460 BRACHIAL PLEXUS. BRANCHES. 



disposition in the formation of the plexus is the following : the fifth 

 and sixth nerves unite to form a common trunk, which soon divides 

 into two branches ; the last cervical and first dorsal also unite imme- 

 diately upon their exit from the intervertebral foramina, and the 

 common trunk resulting from their union after a short course also 

 divides into two branches ; the seventh nerve passes outwards be- 

 tween the common trunks of the two preceding, and opposite the 

 clavicle divides into a superior branch which unites with the inferior 

 division of the superior trunk, and an inferior branch which commu- 

 nicates with the superior division of the inferior trunk : from these 

 divisions and communications the brachial plexus results. The bra- 

 chial plexus communicates with the cervical plexus by means of a 

 branch sent down from the fourth to the fifth nerve, and by the infe- 

 rior branch of origin of the phrenic nerve, and also sends filaments of 

 communication to the sympathetic. The plexus is broad in the neck, 

 narrows as it descends into the axilla, and again enlarges at its lower 

 part where it divides into its six terminal branches. 



Relations. The brachial plexus is in relation in the neck with the 

 two scaleni muscles, between which its nerves issue ; lower down it is 

 placed between the clavicle and subclavius muscle above, and the first 

 rib and first serration of the serratus magnus muscle below. In the 

 axilla, it is situated at first to the outer side and then behind the 

 axillary artery, resting by its outer border against the tendon of the 

 subscapularis muscle. At this point it completely surrounds the artery 

 by means of the two cords which are sent off to form the median 

 nerve. 



Its Branches may be arranged into two groups, humeral and de- 

 scending, 



Humeral Branches. Descending Branches. 



Superior muscular, External cutaneous, 



Short thoracic, Internal cutaneous, 



Long thoracic, Lesser internal cutaneous, 



Supra- scapular, Median, 



Subscapular, Ulnar, 



Inferior muscular. Musculo- spiral, 



Circumflex. 



The superior Muscular nerves are several large branches which are 

 given off by the fifth cervical nerve above the clavicle ; they are, 

 a subclavian branch to the subclavius muscle, which usually sends a 

 communicating filament to the phrenic nerve ; a rhomboid branch to 

 the rhomboidei muscles ; and frequently an angular branch to the 

 levator anguli scapulae. 



The Short thoracic nerves (anterior) are two in number ; they arise 

 from the brachial plexus at a point parallel with the clavicle, and are 

 divisible into an anterior and a posterior branch. The anterior branch 

 passes forwards between the subclavius muscle and the subclavian 

 vein, and is distributed to the pectoralis major muscle, entering it by 



