The Minute Anatomy of the Brachial Plexus. 



425 



uncommon to find it springing from the 5th before the junction is 

 made. When this is the case it contains no fibres from the 6th, but 

 when given off at the junction it usually, though not always, receives 

 a minute fibre from the 6th which passes either behind or through 

 the fibres of the 5th to reach the suprascapular. The 6th therefore 

 exercises sometimes an extremely small influence, and sometimes none 

 at all over the supra- and infra-spinatus. 



The united 5th and 6th then divide into an anterior and a posterior 

 branch.* 



The anterior joins with an anterior branch of the 7th to form the 

 outer cord. Just before their junction the united 5th and 6th gives 

 off one, and the 7th two branches for the pectorals. The upper branch 

 from the 7th joins with that from the 5th and 6th to form the external 

 anterior thoracic, which, piercing the costo-coracoid membrane, supplies 

 the clavicular and the upper piece of the sternal part of the muscle 

 as far down as the fibres which arise from the 2nd costal cartilage. 

 The lower 7th branch forms that which is usually called the communi- 

 cating branch of the external anterior thoracic. It has, however, a 

 separate origin from the 7th trunk, and contains no fibre of the oth or 



Diagram of Anterior Thoracics. 

 Fig. 3. — As seen in Adults. Fig. 4. — As seen in a Foetus. 



V, VI, VII, VIII, IX = 5th, 6th, 7th, 8th cervical and 1st dorsal roots. 

 V + VI, VIII + IX = Trunks formed by union of these roots. 



a = Twigs to clavicular division of pectoralis major. 

 b, c, d = Twigs to upper, middle, and lower parts of the sternal divi- 

 sion of the muscle. 

 X = " Communicating branch," of which Xj goes over, X 2 under, 

 the pectoralis minor. 



6th. It runs under the costo-coracoid membrane over the axillary 

 artery, and at the upper border of the pectoralis minor divides into 

 two, of which one branch passes in front, running with the branches 



* Or the nerves may each divide before they join. 



