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 Princeps Pollicis, or digital artery of the thumb, descends between the abductor indicis and deep head of the shorter flexor pollicis. It then follows the internal margin of the thumb, and anastomoses with the other small digital vessels which run along the dorsal aspect of the thumb.
In some cases the radialis indicis and princeps pollicis arise by a common trunk, which descends to the lower part of the first interosseous space before it bifurcates: this is described as the regular disposition by Cloquet and Boyer. Professor Harrison describes the radial artery as terminating by dividing into three branches; the radialis indicis, princeps pollicis, and palmaris profunda.
This is the proper termination of the radial artery; it passes horizontally inwards, between the metacarpal bones and interosseous muscles which are behind it, and the adductor pollicis and flexor tendons which lie in front. It then unites with the deep terminating branch of the ulnar, thus forming the deep palmar arch.