THE BRACHIAL PLEXUS 133 



flexed beyond that angle, the patient is unable to perform any 

 forward . pushing movements. Under these conditions, his 

 endeavours result in marked "winging" of the scapula. On 

 the other hand, forward pushing movements with the arm 

 flexed to less than a right angle are not only possible, but 

 they do not produce any " winging " of the scapula, which is 

 steadied by the trapezius and the rhomboids (Sherren). 



It, however, either or both of the latter muscles are paralysed 

 in addition to the serratus anterior, forward pushing move- 

 ments carried out in any plane cause " winging " of the 

 scapula. The combined lesion appears to be the less un- 

 common condition. 



When the scapula is fixed by the contraction of the 

 trapezius and the rhomboids, the serratus anterior can help 

 inspiration by elevating the upper eight ribs. Patients suffering 

 from chronic bronchitis and emphysema bring into use all the 

 auxiliary muscles of respiration when they are seized by a fit 

 of coughing, and the digitations of the serratus anterior stand 

 out in relief on the medial wall of the axilla, more especially 

 in spare subjects. 



THE INFRA-CLAVICULAR BRANCHES OF THE BRACHIAL 

 PLEXUS 



(A) Lateral Cord. The Lateral Anterior Thoracic Nerve 



(C. 5, 6 and 7) supplies the whole of the clavicular head and 

 part of the costo-sternal head of the pectoralis major. 



The pectoralis major covers the upper part of the anterior 

 chest wall, and its lower border constitutes the anterior 

 axillary fold. In the female, it is partly obscured by the 

 mammary gland. From its origin the muscle passes laterally 

 to be inserted into the proximal part of the humerus. Its 

 line of pull lies below and anterior to the centre of the 

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

 adductor and medial rotator of the arm. 



Paralysis of the pectoralis major occurs along with paralysis 

 of other muscles in injuries of the upper trunk of the brachial 



