The brachial plexus forms from the anterior rami of cervical nerves C5 to C8 and thoracic nerve T1. The contributions of each of these nerves, which are between the anterior and middle scalene muscles, are the roots of the brachial plexus. As the roots emerge from between these muscles, they form the next component of the brachial plexus (the trunks) as follows: the anterior rami of C5 and C6 form the upper trunk, the anterior ramus of C7 forms the middle trunk, the anterior rami of C8 and T1 form the lower trunk.
The trunks cross the base of the posterior triangle (see Fig. 8.186). Several branches of the brachial plexus may be visible in the posterior triangle (see Fig. 7.54 on pg. 730). These include the: dorsal scapular nerve to the rhomboid muscles, long thoracic nerve to the serratus anterior muscle, nerve to the subclavius muscle, and suprascapular nerve to the supraspinatus and infraspinatus muscles.
Root of the neck
The root of the neck (Fig. 8.190) is the area immediately superior to the superior thoracic aperture and axillary inlets. It is bounded by: the top of the manubrium of the sternum and superior margin of the clavicle anteriorly, and the top of the thoracic vertebra TI and the superior margin of the scapula to the coracoid process posteriorly.
It contains structures passing between the neck, thorax, and upper limb. There is also an extension of the thoracic cavity projecting into the root of the neck (Fig. 8.190). This consists of an upward projection of the pleural cavity, on both sides, and includes the cervical part of the parietal pleura (cupula), and the apical part of the superior lobe of each lung.
Anteriorly, the pleural cavity extends above the top of the manubrium of the sternum and superior border of rib I, while posteriorly, due to the downward slope of the superior thoracic aperture, the pleural cavity remains below the top of vertebra TI.
The subclavian arteries on both sides arch upward out of the thorax to enter the root of the neck (Fig. 8.191).
The right subclavian artery begins posterior to the sternoclavicular joint as one of two terminal branches of the brachiocephalic trunk. It arches superiorly and laterally to pass anterior to the extension of the pleural cavity in the root of the neck and posterior to the anterior scalene muscle. Continuing laterally across rib I, it becomes the axillary artery as it crosses its lateral border.
The left subclavian artery begins lower in the thorax than the right subclavian artery as a direct branch of the arch of the aorta. Lying posterior to the left common carotid artery and lateral to the trachea, it ascends and arches laterally, passing anterior to the extension of the pleural cavity and posterior to the anterior scalene muscle. It continues laterally over rib I, and becomes the axillary artery as it crosses the lateral border of rib I.
Both subclavian arteries are divided into three parts by the anterior scalene muscle (Fig. 8.191):
The first part extends from the origin of the artery to the anterior scalene muscle.
The second part is the part of the artery posterior to the anterior scalene muscle.
The third part is the part lateral to the anterior scalene muscle before the artery reaches the lateral border of rib I.
All branches from the right and left subclavian arteries arise from the first part of the artery, except in the case of one branch (the costocervical trunk) on the right side (Fig. 8.191). The branches include the vertebral artery, the thyrocervical trunk, the internal thoracic artery, and the costocervical trunk.