This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The deep fascia of the arm completely encircles it, like a tube. It is continuous above with the fascia covering the deltoid, pectoralis major and teres major muscles, and axillary fascia. Below, it is continuous with the fascia of the forearm and is attached to the olecranon and internal and external condyles.
On each side of the lower half of the humerus, extending from the condyles and the bone above outward to the deep fascia, are two fibrous partitions. They are the internal and external intermuscular septa. The space in front of them is filled by the flexors, the biceps, and brachialis anticus, and the space behind contains the triceps extensor. The external septum begins at the external condyle and extends above to the tendon of the deltoid, with which it blends. The internal septum begins below at the internal condyle and extends above to the coraco-brachialis. The radial (musculospiral) nerve and anterior terminal branch of the (superior) profunda artery, as they wind around the humerus below the insertion of the deltoid, pierce the external septum. The internal septum is pierced high up by the ulnar nerve and superior ulnar collateral (inferior profunda) artery as they emerge at about the level of the lower portion of the insertion of the coracobrachialis to pass down behind the internal condyle.
Inasmuch as the movements of the elbow-joint are anteroposterior only and not lateral, the muscles are principally on the front and back and not on the sides. Hence on looking at an arm a rounded mass is seen anteriorly and posteriorly, and separating them on the sides can be seen in a spare, muscular individual, distinct furrows called the internal and external bicipital furrows. If these furrows are obscured by fat, one can still feel that the bone is nearer the surface at these points than elsewhere. The anterior muscle mass is formed by the biceps and brachialis anticus muscles, the posterior mass by the triceps. The bone is most readily felt at the insertion of the deltoid at the middle of the outer side of the arm. From this point directly down to the external condyle passes the external intermuscular septum and external bicipital furrow. Winding around from the posterior edge of the insertion of the deltoid is the radial (musculospiral) nerve and (superior) profunda artery. They pierce the external intermuscular septum and pass downward in the groove formed by the brachioradialis (supinator longus) and extensor muscles on the outside and the brachialis anticus on the inside. On the inner side of the arm the bicipital furrow, between the biceps in front and the triceps behind, is quite evident and marks the internal intermuscular septum, which extends to the medial (internal) condyle. In front of it lie the brachial artery and veins, and median and medial antebrachial (internal) cutaneous nerve. At the upper portion of the inside of the arm can be seen the swell formed by the coracobrachialis muscle. The inner or posterior border of the coracobrachialis is continuous with the inner border of the biceps, and the brachial artery follows them. The coracobrachialis muscle ends just below the level of the insertion of the deltoid, and, of course, can neither be seen nor felt below that point. It is here that the ulnar nerve leaves the artery to pierce the internal intermuscular septum in company with the superior ulnar collateral (inferior profanda) artery to reach the groove behind the internal condyle. The brachial artery is covered only by the skin and superficial and deep fascia, and can be felt pulsating along the inner edge of the biceps muscle and tendon; it can be compressed against the bone by pressure directed outwardly above and inclining more posteriorly as the artery progresses down toward the bend of the elbow. It is on the inner side of the arm in the upper two-thirds, and is more anterior in the lower one-third (Fig. 284).
Fig. 283. - Triceps and anconeus muscles.
The cephalic vein runs up the external bicipital furrow and the basilic up the internal. At the junction of the middle and lower thirds of the arm the basilic pierces the deep fascia and from that point runs up beneath it and joins with the internal vena comes opposite the lower border of the teres major or subscapularis.