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 Acromial Or Thoracica Acromialis Artery arises from the axillary in its first stage; it is a short thick axis; it arises a little below the clavicle and passes forwards above the edge of the pectoralis minor muscle, which it separates from the subclavius muscle and ligamentum bicorne. It then advances towards the interval between the deltoid and pectoralis major muscles, and after sending some branches to the serratus magnus, pectoral, and subclavius muscles, it terminates by dividing into a superior and inferior branch. The superior branch passes horizontally outwards beneath the deltoid muscle, and is lost in supplying the latter and the supra-spinatus muscle, and the scapulo-humeral and acromio-clavicular articulations. The inferior branch, or thoracica-humera-ria, turns spirally round the cephalic vein, and descends with it in the areolar interval between the deltoid and great pectoral muscles, and is distributed to these muscles and to the integuments. The acromial artery anastomoses with the supra-scapular and posterior circumflex.
The Thoracica Suprema Artery arises from the first stage of the axillary ; sometimes it arises separately a little beneath the preceding, but more frequently it is a branch of the acromial. It generally runs for some distance along the upper margin of the pectoralis minor, and then descends obliquely inwards between it and the pectoralis major, to both of which muscles it sends several branches: it also supplies the mammary gland and integuments, and anastomoses with the intercostal and internal mammary arteries.