STERNO-CLEIDO-MASTOID 93 



The mylo-hyoid (Fig. 78) is a flat muscle which forms the floor 

 of the mouth, being attached by one border to the inner surface of 

 the body of the mandible, and by the other to the hyoid bone, 

 which, it will be remembered, is on a level with the mandible. 



Action. It can draw the hyoid bone forward in the act of 

 swallowing, thus keeping the larynx out of the way of the food. 



Nerve. Fifth cranial. 



The platysma. As the muscles of the back and side of the neck are covered 

 by the trapezius, so those of the front and side are covered by the platysma, 

 which is a broad thin sheet of muscular fibers attached above to the mandible 

 and the fascia of the side of the face, and below to the deep fascia on the front 

 of the shoulder (Fig. 76). Like the face muscles, it is not covered by deep 

 fascia, and, since it moves the skin, it is like them a muscle of expression. It 

 draws the angle of the mouth downward, and strong contractions of the 

 muscle assist in causing an appearance as of one in a "great rage." The 

 action of this muscle in grazing animals is displayed when used to shake off 

 insects which alight upon the skin of the neck. 



Nerve. Seventh cranial. 



The sterno-cleido-mastoid (Figs. 78). is the most conspic- 

 uous muscle in the side of the neck. Origin. By two divi- 

 sions, one on the sternum (sternal, or medial origin), the other on 

 the clavicula (clavicular, or lateral origin). Insertion. The 

 mastoid process and upper curved line of the occipital bone. 



Action. Principally to pull the mastoid process toward the 

 sternum and clavicula. If the right muscle contracts the right 

 mastoid process comes downward and forward and the chin turns 

 upward to the left. If the left muscle contracts the left mastoid is 

 pulled downward and forward and the chin goes upward to the 

 right. Both muscles together simply bend the head forward, or 

 flex it. 



Nerves. Spinal accessory (and cervical). 



Clinical note. The sterno-mastoid is another muscle which is sometimes 

 the seat of spasmodic contractions, causing wry-neck, or torticollis. 



Levator scapulae. The elevator of the scapula is an important muscle 

 in the side of the neck. Origin. The upper three or four transverse processes. 

 Insertion. The medial angle of the scapula. 



Nerves. Cervical. 



