140 THE NERVOUS SYSTEM 



branches to the brachio-radialis (supinator longus), the extensor 

 carpi radialis longus and the brachialis (p. 134). 



The Triceps arises by three heads. The long head arises from the upper 

 part of the axillary border of the scapula and it can be palpated distal to 

 the posterior fold of the axilla, when the forearm is actively extended. The 

 lateral 'and medial heads arise from the posterior aspect of the humerus. 

 When the lateral head contracts, it forms an oblique ridge on the back of 

 the arm, just below the posterior border of the deltoid. 



The triceps is inserted into the proximal surface of the olecranon, and 

 acts as a powerful extensor of the elbow. In this action it is aided by 

 the anconaeus, a small muscle which passes from the posterior aspect of the 

 lateral epicondyle of the humerus to the lateral aspect of the olecranon. 



The Brachio-radialis is an extremely important muscle in many ways. 

 It arises from the lateral intermuscular septum and the lateral epicondylic 

 ridge of the humerus, and is inserted into the lateral aspect of the radius, 

 just proximal to the styloid process. Its principal action is flexion of the 

 elbow, and when that movement is attempted against resistance, the brachio- 

 radialis forms an unmistakable prominence on the lateral part of the front 

 of the forearm. When the limb is supine, the line of the pull of the 

 brachio-radialis lies medial to the axis of the movement of pronation, and 

 therefore the muscle acts as a pronator, in the initial stage of pronation. 

 But, in the mid-prone position, the line of pull exactly overlies the axis 

 of movement and the muscle ceases to act as a pronator. When the limb 

 is fully pronated, the line of pull lies lateral to the axis of movement, 

 and the brachio-radialis, therefore, may act as a supinator until the mid- 

 prone position is reached. 



The brachio-radialis is paralysed when the radial (musculo-spiral] nerve 

 is injured in the radial groove or in the axilla, but it is not affected in the 

 " wrist-drop'''' paralysis of lead-poisoning. 



The Extensor Carpi Radialis Longus lies under cover of the brachio- 

 radialis. It arises from the humerus and the lateral intermuscular septum just 

 distal to that muscle and is inserted into the dorsum of the base of the second 

 metacarpal bone. It is a powerful extensor of the wrist-joint and, when 

 the forearm is pronated, it assists in flexion of the elbow-joint. Like the 

 brachio-radialis, it commonly escapes in lead-poisoning, but as the other carpal 

 extensors are involved its action may be masked by the tonus of the carpal 

 flexors when an endeavour is made to extend the wrist. If, however, the 

 hand is supported during the movement, the contraction of the extensor carpi 

 radialis longus can be satisfactorily demonstrated. 



The actions and attachments of the Brachialis are described on page 134. 



The Deep Branch of the Radial Nerve supplies the supin- 



