From the usual place of origin the radial was found, in 429 observations, to deviate in the proportion of nearly 1 case in 8. In all it arose higher than usual, with the exception of one case of low division of the brachial artery, and in this the radial artery was joined by a vas aberrans. The brachial artery (most commonly near its upper end) was the source from which the radial proceeded in case of high origin much more frequently than the axillary.

The position of this artery in the upper arm, in the case of its premature origin, having been already mentioned, it remains only to examine the peculiarities of its course in the fore-arm. The radial artery more rarely deviates from its usual position along the fore-arm than the ulnar. It has, however, been found lying upon the fibrous expansion from the tendon of the biceps, and over the fascia of the fore-arm, instead of beneath those structures. This vessel has also been observed to be placed on the surface of the long supinator, instead of on the inner border of that muscle. In turning round the wrist, it has likewise been seen to deviate from its ordinary course by passing over the extensor tendons of the thumb, instead of beneath them. But these several peculiarities are of very rare occurrence. As was previously stated, the vasa aberrantia occasionally derived from the brachial and axillary arteries most commonly end by joining the radial, or one of its branches. 

Some variations in the ulnar and radial arteries compared

Some of the peculiarities observed in these vessels may be usefully contrasted with each other.

In the first place, it has been found that the radial artery much more frequently deviates in some important particular from its usual condition than the ulnar, the proportions in a large and about an equal number of cases being about 1 in 4,5 for the radial artery, and 1 in 10 for the ulnar.

With respect to the place of origin, the radial offered more frequent peculiarities than the ulnar, in the proportion of 13 to 8. The radial artery arose from the axillary twice as often as the ulnar. In taking origin high up from the brachial, the radial artery sprung most frequently from the upper part of that trunk ; while, on the contrary, the ulnar artery most commonly arose from the brachial, near its lower end.

There are certain variations of size presented by the radial and ulnar arteries which may be best explained together, for they exhibit a principle of compensation, according to which, if one be smaller, the other is larger. .

The ulnar artery is the vessel which is the most frequently diminished in size, its deficiency being then usually compensated for on the hand by the radial, as will be shown in considering the arteries of the hand ; but the palmar part of the ulnar artery may, when small, be reinforced by a large median artery, and in the fore-arm the ulnar artery itself has been found to be strengthened by another branch from the anterior interosseous.

A diminution in size of the radial is less frequent than of the ulnar artery.

The defect, when it exists, is compensated, especially in the hand, by an increase in the size of the ulnar. In the fore-arm the radial artery has been observed to be reinforced by the anterior interosseous in front of the wrist, and by the perforating branch of that vessel behind the carpus. The last cases referred to may be regarded as a transition to an extremely rare variety, in which the radial artery ended by giving the recurrents and a few muscular branches, the place of its descending trunk being supplied near the wrist by the interosseous. An example of this arrangement of the vessels is in the Museum of Professor Otto, at Breslau.  

From Quain's Anatomy.