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At a very early stage (7th week) there is developed along the superficial aspect of the median nerve an arterial anastomotic channel fed by a succession of branches which spring from the axial brachial vessel.
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This channel frequently opens up in part, or even in its whole extent and gives rise to the greater number of arterial anomalies met with in the arm.
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The vas aberrans may replace the main artery, being known from the normal brachial artery by the fact that it lies superficial to the median nerve, whereas the usual vessel is deep to that nerve.
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The first ramus of the vas appears between the heads of the median nerve The ulnar or radial artery frequently arises from the brachial artery in the lower third of the arm ; in such cases the upper part of the radial or ulnar vessels will be found to be formed out of the anastomotic channel.
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In the fore-arm the median artery may be of large size, ending in the superficial palmar arch ; it, too, is formed out of the arterial anastomosis which is laid down in foetal life, superficial to the median nerve.