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The arm, or brachium, is the region between the shoulder joint and the elbow. The arm muscles are few, and they are served by certain of the terminal branches of the brachial plexus and portions of the great vascular channels of the limb (see Plates 2-7 to 2-11).

MUSCLES OF UPPER ARM AND ELBOW, Brachial Fascia, Muscles, Coracobrachialis Muscle, Biceps Brachii Muscle, Brachialis Muscle, Triceps Brachii Muscle, Anconeus Muscle, Muscle Actions

Brachial Fascia
A strong tubular investment of the deeper parts of the arm, the brachial fascia is continuous above with the pectoral and axillary fasciae and with the fascial covering of the deltoid and latissimus dorsi muscles. Below, the brachial fascia is attached to the epicondyles of the humerus and to the olecranon and then is continuous with the antebrachial fascia. It is perforated for the passage of the basilic vein, for the medial antebrachial cutaneous nerve, and for many lesser nerves and vessels.
Two intermuscular septa are prolonged upward from the epicondylar attachments of the brachial fascia. These blend with the periosteum of the humerus along its supracondylar ridges and borders and fuse peripherally with the brachial fascia to form the anterior and posterior compartments of the arm. Above, the lateral intermuscular septum ends at the insertion of the deltoid muscle; the medial intermuscular septum ends in continuity with the fascia of the coracobrachialis muscle. The medial intermuscular septum has an additional, weaker anterior lamina, and the anterior and posterior laminae together with the brachial fascia form the neurovascular compartment of the arm (see Plate 2-10).

The muscles of the arm are separated both positionally and functionally by the humerus and the intermuscular septa into an anterior and a posterior group or compartments (see Plates 2-8 and 2-9). The anterior group comprises the coracobrachialis, biceps brachii, and brachialis muscles. The posterior group includes the triceps brachii and anconeus muscles. Important neurovascular structures in the anterior compartment include the musculocutaneous nerve, median nerve, ulnar nerve (proximally), radial nerve (distally), and brachial artery. Important neurovascular structures in the posterior compartment include the ulnar nerve (distally), radial nerve (proximally), and radial recurrent arteries. The median nerve, ulnar nerve, and brachial artery run along the medial aspect of the upper arm, while the radial nerve runs laterally. The musculocuta- neous nerve travels more in the midline, between the biceps brachii and brachialis muscles. Key origin and insertion points of the muscles of the arm are pictured on Plate 2-7.

Coracobrachialis Muscle
A short, bandlike muscle of the upper arm, the coracobrachialis, arises from the tip of the coracoid process and acts to flex and adduct the arm. The short head of the biceps brachii muscle originates from the lateral side of the coracoid process and runs side by side with the coracobrachialis to form the conjoined tendon. The coracobrachialis inserts by a flat tendon into the medial surface of the humerus just proximal to its midlength (see Plate 2-8). The musculocutaneous nerve supplies the coracobrachialis muscle and passes diagonally through the muscle at its midlength.

MUSCLES OF UPPER ARM AND ELBOW, Brachial Fascia, Muscles, Coracobrachialis Muscle, Biceps Brachii Muscle, Brachialis Muscle, Triceps Brachii Muscle, Anconeus Muscle, Muscle Actions

Biceps Brachii Muscle
The biceps brachii is a long, fusiform muscle of the anterior aspect of the arm (see Plate 2-8). Its long head arises as a rounded tendon from the supraglenoid tubercle of the scapula, crosses the head of the humerus within the capsule of the shoulder joint, and emerges from that capsule to travel down the bicipital groove of the proximal humerus, between the greater and lesser tuberosities. It is covered by the intertubercular synovial sheath. The short head of the biceps brachii muscle arises by a thick, flattened tendon from the tip of the coracoid process, in common with the coracobrachialis muscle and, unlike the long head, never has an intra-articular course.
The two bellies of the biceps brachii muscle unite at about the middle of the arm to form the most prominent muscle of the anterior compartment. The tendon of insertion is a strong, vertical cord palpable down the center of the cubital fossa. Here, its deeper part turns its anterior surface lateralward to end on the tuberosity of the radius, separated from the anterior part of the tuberosity by the small bicipitoradial bursa. The variable interosseous cubital bursa may separate the tendon from the ulna and its covering muscles. The musculocutaneous nerve supplies both heads of the biceps brachii, whose function is unique in that it acts across both the shoulder and elbow joints. At the shoulder, the muscle assists in forward flexion, joint stabilization (long head), and adduction (short head). At the elbow, its main actions are to flex the elbow and supinate the forearm. The biceps acts as an elbow flexor particularly when the forearm is supinated and is a strong supinator with the elbow at least partially flexed and with the forearm in a more pronated position.
The bicipital aponeurosis, or lacertus fibrosis, formed from the more anterior and medial tendon fibers of the muscle, arises at the bend of the elbow and passes obliquely over the brachial artery and median nerve to blend with the antebrachial fascia over the flexor group of the forearm (see Plate 2-8). The pull of the bicipital aponeurosis is largely exerted on the ulna.

