described, consists of two distinct muscles united at their attachment to the hyoid 

 bone, the anterior of the two muscles belonging to the trigeminal group, while the 

 posterior is a member of the facial group. It will be convenient to describe the 

 muscle as a whole, even although it belongs only in part to the group under con- 

 sideration. 



I. Mylo-Hyoideus (Fig. 497). 



Attachments. — The mylo-hyoid arises from practically the entire length of the 

 mylo-hyoid ridge of the mandible, from which the fibres pass inward and slightly 

 backward to be inserted for the most part into a median fibrous raphe common to 

 the two muscles of opposite sides, the posterior fibres, however, being attached to 

 the upper border of the body of the hyoid bone. The two muscles, taken together, 

 form a muscular floor for the mouth, the diaphragma oris, upon which the tongue 

 may be said to rest. 



Nerve-Supply. — By the mylo-hyoid from the inferior dental branch of the 

 mandibular division of the trigeminus. 



Action. — To draw the hyoid bone upward and at the same time to raise the 

 floor of the mouth, pressing the tongue against the palate. 



Relations. — The superficial surface of the mylo-hyoid is in relation with the 

 anterior belly of the digastric and with the facial artery. The submaxillary gland 

 curves around its posterior free margin and is thus in relation with both its surfaces, 

 the submaxillary duct running forward upon its deeper surface. This latter surface 

 is also in relation with the genio-hyoid, genio-glossal, hyo-glossal, and stylo-glossal 

 muscles, with the sublingual gland, and with the lingual branch of the trigeminus 

 and the hypoglossal nerve. 



2. Digastricus (Figs. 497, 502). 



Attachments. — The digastric, as its name indicates, consists of two bellies 

 which are united by a strong cylindrical tendon. The anterior belly, which alone 

 belongs to the trigeminal group of muscles, arises from the digastric fossa of the 

 mandible, and is directed downward, backward, and slightly outward to become con- 



