Muscles of the Forearm.—The two bones of the forearm are covered by a series of muscles with fleshy bellies, generally fusiform, terminating inferiorly in tendons which are frequently very long, and become prominent in the region of the wrist. Some of these muscles move the forearm on the arm, or the radius on the ulna, but the action of the greater number is to move the hand on the forearm and the different segments of the fingers on each other. These muscles are divided into five series, each composed of four muscles, making the total number of muscles in the arm twenty altogether. We shall dwell chiefly upon the superficial muscles, a short mention being sufficient for the deeper ones. We distinguish first of all two groups, anterior and posterior: one on the front, the other on the back of the forearm. The anterior group is divisible into: 1st. An anterior superficial layer, of which each muscle will be studied; 2nd. An anterior deep layer, to which we will briefly refer. The posterior group may be subdivided into three: 1st. An external layer; 2nd. A superficial posterior layer, which we must review in detail; and 3rd. A posterior deep layer, respecting which we shall only say sufficient to enable the reader to understand the shape of the wrist formed by the corresponding tendons.

A. ANTERIOR MUSCLES OF THE FOREARM. I. Anterior superficial muscles.—All these muscles arise by a common tendon from the internal condyle of the humerus; their common origin does not appreciably extend above the level of the condyle, so that at the inner side of the elbow, contrary to what takes place on the outer side, the muscles of the forearm do not ascend on the corresponding side of the upper arm. If from the inner condyle we draw four lines, of which the first goes towards the middle of the radius, the second towards the outer border of the wrist, the third towards the middle, and the fourth towards the inner border of the wrist; these four lines, of which the first is very oblique and the others gradually approach the vertical, will give us the direction of each of these four anterior superficial muscles of the forearm, which are, in the order of the lines, from without inwards, the pronator radii teres, the flexor carpi radialis, the palmaris longus, and the flexor carpi ulnaris.

The pronator teres (6, Fig. 77) is fleshy throughout the entire extent in which it is visible on the model; arising from the internal condyle, it is directed obliquely downwards and outwards, and disappears beneath the external muscles of the forearm (supinator longus) to be inserted into the radius (around which it is slightly twisted) in the middle of its external surface (impression for the pronator teres, page 78). Contracting, the pronator teres turns the radius forward and inwards, producing pronation. This muscle forms the inner side, very oblique, of a triangular pit, known as the hollow of the elbow, of which the outer side is formed by the supinator longus (12, Fig. 77). In this hollow the tendon of the biceps dips down (3, Fig. 77) along with the brachialis anticus (4, 4, Fig. 77) to be inserted into the bones of the forearm. The upper part of the pronator teres is crossed by the aponeurotic expansion of the biceps (2, Fig. 77), and we have already dwelt on the particulars of external form which result from this arrangement.

The flexor carpi radialis (7, Fig. 77) arises from the inner condyle by a fusiform fleshy belly which descends obliquely to the middle of the forearm. It is replaced by a narrow, strong tendon which gains the outer part of the wrist, and then disappears beneath the annular ligament of the carpus, and, lying in a groove on the anterior surface of the trapezium, is inserted into the bases of the metacarpal bones of the fore and middle fingers. This muscle flexes the hand on the forearm; when it contracts, its tendon becomes very prominent and raises the skin at the lower part of the front of the forearm; it forms the first tendinous prominence that we meet at this level in passing from the radial to the ulnar border, and lies just internal to the groove in which the radial artery pulsates.

The palmaris longus (8, 9, Fig. 77) is much smaller than the preceding muscle; arising from the internal condyle, it forms a short fusiform fleshy belly (8) to which a long slender tendon succeeds. Descending almost vertically to the middle of the wrist, the tendon ends by being inserted into the annular ligament of the carpus and the fascia of the palm (9, Fig. 77). The palmaris longus flexes the hand on the forearm, and its tendon forms in contraction of the muscle a well-marked prominence situated in the middle of the lower part of the front of the forearm on the inner side of the tendon of the flexor carpi radialis. In some subjects this muscle is absent, and it is frequently subject to variations of form.

The flexor carpi ulnaris (10, 11, Fig. 77) arises not only from the inner condyle, like the three preceding muscles, but also (18, Fig. 78) from the posterior border of the ulna (including the olecranon) process; it descends vertically over the ulna, and is inserted by tendinous and fleshy fibres into the pisiform bone of the carpus (11, Fig. 77). As its fleshy fibres continue down to its insertion the form of the muscle is not marked by a prominence such as those produced by the tendons of the preceding muscles; this muscle, therefore, helps to give a rounded form to the whole extent of the inner border of the forearm. It flexes the hand, and at the same time draws it inwards towards the inner side of the wrist.

