The muscles of the arm form two distinct fleshy masses, a mass in front formed by the biceps, which occupies the entire length of the arm; by the coraco-brachialis, which occupies only the upper part; and lastly, by the brachialis anticus, which occupies the lower part; and a mass behind, formed by one muscle only, the triceps.

The biceps (12, Fig. 75, and 21, Fig. 70) is so called because it possesses two heads of origin, which are known by the names of the long and short heads. The long head of the muscle presents the form of a long tendon, which arises from the upper limit of the glenoid cavity of the scapula, and descends through the shoulder-joint and the bicipital groove of the humerus to the arm. The short head of the biceps has a less complex course, and arises from the summit of the coracoid process, in common with the coraco-brachialis.

These two tendons (long and short head) descend in the outer wall of the axilla, covered by the great pectoral muscle (Fig. 70, page 197); a little above the lower border of this muscle the fleshy fibres appear and form two cylindrical bellies which descend and become united, at the level of the middle of the anterior surface of the arm, in one large mass, very marked in muscular subjects (12, Fig. 75). A short distance above the elbow-joint, a flat tendon of insertion succeeds to the fleshy muscle; it is at first broad, but soon divides into two parts, one aponeurotic, called the semi-lunar fascia of the biceps; the other, a rounded tendon forming the chief insertion of the muscle. The aponeurotic expansion (2, Fig. 77) of the bicipital fascia is directed downwards and inwards over the common mass of the flexor muscles of the forearm, and soon becomes blended with the aponeurosis of the forearm; the tendon proper (3, Fig. 74) dips down in the hollow of the elbow between the internal and external muscles of the forearm, and is fixed to the bicipital tubercle of the radius (page 78, and Fig. 21), upon which it turns to be inserted into the posterior part of the tubercle.

The biceps muscle is essentially the flexor of the forearm on the arm. This action is evident, and known to all, and it is useless to dwell on it, except to recall the fact that the biceps, in acting on the forearm, is inserted perpendicularly on the lever which it moves, so that it is in the most favourable position for the exercise of all its power. But the contraction of the biceps produces along with flexion of the elbow two other movements to which it is important to direct attention. 1st. If the forearm is in pronation the tendon of the biceps is obviously twisted round the upper part of the radius, since it is inserted into the posterior part of the bicipital tubercle; the first effect produced, therefore, by its contraction is a rotation of the radius outwards, and a movement of supination; the biceps is, therefore, a supinator, and one of the most powerful. 2nd. Besides flexion at the elbow-joint and this movement of supination, the biceps muscle raises and draws inwards the humerus by its action on the shoulder-joint. The three separate movements are found in association with one another in such a movement as the act of raising the hand to the mouth.

During the contraction of the biceps, its aponeurotic expansion is tightened, and binds down the mass of the flexor muscles of the forearm over which it passes; hence it marks on the inner surface of the fleshy part of this portion of the limb, two fingers’ breadth below the internal condyloid ridge, a distinct furrow, during the contraction of the muscle.

The changes of form in the arm which accompany the contraction of the biceps are well known, and it is sufficient to recall the fact that the fleshy belly of this fusiform muscle is lengthened in the state of repose, becoming short and globular during contraction. Nothing is more striking, nor gives a better idea of the change of form in a muscle during contraction, than to examine the biceps in a model, who brings it gradually into action by flexing the forearm on the arm (Fig. 76). We then see on the anterior surface of the arm a kind of fleshy ball becoming more and more clearly marked, which hardens and swells up at the same time, so that it seems to mount up towards the upper part of the arm, to the inferior border of the great pectoral muscle.

The coraco-brachialis muscle forms a narrow fusiform fleshy mass, which occupies the upper part of the inner surface of the arm. It arises from the coracoid process of the scapula (alongside the short head of the biceps), and is inserted into the middle of the internal border of the humerus. When the arm is hanging loosely the lower half only of this muscle is visible on the subject, and its prominence is blended with that of the biceps, while its upper half is hidden within the axillary space, beneath the great pectoral muscle. It becomes visible beneath the skin of the armpit, however, when the arms are raised, as in the position on the cross, and we have already dwelt upon the contour which the fusiform muscular belly of this muscle presents in the outer wall of the axilla. When the coraco-brachialis contracts, its form becomes more prominent, like that of every muscle in contraction; but it is not more visible on this account, for as it draws the arm to the thorax against which it is applied, it conceals by this movement the region in which its prominence is marked.

