134 THE NERVOUS SYSTEM 



plexus and in lesions of the spinal medulla, etc., but as an 

 isolated condition it is practically unknown. 



The costo-sternal head may be removed in the complete 

 operation for scirrhus mammae or it may be absent congenitally 

 without causing any noticeable disability. When this part of 

 the muscle is absent congenitally, there is visible deformity 

 and the chest appears to be much flatter on the affected side. 

 The condition, however, does not necessarily predispose to 

 phthisis, and the patient may have an otherwise normal and 

 healthy chest. 



The Musculo-cutaneous Nerve (C. 5, 6 and 7) contains 

 both motor and sensory fibres. Its motor branches supply 

 the coraco-brachialis, the biceps and the brachialis (b. anticus) 

 muscles. 



The Coraco-brachialis can be seen and felt on the lateral wall of the 

 axilla, when the arm is fully abducted. It arises from the tip of the 

 coracoid process and is inserted into the middle of the medial aspect of 

 the shaft of the humerus. Its line of pull lies anterior and a little medial 

 to the centre of the shoulder-joint, and the muscle therefore acts as a flexor, 

 adductor and medial rotator of the arm. 



The Biceps arises from the tip of the coracoid process and from the 

 upper border of the glenoid cavity. As it passes to be inserted into the 

 radial tuberosity, it forms a well-marked elevation on the front of the arm. 

 The biceps is a powerful flexor and supinator of the forearm and it also 

 acts as a weak flexor of the shoulder-joint. 



The Brachialis lies behind the lower part of the biceps. It arises from 

 the anterior aspect of the humerus and is inserted into the coronoid process 

 of the ulna. It is a powerful flexor of the elbow-joint, but it does not 

 depend for its nerve-supply on the musculo-cutaneous nerve alone, as it 

 also receives a branch from the radial (musculo-spiral) nerve. 



The motor symptoms in paralysis of the musculo-cutaneous 

 nerve are great weakness in the movement of flexion at the 

 elbow and impairment in the power of supination, which is 

 normally a much stronger movement than pronation. 



The sensory part of the musculo-cutaneous nerve 

 constitutes the lateral cutaneous nerve of the forearm. 

 It divides into volar (anterior] and dorsal branches, which 

 supply the corresponding surfaces of the lateral aspect of the 



