Quadrangular space (from posterior)
Because this space is below the inferior margin of the teres major, which defines the inferior boundary of the axilla, the triangular interval serves as a passageway between the anterior and posterior compartments of the arm and between the posterior compartment of the arm and the axilla. The radial nerve, the profunda brachii artery (deep artery of arm), and associated veins pass through it.
The two major nerves of the posterior scapular region are the suprascapular and axillary nerves, both of which originate from the brachial plexus in the axilla.
The suprascapular nerve originates in the base of the neck from the superior trunk of the brachial plexus. It passes through the suprascapular foramen to reach the posterior scapular region, where it lies in the plane between bone and muscle. It innervates the supraspinatus muscle, then terminates in and innervates the infraspinatus muscle. Generally, the suprascapular nerve has no cutaneous branches.
The axillary nerve originates from the posterior cord of the brachial plexus. It exits the axilla by passing through the quadrangular space in the posterior wall of the axilla, and enters the posterior scapular region. Together with the posterior circumflex humeral artery and vein, it is directly related to the posterior surface of the surgical neck of the humerus.The axillary nerve innervates the deltoid and teres minor muscles. In addition, it has a cutaneous branch, the superior lateral cutaneous nerve of the arm, which carries general sensation from the skin over the inferior part of the deltoid muscle.
Arteries and veins
Three major arteries are found in the posterior scapular region: the suprascapular, posterior circumflex humeral, and circumflex scapular arteries. These arteries contribute to an interconnected vascular network around the scapula. Veins in the posterior scapular region generally follow the arteries and connect with vessels in the neck, back, arm, and axilla.
Anastomosis in the shoulder
Formation of anastomosis around the surgical neck of humerus
(See for more info @ http://www.slideshare.net/ananthatiger/3-anastomosis-around-the-surgical-neck-of-humerus1)
Anterior circumflex humeral artery and posterior circumflex humeral artery are both branches of the third part of the axillary artery.The posterior circumflex humeral artery anastomoses with anterior circumflex humeral artery and also with branches from profunda brachii (a branch of brachial artery), suprascapular (a branch of subclavian artery) and thoracoacromial (a branch of axillary artery) arteries.
The scapular anastomosis system: is a system connecting each subclavian artery and the corresponding axillary artery, forming an anastomosis around the scapula. It allows blood to flow past the joint regardless of the position of the arm. It includes:
transverse cervical artery (subclavian artery)
transverse scapular artery (subclavian artery)
subscapular artery (axillary artery)
branches of thoracic aorta
The subscapular artery gives off a circumflex scapular branch that enters the infraspinous fossa on the dorsal surface of the bone, grooving the axillary border.
All these vessels anastamose or join to connect the first part of the subclavian with the third part of the axillary, providing a collateral circulation. This collateral circulation allows for blood to continue circulating if the subclavian is obstructed.