IV. THE MUSCLES OF THE UPPER ARM
| Coracobrachialis | Brachialis |
| Biceps (brachii) | Triceps |
The brachial fascia or deep fascia of the arm is continuous with that covering the Deltoid and the Pectoralis major.; it forms a thin, loose sheath for the muscles of the upper arm, and sends septa between them; it is composed of fibers disposed in a circular or spiral direction, and connected together by vertical and oblique fibers. It is thin over the Biceps, but thicker inhere it covers the Triceps; and over the epicondyles of the humerus: it is strengthened by fibrous aponeuroses, derived from the Pectoralis major and Latissimus dorsi medially; and from the Deltoid laterally. On each side it gives off a strong intermuscular septum; which is attached to the corresponding supracondylar ridge and epicondyle of the humerus.
The lateral intermuscular septum extends from the lower part of the, lateral lip of the bicipital groove. (intertubercular sulcus), along the lateral supracondylar ridge, to the epicondyle: it is blended with the tendon of the Deltoid, gives attachment to the Triceps behind, to the Brachialis, Brachioradialis and Extensor carpi radialis longus in front, and is perforated at the junction of its upper and middle thirds by the radial nerve and the anterior descending branch of the arteria profunda brachii. The medial intermuscular septum, thicker than the preceding, extends from the lower pert of the medial lip of’ the bicipital groove (intertubercular sulcus) below the Teres major, along the medial supracondylar ridge to the epicondyle; it is blended with the tendon of the Coracobracbialis, and affords attachment to the Triceps behind and the Brachialis in front. It is perforated by the ulnar nerve, the ulnar collateral artery, and the posterior branch of the supratrochlear (inferior ulnar collateral) artery.
At the elbow, the brachial fascia is attached to the epicondyles of the humerus and the olecranon of the ulna, and is continuous with the antebrachial fascia. Just below the middle of the medial side of the upper arm, an oval opening in the fascia: transmits the basilic vein and some lymphatic vessels.
The Coracobrachialis (figs. 620; 622) is situated at the upper and medial part of the arm. It arises from the apex of the corticoid process, in common with the tendon of the short head of the Biceps, and by muscular fibers from the upper 10 cm. of this tendon; it is inserted into an impression, from 3 to 5 cm. in length, at the middle part of the medial border of the shaft of the humerus between the origins of the Triceps and Brachialis.
Relations.-It is perforated by the musculocutaneous nerve, and is in relation, in front, with the Pectoralis major above, and at its insertion with the brachial vessels and median nerve, which cross it; behind with the tendons of the Subscapularis, Latissimus dorsi, and Teres major, the medial bead of the Triceps, the humerus and the anterior circumflex humeral vessels; by its medial border, with the third part of the axillary artery, the upper part of the brachial artery, the median and musculocutaneous nerves; by its lateral border, with the Biceps and Brachialis.
Nerve-supply.-The Coracobrachialis is supplied by the musculocutaneous nerve (C. 7).
Action.-The Coracobrachialis draws the arm forwards and medially.
The Biceps (brachii) (figs. 620, 622, 623), a long, fusiform muscle placed on the front of the arm, has received its name from the circumstance that it has two heads of origin. The short head arises by a thick flattened tendon from the apex of the coracoid process; in common with the Coracobrachialis. The long head takes origin within the capsule of the shoulder-joint. It arises by a long narrow tendon from the supraglenoid tubercle at the apex of the glenoid cavity, and is continuous with the glenoidal labrum. The tendon of the long head, enclosed in a sheath of the synovial membrane of the shoulder joint; arches over the head of the humerus; it emerges from the joint through an opening in the capsular ligament close to its humeral attachment and descends in the bicipital groove (intertubercular sulcus); it is retained in the groove by the transverse humeral ligament and by a fibrous expansion from the tendon of the Pectoralis major. Each tendon is succeeded by an elongated muscular belly, and the two bellies, although closely applied to each other, can be readily separated until within about 7.5 cm. of the elbow-joint. Here they end in a flattened tendon, which is inserted into the rough, posterior portion of the tuberosity of the radius, a bursa being interposed between the tendon and the front part of the tuberosity. As the tendon of the muscle approaches the radius it is twisted upon itself, so that its anterior surface becomes lateral and is applied to the tuberosity of the radius at its insertion. Opposite the bend of the elbow the tendon gives off; from its medial side, a broad aponeurosis, named the bicipital aponeurosis (lacertus fibrosus), which passes obliquely downwards and medially across the brachial artery and is continuous with the deep fascia covering the origins of the flexor muscles of the forearm (fig. 619). With very little force the tendon of insertion can be split down to the radial tuberosity, when it can be seen that the anterior portion of the tendon receives the fibers of the short head, and the posterior portion those of the long head.
