THE BRACHIAL PLEXUS. 



1297 



Variations. The phrenic nerve may give off a branch to the subclavius or may receive a fila- 

 ment from the nerve to the subclavius. A branch of communication with the external anterior 

 thoracic and a branch to the clavicular head of the sterno-cleido-mastoid have been noted. 



5. The communicating branch to the phrenic nerve (Fig. 1090) arises 

 usually from the fifth cervical nerve, sometimes from the fifth and sixth. Originating 

 at the outer margin of the scalenus anticus it passes inward and joins the phrenic. If 

 this nerve is not present the nerve to the subclavius usually supplies the deficiency. 



II. The Infraclavicular Branches. These branches comprise those given 

 off by the three cords of the plexus after the latter has passed beneath the clavicle 

 into the axilla. 



6. THE EXTERNAL ANTERIOR THORACIC NERVE. 



The external anterior thoracic nerve (n. thoracalis anterior lateralis) (Fig. 1093) 

 receives its fibres from the fifth, sixth and seventh cervical nerves. Leaving the outer 

 cord beneath the clavicle, it passes mesially over the axillary artery and, after giving 



FIG. 1094. 



Musculo-spiral 

 nerve 

 Ulnar nerve 



Lesser internal cutaneous 



nerve 

 Internal cutaneous nerve 



Common 

 carotid artery 



Phrenic nerve 



Scalenus 

 'anticus muscle 



Subclavian artery Subclavian vein 



Dissection of right axilla, showing relation of brachial plexus to Subclavian and axillary vessels 



with arm abducted. 



off a filament which unites with a similar structure from the internal anterior thoracic 

 nerve, divides into two branches which pierce the costo-coracoid membrane and enter 

 the deep surface of the pectoralis major. The upper branch supplies the clavicular 

 portion of the muscle and the lower branch the upper part of the sternal portion. 



The loop between the anterior thoracic nerves gives off a filament which pierces 

 the pectoralis minor and ends in the sternal part of the pectoralis major, to both of 

 which muscles it is distributed. 



Variations. This nerve may supply fibres to the clavicular portion of the deltoid and to the 

 acromio-clavicular articulation. 



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