322 PRACTICAL ANATOMY. 



and the rest of the formula becomes clear, forming 2 

 trunks plus 3 cords, or in full (3N + 2N = 2T + 3C), 

 and from the cords the principal branches of the brachial 

 plexus are given off. The figure 5 is the key to the 

 plexus. The branches of the brachial plexus are divided 

 into those given off above the clavicle and those below. 

 Those above the clavicle are: 



1. Muscular, to muscles of neck. 



2. Communicating, a filament from fifth cervical to 

 phrenic. 



3. External respiratory, from the fifth and sixth cer- 

 vical ; supplies serratus magnus muscle. 



4. Supra-scapular, from fifth and sixth cervical ; runs 

 through the supra-spinous foramen, supplying the pos- 

 terior scapular muscles. 



Below the clavicle the branches are numerous, and 

 are distributed to the trunk and shoulder and the arm, 

 forearm, and hand. They are : 



1. The anterior thoracic, two or three in number, 

 arise from the upper and lower cords, and are distributed 

 to the chest-muscles. 



2. The circumflex, arises from the posterior cord, 

 winds around the neck of the humerus, and is distrib- 

 uted to the shoulder-joint, deltoid, teres minor, and in- 

 tegument. 



3. The subscapular, two or three branches from the 

 posterior cord, supply the subscapular muscles. 



4. The external cutaneous, from the outer cord of 

 brachial plexus, pierces the coraco-brachial muscle, passes 

 beneath the biceps, and appears beneath the medio- 

 cephalic vein. It supplies the elbow-joint and integu- 

 ment of elbow along outer side of forearm. 



5. The median is formed by a branch from the outer 

 and one from the inner cord. These unite and form the 