A third head to the Biceps is occasionally found, arising at the upper and medial part of the Brachialis, with which it is blended, and inserted into the bicipital aponeurosis and medial side of the tendon of the muscle , in most cases this additional slip lies behind the brachial artery. In some instances the third head consists of two slips, which pass flown, one in front of, the other behind the artery.
Nerve-supply-The Biceps is supplied by the musculocutaneous nerve (C. 5 and 6).
Actions,-The Biceps is a powerful supinator of the forearm; it also flexes the elbow-joint, and to a slight extent the shoulder-joint. Through the bicipital aponeurosis it is a tensor of the antebrachial fascia.
Applied Anatomy.-The long tendon of the Biceps is sometimes dislocated from its groove on the humerus. When this occurs, the arm is fixed in a position of abduction, but the head of the humerus can be felt in its proper position. The tendon can generally be replaced by flexing the forearm on the arm and rotating the limb. Rupture of the long tendon of the Biceps may also take place.
The Brachialis (figs. 620; 623, 624) covers the front, of the elbow-joint and the lower one-half of the humerus. It arises from the lower one-half of the front of the .humerus, commencing above at the insertion of the Deltoid, which it embraces by two pointed processes, and extending below to within 2.5 cm. of the margin of the articular surface. It also arises from the intermuscular septa but more extensively from the medial than from the lateral; it is separated from the lower part of the lateral intermuscular septum by the Brachioradialis and Extensor carpi radialis longus. Its fibers converge to a thick tendon, which is inserted into the tuberosity of the ulna and the rough depression on the anterior surface of the coronoid process.
Relations.–It is in relation, in front, with the Biceps, the brachial vessels, musculocutaneous and median nerves; behind, with the humerus and articular capsule of the elbow-joint; by its medial border, with the Pronator teres, and with the medial intermuscular septum, which separates it from the Triceps and the ulnar nerve; by its lateral border, with the radial nerve, radial recurrent and the anterior descending branch of the profunda brachii arteries, the Brachioradialis and Extensor carpi radialis longus.
Action.-The Brachialis flexes the elbow-joint.
The Triceps (figs. 621 to 624), situated on the back of the upper arm, is of large size, and arises by three heads (long, lateral and medial), hence its name.
The long head arises by a flattened tendon from the infraglenoid tubercle of the scapula, being blended at its upper part with the capsule of the shoulder-joint; the muscular fibers pass downwards along the medial side of the lateral head and superficial to the medial head, and join with them in the tendon of insertion.
The lateral head arises from a narrow ridge on the posterior surface of the shaft of the humerus, extending from near the insertion of the Teres minor to the upper part of the spiral groove (sulcus for the radial nerve), and from the lateral border of the humerus and the lateral intermuscular septum; the fibers from this origin converge towards the tendon of insertion.
The, medial head, which is covered posteriorly by the lateral and the long heads, arises from the posterior surface of the shaft of the humerus, below the spiral groove; it is narrow and pointed above, and extends from the insertion of the Teres major to within 2.5 cm. of the trochlea of the humerus; it also arises from the medial border of the bone and from the back of the whole length of the medial intermuscular septum. Some of the fibers are directed downwards to the olecranon, while others converge to the tendon of insertion.
The tendon of insertion of the Triceps begins about the middle of the muscle. It consists of two aponeurotic laminae, one of which covers the back of the lower one-half of the muscle; the other is more deeply seated in the substance of the muscle. After receiving the attachment of the muscular fibers, the two lamellae unite above the elbow, and are inserted, for the most part, into the posterior portion of the upper surface of the olecranon; on the lateral side a band of fibers is continued downwards, over the Anconeus, to blend with the antebrachial fascia.
The Subanconeus is the name given to a few fibers which spring from the deep surface of the lower part of the Triceps, and are inserted into the posterior part of the articular capsule of the elbow-joint.
Nerves.-The Triceps is supplied by the radial nerve (C. 6, 7, and 8).
Actions.-The Triceps is the great extensor muscle of the forearm. When the arm is extended, the long head of the muscle may assist in drawing the humerus backwards and in adducting it to the thorax. The long head supports the under part of the shoulder-joint. The Subanconeus draws tip the posterior part of the articular capsule of the elbow-joint during extension of the forearm.
Applied Anatomy.-The insertion of the Triceps into the deep fascia of the forearm is of importance in the operation of excision of the elbow, and should always be carefully preserved. By means of it the patient is enabled to extend the forearm, a movement which would otherwise be accomplished mainly by gravity-that is to say by allowing the forearm to drop by its own weight.
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