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.
In addition to the great number of varieties already noticed, we shall only add, in this place, that, in some cases, the subclavian artery passes in front of the scalenus anticus muscle along with the vein,* whilst, on the other hand, this latter vessel may be found behind the muscle together with the artery. Both of these varieties have been observed by Vel-peau. M. Robert has observed, that the little scalenus, when present, in passing to its insertion into the rib, frequently separates the two inferior fasciculi of the brachial plexus, and pushes them forwards against the artery. In some cases, the muscular relations differ: the omo-hyoid muscle may have an additional origin from, or attachment to, the clavicle.
Bouillaud mentions that Breschet observed a very remarkable anomaly, in which the left subclavian artery arose from the pulmonary artery. In some cases the thyroid axis is deficient, and its usual branches arise by two or more separate trunks. In its third stage, the subclavian frequently gives off the posterior scapular. Professor Hargrave has seen the internal mammary arise in this situation and descend in front of the scalenus anticus muscle.
* Hird, Lond. Med. Gazette, Feb. 4, 1837.
Varieties of the Vertebral as to its origin have been alluded to on page 380.
This artery may arise from the arch of the aorta, arteria innominata, thyroid axis, and even from the third stage of the subclavian, as observed by Professor Hargrave. Bichat has seen its comes nervi phrenici branch as large as the trunk of the internal mammary, and Cruveilhier met a subject in which its third intercostal branch was large enough to appear a bifurcation of it.
We have already seen that this artery, in its normal state, arises from the thyroid axis, but that sometimes it arises from the subclavian in its third stage; in such cases it may pass through the brachial plexus of nerves, and take its ordinary place along the vertebral margin of the scapula. In cases of this description of variety, the cervicalis superficialis, as already stated, forms a distinct branch of the thyroid axis.
This artery sometimes arises in common with the posterior circumflex, and Dr. Monro has seen it arising from the inferior thyroid.
In some cases this artery winds round the humerus by passing beneath the tendons of the teres major and latissimus dorsi, and not through the quadrangular space already described. In such cases it usually gives off the superior profunda artery.