SPECIAL PHYSIOLOGY OF MUSCLES. 65 



passes upward, and is attached by its continuations to all the ver- 

 tebrae. Its action is to extend the vertebral column and to maintain 

 the erect position. The head is balanced upon the top of the ver- 

 tebral column by the combined action of the trapezius and suboccipi- 

 tal muscles forming the nape of the neck, and by the sterno-cleido- 

 mastoid muscle. This latter muscle arises from the inner third of 

 the clavicle and upper border of the sternum. It is inserted into the 

 temporal bone just behind the ear. Its action is to flex the head 

 laterally and to rotate the face to the opposite side. When both 

 muscles act simultaneously, the head and neck are flexed upon the 

 thorax. 



The temporal and masseter muscles, situated at the side of the 

 head, arise respectively from the temporal fossa and the zygomatic 

 arch, and are inserted into the ramus of the lower jaw. Their action 

 is to close the mouth and to assist in mastication. The occipito- 

 frontalis, the orbicularis palpebrarum, and orbicularis oris muscles 

 are largely concerned in wrinkling the forehead, closing the eyes and 

 mouth, and in giving various expressions to the face. 



The deltoid is a thick, triangular muscle covering the shoulder- 

 joint. Arising from the outer third of the clavicle, the acromial 

 process, and the spine of the scapula, its fibers converge to be in- 

 serted into the humerus just above its middle point. Its action is to 

 elevate the arm through a right angle. Owing to its point of inser- 

 tion it acts as a lever of the third order, but, notwithstanding the 

 advantageous points of insertion, it acts at a considerable disad- 

 vantage, owing to the obliquity of its direction. 



The biceps muscle, situated on the anterior aspect of the arm, arises 

 from the upper border of the glenoid fossa and the coracoid process, 

 and is inserted into the radius just beyond the elbow-joint. Its 

 action is to flex and supinate the forearm and to place it in the most 

 favorable position for striking a blow. When the forearm is fixed, 

 it assists in flexing the arm, as in climbing. 



The triceps muscle, situated on the back of the arm, arises from 

 the scapula and the posterior surface of the humerus, and is inserted 

 in the olecranon process of the ulna. In its action it directly an- 

 tagonizes the biceps, namely, extending the forearm. In so doing it 

 acts as a lever of the first order. The short distance between the 

 muscular insertion and the fulcrum causes it to act at a great 

 mechanical disadvantage, but there is a corresponding gain in both 

 speed and range of movement. The muscles of the forearm are 

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