II. Anterior deep muscles.—These muscles constitute a fleshy mass lying beneath the preceding superficial muscles, and terminate near the wrist in numerous tendons, which pass to the fingers; the tendons of the muscles appear to a slight extent on the model in the grooves between the tendons of the superficial muscles (19, 19, Fig. 77). Lower down the tendons of these muscles lie in the anterior groove of the carpus, bound down by the corresponding annular ligament, and after traversing the hollow of the palm of the hand, they are inserted into the phalanges of the fingers, presenting characteristic features which we will rapidly point out in the enumeration of these muscles.

These deep muscles comprise, 1st, the flexor sublimis digitorum (19, Fig. 77), which divides below into four tendons, one for each finger (except the thumb); 2nd, the flexor profundus digitorum, which also divides into four tendons, one for each finger (except the thumb). On the anterior surface of each finger, therefore, we see two tendons—one superficial (23, Fig. 77) and the other deep. The first (flexor sublimis) presents at the level of the first phalanx (20, Fig. 77) a slit or button-hole, through which the second passes (flexor profundus); owing to this arrangement the tendon of the deep flexor is inserted into the base of the third phalanx (22, Fig. 77). superficial flexor is inserted into the base of the second phalanx (21, There is, indeed, a flexor muscle for each of the phalanges; the first phalanges of the fingers having special flexors in addition, the small lumbricales muscles of the palm of the hand, which arise from the sides of the tendons of the flexor profundus digitorum in the palm of the hand.

3rd. The flexor longus pollicis (26, Fig. 77) arises in the forearm from the radius, and its tendon is inserted into the base of the second or last phalanx of the thumb. It gives no evidence of its existence on the surface. 4th. The pronator quadratus muscle, a quadrilateral fleshy muscle, is disposed in a different manner to the preceding muscles, which must be raised in order to see it. It is formed by transverse fibres placed in the lower fourth of the forearm, and it passes from the anterior surface of the ulna to that of the radius. Its contraction produces pronation, rotating the radius over the lower part of the shaft of the ulna.

B. POSTERIOR MUSCLES OF THE FOREARM. I. External muscles.—These form the fleshy mass which covers the lower third of the outer border of the arm (Fig. 75, page 225) and descends along the outer border of the radius in the forearm. The group comprises four muscles—the supinator longus (brachio-radialis), two radial extensors of the carpus, and the supinator radii brevis.

Of these four muscles, one only is visible on the model throughout its entire extent—viz., 1st, the supinator longus or brachio-radialis (14, Fig. 75; 12, 13, Fig. 77), which arises from the outer supra-condyloid border of the humerus, between the brachialis anticus in front and the triceps behind. It is noteworthy that on this side the muscles of the forearm ascend on the arm almost up to the insertion of the deltoid. The supinator longus enlarges as it descends, so that it presents its greatest size at the level of the external condyle, the prominence of which it completely conceals. It forms the outer vertical boundary (page 234) of the triangular hollow at the bend of the elbow. Just below the middle of the forearm, the fleshy fibres are replaced by a long tendon which lies on the radius, and (13, Fig. 77) is finally inserted into the base of the styloid process of that bone. Notwithstanding its name, this muscle is not essentially a supinator; it acts in this way only when the forearm is in a position of complete pronation, and its proper action in this respect is to bring the forearm into a position between supination and pronation. Its principal function, however, is the flexion of the forearm on the arm, and in this movement its form is clearly shown externally in the form of a prominent band, which rises from the arm, and forms, on the antero-external part of the elbow, a strong fleshy mass, filling up the hollow produced by the flexion of the forearm on the arm. The supinator longus is the most important of the muscles of the forearm with regard to the part it takes in the external form of this region.

