BEACHIAL PLEXUS. 



'01 



nerves are nearly equal in size. The posterior trunk of the eighth cervical nerve 

 is much smaller. The 'posterior trunk of the first thoracic nerve is very minute, 

 and may not be present at all. 



(3) The secondary cords of the plexus are formed by combinations of these 

 anterior and posterior trunks, in relation to the axillary artery. They are three 

 in number. The lateral cord is formed by a combination of the anterior trunks 

 of the fifth, sixth, and seventh nerves, and lies on the lateral side of the axillary 

 artery. The medial cord is formed by a combination of the anterior trunk of the 

 eighth cervical with the part of the first thoracic nerve engaged in the formation 

 of the plexus; it lies on the medial side of the axillary artery. The posterior 

 cord is made up of all the posterior trunks from the fifth, sixth, seventh, and 

 eighth cervical and first thoracic nerves, and lies behind the axillary artery. 



N. INTERCOSTAL.S 



FIG. 614. THE NERVES OF THE BRACHIAL PLEXUS. 



ie first" thoracic nerve may not contribute to the posterior cord, and the branch, 

 rhen present, is a very small nerve. 



(4) The nerves of distribution for the shoulder and arm are derived from these 

 secondary cords, and receive in this way various contributions from the constituent 

 spinal nerves. From the lateral cord arise the lateral anterj r>r t.hnrapjn and muscuJQ- 

 cutaneous nerves, and the lateral head of the median nerve. From the, medial cord 

 e the medial head of the median nerve, the ulnar nerve, medial cutaneous 

 iiftt tfpHfttG forearm, medial cutaneous nerve of the arm, and the medial anterior 

 icic nerve. From the posterior cord arise the axillary nerve, the two sub- 

 ilar nerves, the thoraco-dorsal nerve, and the radial nerve. 



I to be remembered that, although derived from a secondary cord formed by a certain 



it of spinal nerves, any given nerve does not necessarily contain fibres from all the constituent 



B^jrres ; e.g., both the musculo-cutaneous and axillary nerves, from the lateral and posterior cords 



respectively, are ultimately derived only from the fifth and sixth cervical nerves. In other 



* words, the secondary cords are merely collections of nerves of distribution bound together in a 

 common sheath in their passage through the axilla. 



5 THE BRANCHES OF THE BRACHIAL PLEXUS. 



It is customary to separate artificially the nerves of distribution of the brachial 

 xus into two sets : (1) supra-clavicular and (2) infra-clavicular. Clinically it is 



y 



