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 Supra Scapular, Or Transversalis Humeri Artery, runs horizontally outwards, in front of the anterior scalenus muscle, the phrenic nerve, the brachial plexus, and the posterior scalenus muscle, being covered anteriorly by the clavicle and the sterno-mastoid and trapezius muscles. In this course it gives off a thoracic and acromial branch; and then passes over the ligament of the notch in the superior margin of the scapula, placed between the origin of the omo-hyoid muscle and the apex of the conoid ligament: from this it dips into the supra-spinata fossa, where it terminates by dividing into the supra-spinata and infra-spinata arteries. The nerve corresponding to the supra-scapular artery usually passes under the ligament of the notch. Sometimes, however, though rarely, we find their position reversed, the artery passing beneath and the nerve above the ligament, or both may go together beneath it. The thoracic branch is small; it descends through the substance of the subclavian muscle, to communicate with the thoracic branches of the axillary artery. The acromial branch is considerable ; it usually arises from the supra-scapular, as it is passing into the supra-spinata fossa, but may arise from it in any part of its course; it supplies the trapezius and supra-spinatus muscles, and the periosteum and integuments covering the acromion process. The supra-spinata artery is entirely lost in the muscle of the same name. The infra-spinata artery descends in front of the spine of the scapula and beneath the spino-glenoid ligament of Sir A. Cooper: having arrived in the infra-spinata fossa, it gives off several branches to the muscles of this region, and then forms a curve to anastomose with the posterior branch of the sub-scapular artery: it also sends a delicate branch along the axillary margin of the scapula, towards its inferior angle, where it anastomoses with the posterior scapular artery.