458 



THE MUSCULAK SYSTEM. 



a. Opening of the Mouth. 



Weight of the mandible 

 Digastric 

 Mylo-hyoid 

 Genio-hyoid 



Genioglossus 

 Infra-hyoid 



muscles 



b. Protrusion of the Mandible. 



External pterygoid 

 Internal pterygoid 

 Temporal (anterior fibres) 



Closure of the Mouth. 

 Masseter 

 Temporal 

 Internal pterygoid 



Retraction of the Mandible. 



Temporal (posterior fibres) 



c. Lateral Movement of the Mandible. 



External pter 

 Internal 



one side) 



THE MUSCLES OF THE NECK. 



In addition to those included among the muscles of the back (p. 438), the 

 following series of muscles occur in the neck : (1) sterno-cleido-mastoid ; (2) the 

 muscles of the hyoid bone (supra-hyoid and infra-hyoid) ; (3) the muscles of the 

 tongue (extrinsic and intrinsic) ; (4) the muscles of the pharynx and soft palate ; 

 and (5) the prse vertebral muscles. 



M. Sternocleidomastoideus. The sterno-mastoid muscle is the prominent 

 muscle projecting on the side of the neck, and separating the anterior from the 

 posterior triangle. It arises by two heads (1) a narrow, tendinous, sternal head, 

 from the anterior surface of the manubrium sterni (Fig. 330, p. 370), and (2) a 

 broader clavicular origin, partly tendinous, partly fleshy, from the superior surface 

 of the clavicle in its medial third (Fig. 327, p. 366). The muscle is inserted into 

 the lateral surface of the mastoid portion of the temporal bone and into the 

 superior nuchal line of the occipital bone (Fig. 396, p. 444). 



The muscle passes obliquely over the side of the neck, separating the anterior 

 from the posterior triangle. It is almost superficial in its whole extent, but is 

 overlapped superiorly by the parotid gland and is covered in its inferior part by the 

 platysma. It is crossed by the external jugular vein, and by superficial branches 

 of the cervical plexus. Its deep surface is in contact with: (a) in its lower third, the 

 infra-hyoid muscles, which separate it from the common carotid artery, and the 

 subclavian artery and the internal jugular vein ; (&) in its middle third, with th* 

 cervical nerves which emerge between the transverse processes of the cervic* 

 vertebrae to form the cervical plexus ; and (c) in its superior third, with the splenii 

 capitis muscle, and the accessory nerve, which there pierces the deep surface of tl 

 muscle. Near its insertion the muscle is related to the splenius capitis, longissimu* 

 capitis, the posterior belly of the digastric, and the occipital artery. 



The sterno-cleido-mastoid muscle is properly divisible into three parts : (1) sterno-mastoii 

 placed superficially, and passing obliquely from the sternum to the mastoid process ; (2) cleido- 

 mastoid, placed more deeply, and directed vertically upwards from the clavicle to the mastoic 

 process ; and (3) cleido-occipitalis, passing obliquely upwards and backwards behind the cleido 

 mastoid to the superior nuchal line of the occipital bone. 



Nerve-Supply. The sterno-mastoid muscle is innervated by the accessory nerve, joined 

 a branch from the cervical plexus (C. 2.). 



Actions. When one muscle acts alone, it flexes the head laterally, and rotates it to th 

 opposite side. The two muscles acting together (1) flex the head in a forward direction, and 

 act as extraordinary muscles of inspiration, by raising the sternum and clavicles. 



The Muscles of the Hyoid Bone. 



The muscles attached to the hyoid bone are in three series : (1) infra-hyoi' 

 muscles, connecting the hyoid bone to the scapula, the wall of the thorax, an 



