In the lower part of the posterior triangle, the external jugular vein pierces the investing layer of cervical fascia and ends in the subclavian vein.
Tributaries to the external jugular vein while it traverses the posterior triangle of the neck include the transverse cervical, suprascapular, and anterior jugular veins.
Several arteries are found within the boundaries of the posterior triangle of the neck. The largest is the third part of the subclavian artery as it crosses the base of the posterior triangle (Fig. 8.186).
The first part of the subclavian artery ascends to the medial border of the anterior scalene muscle from either the brachiocephalic trunk on the right side or directly from the arch of the aorta on the left side. It has numerous branches.
The second part of the subclavian artery passes laterally between the anterior and middle scalene muscles, and one branch may arise from it.
The third part of the subclavian artery emerges from between the anterior and middle scalene muscles to cross the base of the posterior triangle (Fig. 8.186). It extends from the lateral border of the anterior scalene muscle to the lateral border of rib I where it becomes the axillary artery and continues into the upper limb.
A single branch (the dorsal scapular artery) may arise from the third part of the subclavian artery. This branch passes posterolaterally to reach the superior angle of the scapula where it descends along the medial border of the scapula posterior to the rhomboid muscles.
Two other small arteries also cross the base of the posterior triangle. These are the transverse cervical and the suprascapular arteries (Fig. 8.186). They are both branches of the thyrocervical trunk, which arises from the first part of the subclavian artery.
After branching from the thyrocervical trunk, the transverse cervical artery passes laterally and slightly posteriorly across the base of the posterior triangle anterior to the anterior scalene muscle and the brachial plexus. Reaching the deep surface of the trapezius muscle, it divides into superficial and deep branches:
The superficial branch continues on the deep surface of the trapezius muscle.
The deep branch continues on the deep surface of the rhomboid muscles near the medial border of the scapula.
The suprascapular artery, also a branch of the thyrocervical trunk, passes laterally, in a slightly downward direction across the lowest part of the posterior triangle, and ends up posterior to the clavicle (Fig. 8.186). Approaching the scapula, it passes over the superior transverse scapular ligament and distributes branches to muscles on the posterior surface of the scapula.
Veins accompany all the arteries described previously.
The subclavian vein is a continuation of the axillary vein and begins at the lateral border of rib I. As it crosses the base of the posterior triangle, the external jugular, and, possibly, the suprascapular and transverse cervical veins enter it (Fig. 8.185). It ends by joining with the internal jugular vein to form the brachiocephalic vein near the sternoclavicular joint. In the posterior triangle it is anterior to, and slightly lower than, the subclavian artery and passes anterior to the anterior scalene muscle.
Transverse cervical and suprascapular veins travel with each of the similarly named arteries. These veins become tributaries to either the external jugular vein or the initial part of the subclavian vein.
A variety of nerves pass through or are within the posterior triangle. These include the accessory nerve [XI], branches of the cervical plexus, components forming the brachial plexus, and branches of the brachial plexus.