280 A MANUAL OF ANATOMY. 



thirds by nine (or ten) fleshy slips, two arising from the 

 second rib. 



Insertion. Into the anterior aspect of the vertebral bor- 

 der and superior and inferior angles of the scapula. 



Nerve Supply. The posterior or long thoracic (external 

 respiratory of Bell), which brings filaments from the fifth, 

 sixth, and seventh cervical nerves. 



Action. (i) On the scapula. The muscle draws the 

 scapula forward, and owing to the lower portion of the 

 muscle being longer than the upper, the lower angle of the 

 scapula will be advanced farther than its upper angle, thus 

 producing a rotation of the scapula around a central axis 

 perpendicular to its plane surface. This action is further 

 developed in raising the arm to a vertical position, the 

 deltoid carrying it to a right angle, the trapezius and serra- 

 tus magnus completing the action. (2) On the ribs. If 

 the scapula is fixed backward, the muscle would tend to 

 draw the ribs outward, and so enlarge the chest laterally 

 and aid inspiration. 



The Posterior Thoracic Nerve. Figs. 63, 64. 



For formation, see description of brachial plexus, page 315. 



Appears in the axilla posterior to the axillary artery and 

 vein, continues downward across the middle of the serratus 

 magnus muscle to the various digitations to which it is 

 distributed. 



The Intercostohumeral Nerve. Figs. 62, 63, 64. 



This comes through the second intercostal space. It is 

 the lateral cutaneous branch of the second intercostal nerve. 

 In the floor of the axilla some of its filaments interlace with 

 others from the lesser internal cutaneous nerve to form a 

 loose plexus. The nerve is then distributed to the integu- 

 ment of the inner, upper, and posterior part of the arm. 



