From their comparative frequency and surgical interest, the peculiarities of this artery, especially such as affect its trunk, deserve particular attention.

The brachial artery has been seen, though rarely, to deviate from its ordinary course in the following manner. At first it descends, accompanied by the median nerve, towards the inner condyle of the humerus as far as the origin of the pronator teres muscle, which is broader than usual, and then it inclines outwards under cover of or through that muscle, to gain its usual position at the bend of the elbow. In these cases the vessel may be found to turn round a prominence of bone, to which it is bound down by a fibrous band. This deviation of the artery and its connection with the bony prominence, may be regarded as analogous to the ordinary condition of the vessel in some carnivorous animals, in which it is directed to the inner side of the humerus, and passes through an osseous ring, a short distance above the inner condyle of that bone.

The most frequent change from the ordinary arrangement of the brachial artery relates to its place of division into terminal branches. In 386 out of 481 examples recorded from observations made some on the right and some on the left side of the body, the vessel was found to divide at its usual position, a little below the elbow-joint. In one case only (and that complicated by another peculiarity, viz., the existence of a " vas aberrans" proceeding from the axillary to the radial,) was the place of division lower than usual, or between two and three inches lower than the elbow-joint. In 64 cases the brachial artery divided above the usual point, at various heights upwards to the lower border of the axilla. The branch prematurely given off from an early division is most frequently (in the proportion of nearly 3 cases out of 4) the radial artery ; sometimes the ulnar is thus given off, and rarely the interosseous of the fore-arm, or a " vas aberrans."

Now in all these cases it is evident that two arteries must exist in a certain portion'of the arm, instead of one, as usual f and the extent to which they were found varied, of course, according to the height at which the artery divided. The point of division, in the entire number of cases, without reference to the particular branch given off, was most frequently in the upper, less so in the lower, and least so in the middle third of the arm. But the early division of the main artery of the upper limb may, as mentioned in speaking of the varieties of the axillary artery take place within the axilla, in which case it follows that the brachial portion of the vessel is represented, throughout its whole extent, by two separate trunks. In 94 cases out of 481, or about 1 in 5 1/9, there were two arteries instead of one in some parts or in the whole of the arm.

The position of the two arteries, in these cases, is of much surgical interest : we shall here consider their position in the arm, and subsequently trace them in their irregular course in the fore-arm. Usually they are close together, and occupy the ordinary position of the brachial artery; but there are some peculiarities in their position which require to be noticed.

The radial artery, when thus given off in the arm, often arises from the inner side of the brachial, then runs parallel with the larger vessel (the brachial or ulnar-interosseous), and crosses over it, sometimes suddenly, opposite to the bend of the elbow, still covered by the fascia. It has been found to perforate the fascia, and run immediately under the skin, near the bend of the elbow j but very few instances of this arrangement have been recorded.

When the ulnar is the branch given off high from the brachial, it often inclines from the position of the brachial, at the lower part of the arm, towards the inner condyle of the humerus. This vessel generally lies beneath the fascia as it descends, and superficially to the flexor muscles. It is occasionally placed between the integuments and the fascia ; and in a single instance was found beneath the muscles.

The interosseous, after arising from the axillary or brachial artery, is commonly situated behind the main artery, and, on reaching the bend of the elbow, passes deeply between the muscles, to assume its usual position in the fore-arm.

Lastly, when the radial has arisen high in the arm, the residuary portion of the brachial (brachial: ulnar-interosseous) has occasionally been observed descending, accompanied by the median nerve, along the inter-muscular septum towards the inner condyle of the humerus, as far as the origin of the pronator teres (which in the case recorded was found broader than usual), whence it turned outwards, under cover of the muscle, to gain the usual position at the middle of the bend of the elbow.

The two arteries connected or reunited. — -The two arteries representing the brachial are in some instances connected near the bend of the arm by an intervening trunk, which proceeds from the larger (or ulnar-interosseous) artery to the radial, or the radial recurrent, and varies somewhat in its size, form, and course. More rarely the two unusual arteries are actually reunited.

Vasa aberrantia

The " vasa aberrantia," alluded to in the preceding remarks, are long slender vessels, which arise either from the brachial or the axillary artery, and end by joining one of the arteries of the fore-arm, or a branch of these. In eight cases out of nine, — the total number observed, — this unusual vessel joined the radial ; in the remaining case it joined the radial recurrent, which arose irregularly from the ulnar artery. Monro and Meckel have in one case each seen an aberrant vessel join the ulnar. This peculiarity may be regarded, perhaps, as an approach to that condition in which there is division of the brachial artery and subsequent connection of its two parts by an intervening branch.

State of arteries in both limbs. — In most cases there is no correspondence be- tween both arms of the same person with respect to the high division of the arteries. For, in 61 bodies in which the high division existed, it occurred on one side only in 43 ; on both sides, in different positions, in 13; and on both sides, in the same position, in the remaining 5.

 From Quain's Anatomy.