For the sake of a more correct description we divide the artery into thirds, viz: the upper, middle and lower thirds. The upper third begins at the extreme upper part of the arm and extends one third of the way to the elbow, the middle and lower thirds occupy the remainder of the artery. In the upper third we have the superior and inferior profunda arteries coming off; their position is not always the same, and in the extreme lower third the anastomotica magna artery. These arteries continue down the outer and inner arm and anastomose with the recurrent radial and ulnar arteries, thus furnishing collateral circulation. Thus if the fluid is injected in the middle third, toward the heart, these branches that come off the brachial in the upper third will convey the fluid down the arm, filling the branches below the point of injection, which supply the forearm and the hand.
[Illustration: FIG. 48—The brachial artery. (Gray)]
The brachial artery is one continuous vessel, the entire length of the upper arm, and varies in size according to the size of the person and the development of the arm. It is accompanied by the venae comites or deep brachial veins, the one to the inner side of the artery about one-third to one-half the size of the artery, the other about one-half its size lies directly underneath. All are encased in the same common sheath of deep fascia that surrounds and holds them together. Great care, then, should be taken to separate the artery from these veins before cutting the artery for injection.
The artery lies along the inner and under border of the large muscle on top of the arm known as the biceps. The biceps is the muscle used when lifting a weight. To those whose occupation is to exercise the muscular tissue of the body liberally, this muscle becomes quite large, and generally the artery is proportionally large.
_Linear Guide._—The course of the brachial artery may be marked out by drawing a line from the middle of the axillary space (arm pit) to the center of the elbow, provided the palm of the hand be turned upward. This line will be immediately over the artery, which will be found by cutting through the skin at any point on the line, and dissecting through the subcutaneous tissue toward the center of the arm.