582 | THE NERVOUS SYSTEM. 
dorsal trunks, from the fifth, sixth, seventh, and eighth cervical nerves, and lies 
behind the axillary artery. The first thoracic nerve does not usually contribute to 
the posterior cord. 
(4) The nerves of distribution for the shoulder and arm are derived from these 
secondary cords, receiving in this way various contributions from the constituent 
spinal nerves. From the outer cord arise the external anterior thoracic, musculo- 
cutaneous nerve, and the outer head of the median nerve. From the inner cord 
arise the inner head of the median, the ulnar, internal cutaneous, lesser internal 
cutaneous, and the internal anterior thoracic nerves. vom the posterior cord arise 
the circumflex, three subscapular, and the musculo-spiral nerves. 
It is to be remembered that, although derived from a secondary cord formed by a certain 
set of spinal nerves, any given nerve does not necess: wily contain fibres from all the constituent 
nerves ; eg. both the musculo-cutaneous and circumflex nerves, from the outer and posterior 
cords, are ultimately derived only from the fifth and sixth cervical nerves. 
THE BRANCHES OF THE BRACHIAL PLEXUS. 
It is customary to separate artificially the nerves of distribution of the brachial 
plexus into two sets: (1) supra-clavicular and (2) infra-clavicular. 
Supra-clavicular Nerves.—The nerves derived from the plexus above the 
level of the clavicle are, like the main trunks, divisible into two series; anterior 
branches arising from the front, and posterior branches arising from the back of the 
plexus (Fig. 429, p. 581). 
Anterior Branches. Posterior Branches. 
1. Nerves to scalenus anticus and 1. Nerves to scaleni, medius and 
longus colli. posticus. 
2. Communicating nerve to join 2. Posterior scapular nerve. 
the phrenic nerve. 3. Long thoracic nerve. 
3. Nerve to the subclavius muscle. 4. Supra-scapular nerve. 
The muscular twigs to the scalenus anticus and longus colli arise from the lower 
four cervical nerves, as they emerge from the intervertebral foramina. 
The communicating nerve to the phrenic arises usually from the fifth cervical 
nerve at the outer border of the scalenus anticus. It is sometimes absent, and 
occasionally an additional root is present from the sixth cervical nerve. In some 
instances the nerve is replaced by a branch from the nerve to the subelavius, 
which passes inwards behind the sterno-mastoid muscle to join the phrenic at the 
inlet of the thorax. 
The nerve to the subclavius is a slender nerve, which arises from the front of 
the cord formed by the fifth and sixth cervical nerves. It descends in the posterior 
triangle of the neck over the third part of the subclavian artery. It often com- 
municates with the phrenic nerve. 
The branches to the scaleni, medius and posticus, are small trunks which arise 
from the lower four cervical nerves as they emerge from the intervertebral foramina. 
The posterior scapular nerve (n. dorsalis scapule, nerve to the rhomboids) 
arises from the back of the fifth cervical nerve, as it emerges from the intervertebral 
foramen. It appears in the posterior triangle of the neck, after piercing the 
scalenus medius muscle. It is directed downwards, under cover of the levator 
anguli scapulee and rhomboid muscles, and along the vertebral border of the 
scapula, to be distributed to the levator anguli scapule, rhomboideus minor, and 
rhomboideus major muscles. It occasionally pierces the levator anguli scapule. 
The long thoracic nerve (n. thoracalis longus, external respiratory nerve of 
Bell) arises by three roots, of which the middle one is usually the largest, from the 
back of the fifth, sixth, and seventh nerves, as they emerge trom the intervertebral 
foramina. The nerve pierces the scalenus medius in one or two trunks, and, 
descending along the side of the neck behind the cords of the brachial plexus, it 
enters the axilla between the upper edge of the serratus magnus and the axillary 
artery. It continues its downward course over the outer surface of the serratus 
magnus, to which it is distributed. 