The brachialis anticus muscle (13, Fig. 75, and 4, 4, Fig. 77).—Situated beneath the lower half of the biceps, which it overlaps on each side, this muscle is deeply placed, and covers the lower two-thirds of the anterior surface of the humerus, to which it is attached. The fleshy fibres descend almost to the level of the elbow, where they are replaced by a flat tendon, inserted into the front of the coronoid process of the ulna. As the ulna does not exhibit any of the lateral movements comprised in supination and pronation, the brachialis anticus is simply a flexor of the forearm, and when this movement is forcibly accomplished, it may be seen to swell up on each side of the lower part of the biceps.

Triceps muscle.—This muscle (21, 22, Fig. 73; 1, 2, 3, Fig. 75), which forms alone the entire muscular structure of the posterior surface of the arm, has been called the triceps because it arises by three heads, separated above, united below; one central or median, called the long head, and two lateral parts, distinguished as the external and internal heads. The long head (2, Fig. 75), forming a thick and fusiform fleshy belly, arises by a stout tendon from the upper part of the axillary border of the scapula immediately below the glenoid cavity, and passes between the teres major and teres minor muscles to the back of the arm (page 223). At the level of the junction of the middle with the lower third of the arm, the fleshy belly terminates in a broad, flat tendon (1, Figs. 75 and 78) which forms the main insertion of the muscle, and receives the insertions of the two other portions of the muscle. The external head (3, Fig. 75) arises from the upper part of the posterior surface of the humerus (above and to the outer side of the spiral groove), and is directed obliquely downwards and inwards to be attached to the external border of the main tendon almost as low down as the elbow. Lastly, the internal head (22, Fig. 73) arises from the lower part of the posterior surface of the humerus (below and to the inner side of the spiral groove), and is attached to the internal border and deep surface of the inferior common tendon. This tendon is inserted (4, Fig. 75) into the upper surface of the olecranon process of the ulna, and on either side of this it blends with the deep fascia of the forearm.

The form of the posterior surface of the arm is greatly influenced by the presence of the inferior common tendon of the triceps, which, narrow and pointed towards its upper end, forms a large flat surface at the lower part of the arm. This flat surface is overlapped on each side by the prominence of the fleshy parts of the inner and outer heads. Above, in the upper two-thirds of the posterior surface of the arm, two fleshy bellies may be seen side by side, an outer one formed by the outer head, an inner one formed by the long head of the muscle. The inner head, more deeply placed, is only obvious in the lower third of the arm. These various details—viz., the tendinous flat surface above the olecranon, the muscular prominences which bound it on each side, and the two fleshy bellies which ascend above it—become visible in a marked degree when the subject forcibly extends the forearm on the arm, as in struggling against any resistance which causes the position of flexion to be maintained. It is hardly necessary to say that the triceps is essentially the extensor muscle of the forearm on the arm.

It is not only for the purpose of a regular enumeration, but also with regard to the anatomical interpretation of their external forms, that we have classed the muscles of the arm as anterior and posterior. On each side of the arm at both inner and outer borders, a groove runs which separates the anterior from the posterior muscles. In each of these grooves is a fibrous partition called the internal and external intermuscular septum, which extends from the surface to the corresponding supra-condyloid ridge of the humerus, and serves to increase the area of muscular attachment. In muscular contraction, therefore, each septum or aponeurosis is slightly drawn towards the humerus, and on the surface two grooves are clearly seen, each corresponding to an interval, internally or externally, between the anterior and posterior muscles.

The internal groove commences at the inferior extremity of the coraco-brachialis, and descends almost to the inner condyle. Above it is faintly marked, because the numerous nerves and vessels surrounded by cellular tissue fill up the intermuscular space at this level; below it spreads out, and tends to be confounded with the form of the internal part of the brachialis anticus.

The external groove (Fig. 75) is short. It begins at the level of the lower extremity of the deltoid, and does not descend to the external condyle, because the highest muscles of the extensor group belonging to the forearm arise from the lower part of the outer border of the humerus, so that the groove is filled up by these muscles as they curve forwards towards the bend of the elbow.