The two next muscles (14 and 15, Fig. 77) are partly hidden by the preceding; they are the radial extensors of the carpus (2 and 3), distinguished as the extensor carpi radialis, longior and brevior (15, Fig. 75). They arise from the lower part of the outer supra-condyloid ridge of the humerus and from the outer condyle, and form fleshy masses (3 and 5, Fig. 78), which increase the prominence of the supinator longus, and help to conceal the outer condyle. At about the same level as in the case of the supinator longus, a tendon succeeds the fleshy belly of each of these muscles, and inclines a little backward (Fig. 78); having been crossed by the extensor ossis metacarpi pollicis, by the extensor brevis pollicis (7 and 8, Fig. 78), and by the extensor longus pollicis; each reaches the dorsal surface of the wrist, and is inserted into the base of the metacarpal bone of the index finger (extensor carpi radialis longior), and into the base of the metacarpal bone of the middle finger (extensor carpi radialis brevior, 6, Fig. 78).

4th. Surrounding the upper part of the radius is a small deep muscle, which does not show on the model, and which we mention here only to point out that its presence increases the prominence of the muscular mass on the outer side of the elbow; this is the supinator radii brevis, formed by fibres which, arising from the humerus, elbow-joint, and ulna, are rolled round the back and outer side of the radius, and which acts so as to turn the bone backwards and thus produce supination.

II. Posterior superficial muscles (Fig. 78).—This group comprises the extensor cemmunis digitorum, the extensor minimi digiti, the extensor carpi ulnaris, and the anconeus. These four muscles arise from the external condyle, to which they are attached by a common tendon; from this origin they are directed downwards, the first almost vertically, the last (anconeus) very obliquely backwards and inwards.

1st. The extensor communis digitorum (11, Fig. 78) forms a long fusiform fleshy belly. In the lower third of the posterior surface of the forearm, it gives place to a tendon which soon subdivides into four bands. These remain in contact until they have passed a groove situated in the centre of the inferior extremity of the radius, and separate on the dorsal surface of the wrist, diverging so as to be attached to each of the four fingers. On the back of the hand the tendons of the middle, ring, and little fingers are joined together by two oblique tendinous slips directed obliquely downwards and outwards, from the fourth to the third, and from the third to the second tendon respectively. On the dorsal surface of the first phalanx of each finger (12, Fig. 78) the extensor tendon forms a membranous expansion, and thereafter divides into three slips, a central one inserted into the base of the second phalanx, and two lateral slips which unite again to be inserted into the base of the third phalanx. The tendons cover the backs of the knuckles and of the joints of the fingers.

2nd. The extensor minimi digiti is a small bundle of fleshy fibres lying adjacent to the preceding muscle, but distinctly detached from it. It gives rise to a separate tendon, which passes through a special groove beneath the posterior annular ligament between the lower ends of the radius and ulna. From the wrist it is directed towards the posterior surface of the little finger (14, Fig. 78), where it unites with the tendinous fasciculi from the common extensor tendon for the little finger, and joins in the formation of the membranous expansion on the dorsum of the first phalanx.

3rd. The extensor carpi ulnaris (15, Fig. 78) is a fusiform muscle arising from the external condyle and the posterior surface of the ulna. It is replaced by a tendon in the lower fourth of the forearm (15, Fig. 78). This tendon passes beneath the posterior annular ligament in a groove on the posterior surface of the ulna (10, Fig. 78), and at the inner part of the dorsal surface of the carpus it terminates almost immediately by being inserted into the base of the metacarpal bone of the little finger (16, Fig. 78).

These three muscles are extensors of the fingers and hand. If we examine a living model which has the arm folded on the trunk, the dorsal surface of the forearm being turned forwards, and which quickly moves the fingers and hand, we see clearly the movements of the fingers marked by muscular movements in the upper two-thirds of the posterior surface of the forearm. We can, by following their prominences in contraction, clearly recognise the fusiform bellies of these muscles.

4th. The anconeus occupies the upper part of the posterior surface of the forearm. As its name indicates (ἀγκὼν, olecranon, elbow), it is a muscle of the region of the elbow; it forms (17, Fig. 78) a triangular fleshy mass, of which the apex is attached to the external condyle, and the base is inserted into the external surface of the olecranon, and corresponding part of the ulna (19, Fig. 78). As the ulna does not possess appreciably the movement of rotation, but shares only in flexion and extension of the elbow-joint, the anconeus situated behind the joint has no other action than that of extending the forearm on the arm. When this movement is forcibly produced, we see the anconeus clearly marked by a triangular prominence, of which the upper border, the shortest, is united with the prominence of the triceps; and we have already mentioned the anatomical fact that the inferior portion of the triceps is directly continuous in the forearm with the anconeus muscle.