Brachialis Muscle
This muscle arises from the lower half of the anterior surface of the humerus and the two intermuscular septa and lies deep to the biceps. Its upper extent has two pointed processes positioned on either side of the insertion of the deltoid muscle (see Plate 2-8). The muscular fibers converge to a thick tendon, which adheres to the capsule of the elbow joint and inserts on the tuberosity of the ulna and on the anterior surface of its coronoid process. Its major attachment is to the coronoid process about 2 mm distal from the articular margin. This muscle bulges beyond the biceps brachii muscle on either side, and anterior to its medial border lie the brachial vessels and the median nerve. The medial half of this muscle is supplied by the musculocutaneous nerve, whereas the lateral portion is supplied by the radial nerve. The muscle’s main action is to flex the elbow. The brachialis has the largest cross-sectional area of any of the muscles that flex the elbow, but it has a poor mechanical advantage owing to its close proximity to the axis of rotation. This natural internervous plane within the muscle allows it to be split during a routine anterolateral surgical approach to the humerus to come down on the anterior surface of the humerus, typically for fixation of a humeral shaft fracture.

MUSCLES OF UPPER ARM AND ELBOW, Brachial Fascia, Muscles, Coracobrachialis Muscle, Biceps Brachii Muscle, Brachialis Muscle, Triceps Brachii Muscle, Anconeus Muscle, Muscle Actions

Triceps Brachii Muscle
This large muscle with three heads occupies the entire dorsum of the arm (see Plate 2-9). Each head of the triceps (long, lateral, medial) originates distal to the other, with a progressively larger area of origin. The long head arises by a strong tendon from the infraglenoid tubercle of the scapula. Its belly descends between the teres major and teres minor muscles and joins the lateral and medial heads of the triceps in a common insertion on the olecranon. The long head is a defining border of the triangular space (lateral border), which contains the circumflex scapular artery; the quadrangular space (medial border), which contains the axillary nerve and the posterior humeral circumflex artery; and triangular interval (medial border), which contains the radial nerve and the profunda brachii artery. The lateral head takes origin from the posterior surface and lateral border of the humerus above and lateral to the radial groove and from the lateral intermuscular septum. Crossing the groove and concealing the radial nerve and deep brachial vessels, its fibers join in the common tendon insertion on the olecranon. The lateral head is the lateral border of both the quadrangular space and triangular interval. The medial head arises from the humerus entirely medial and below the radial groove from as high as the insertion of the teres major muscle to as low as the olecranon fossa of the humerus (see Plate 2-9). It also takes origin from the entire length of the medial intermuscular septum and from the lateral septum below the radial nerve groove. The medial head is deep to the other heads and is hidden by them. The tendon of the muscle appears as a flat band covering its distal two fifths. It inserts on the posterior part of the olecranon and into the deep fascia of the forearm on either side of it.
All three heads of the triceps brachii are innervated by the radial nerve and have a primary action of elbow extension. The long and lateral heads are innervated by branches of the radial nerve that arise proximal to the radial groove, whereas the branch to the medial head originates distal to the radial groove and also innervates the anconeus. A radial nerve injury at the midshaft of the humerus, therefore, usually does not disrupt function of the more proximally innervated long and lateral heads of the triceps. The humeral shaft can be surgically approached posteriorly by splitting the triceps muscle to come down onto the posterior cortex or by elevating the muscle along its lateral border and reflecting all three heads of the muscle medially. This approach may afford more exposure than an anterolateral approach in more distal fractures of the humeral shaft.

Anconeus Muscle
This is a small, triangular muscle that arises from a broad site on the posterior aspect of the lateral epicondyle of the humerus (see Plate 2-9). Its fibers diverge from this origin and insert into the side of the olecranon and the adjacent one fourth of the posterior surface of the ulna. The muscle is deep to the dorsal antebrachial fascia and extends across the elbow and the superior radioulnar joints. It is innervated by the terminal branch of the radial nerve that also innervates the medial head of the triceps. The function of this muscle has been the subject of some debate and includes assisting in elbow extension and stabilizing the elbow joint. Joint stabilization may be its primary role. The anconeus serves as a key landmark in the lateral or Kocher approach to the elbow, in which the interval between the extensor carpi ulnaris anteriorly and the anconeus posteriorly is uti- lized to approach the lateral or posterolateral aspect of the elbow.

Muscle Actions
The principal movements produced by the muscles of the arm are flexion and extension of the forearm at the elbow. The brachialis and biceps brachii muscles are the principal flexors. In this action, the brachialis muscle is always active; the biceps brachii muscle becomes active against resistance and is most effective when flexion of the forearm is combined with supination. It is a powerful supinator of the forearm and one of the primary muscles for producing this movement. Extension of the forearm is produced by the triceps brachii muscle and assisted by the anconeus muscle. The medial  head  of  the triceps  brachii  muscle is  usually active, and the lateral and long heads are recruited for extra powers.
Certain heads of these muscles are active at the shoulder joint. The long head of the biceps brachii muscle flexes the arm at the shoulder, and its tendon aids in stabilization of the joint. The long head of the triceps brac ii muscle assists in extension and adduction of the arm.