This section is from the book "Anatomy Of The Arteries Of The Human Body", by John Hatch Power. Also available from Amazon: Anatomy of the Arteries of the Human Body, with the Descriptive Anatomy of the Heart.
The Infra Or Sub Scapular Artery is of considerable size : it arises from the third stage of the axillary, opposite the inferior margin of the sub-scapular muscle, to which it sends one or two branches, and then descends along the inferior margin till it reaches the internal edge of the long head of the triceps. Here it divides into an inferior and posterior branch: the inferior branch continues in the direction of the trunk, and descends between the serratus magnus and latissimus dorsi muscles, to both of which, and to the teres major, its branches are distributed: at the inferior angle of the scapula it anastomoses with the posterior scapular artery. The posterior branch, larger than the inferior, sinks into a triangular space bounded above by the teres minor and sub-scapular muscles, below by the teres major and latissimus dorsi, and externally by the long head of the triceps, which, in this situation, separates it from the posterior circumflex artery: this branch supplies freely the muscles bounding this triangular space, and then curves round the axillary margin of the scapula to arrive in the fossa infra-spinata, being in this part of its course covered by the teres minor and by the infra-spinatus muscle. Here, lying close to bone, it divides into many branches, which supply the infra-spinatus muscle and shoulder-joint and anastomose with the posterior and superior scapular arteries. The infra or sub-scapular artery sometimes arises in common with the posterior circumflex.