III. The deep posterior muscles of the forearm, with regard to external form, are important only in the arrangement of their tendons in the wrist and hand; for this reason we describe them in this chapter along with the muscular structures of the hand and fingers.

As in the other regions, we find in the deep part of the back of the forearm, four muscles. Proceeding from without inwards they are—the extensor ossis metacarpi pollicis, the extensor brevis pollicis, the extensor longus pollicis, and the extensor indicis. The fleshy bellies of these muscles are almost completely hidden beneath the posterior superficial muscles; but their tendons, at least those of the first three, emerge beneath the external border of the common extensor of the fingers, and their form is shown (7 and 8, Fig. 78) by details of great importance on the outer side of the dorsal aspect of the wrist.

The two first (7 and 8, Fig. 78), the extensor ossis metacarpi pollicis (7) and the extensor brevis pollicis (8), must be described together, as their fleshy parts and tendons are placed together and are almost united throughout the greater extent of their course. These two muscles emerge in the lower third of the forearm, at the outer border of the common extensor of the fingers; and they form at this point, at the junction of the posterior surface with the outer border of the forearm, an oblong eminence, which is soon succeeded by a double tendon. Crossing the tendons of the radial extensors of the wrist, this double tendon is directed towards the outer surface of the lower end of the radius, where it passes through a groove (10) converted into a canal by the posterior annular ligament of the wrist. At the outer border of the wrist these two tendons form a prominence, well marked beneath the skin, when the thumb is separated from the other fingers. Finally, at the base of the metacarpal bone of the thumb these tendons separate, one, that of the extensor ossis metacarpi pollicis, being inserted into the base of the metacarpal bone, while the other, that of the extensor brevis pollicis, passes over the first metacarpal bone to be inserted into the base of the first phalanx of the thumb (8, at the level of the thumb, Fig. 78).

The extensor longus pollicis (9, Fig. 78) emerges, like the preceding tendons, on the outer side of the common extensor, but lower down, on the back of the wrist; its tendon only becomes superficial at this point, and it is directed obliquely downwards through a prominent deep groove on the posterior surface of the lower end of the radius, where it is covered by the posterior annular ligament. On the back of the wrist the tendon is directed obliquely outwards, crossing the tendons of the radial extensors of the wrist, to reach the base of the metacarpal bone of the thumb, where it lies parallel to the tendon of the extensor brevis pollicis. Descending lower than this tendon it is finally inserted into the second or terminal phalanx of the thumb (9, Fig. 78).

The tendons of the extensor ossis metacarpi and extensor brevis pollicis on the one hand, and the extensor longus pollicis on the other, form on the outer part of the back of the wrist a triangular figure, of which the apex corresponds to the upper end of the thumb, and the base to the lower end of the radius. When we separate the thumb strongly from the index finger—that is to say, when we contract the three small muscles which we have been studying—the corresponding tendons mark the borders of this triangle in the form of prominent cords, between which is a deep concavity; to this depression is given the name of the anatomical snuff-box (4, Fig. 78).

The extensor indicis is not visible on the model; it is deeply situated beneath the common extensor of the fingers, and terminates in a tendon (13, Fig. 78) which unites with the tendinous fibres from the common extensor to the index finger. It is to this muscle that the index finger owes its power of extension independent of the other fingers, and of performing the functions which have given it the name of the index or indicating finger. It is to be noted that in separate extension of the index finger (with flexion of the others) the tendon of the extensor communis digitorum in the back of the hand gets curved outwards, by being pulled into line with the extended index finger by the traction of the tendon of the extensor indicis (Macewen).

Muscles of the hand.—The numerous muscles belonging to the hand form an interesting study in relation to the mechanism of the multiple and delicate movements of the fingers, but as the various details of their complex arrangement do not show very plainly on the surface we may confine our study of them to an enumeration of their principal features.

The dorsal region of the hand (Fig. 78) does not possess any fleshy muscles, but only presents the tendons belonging to the muscles of the forearm. The arrangement of the tendons after passing beneath the posterior annular ligament has already been described (pp. 245–6). Notice should be taken of an arch of veins lying superficial to the tendons. This venous arch receives the veins from the fingers, and is variable in its arrangement and outline.

On the other hand, the anterior or palmar region of the hand possesses, beside the tendons of the muscles of the forearm, numerous small muscles, which are divided into three groups:—1st, an external group associated with the thumb, forming the fleshy prominence known as the thenar eminence; 2nd, an internal group, belonging to the little finger, forming the hypothenar eminence; 3rd, a middle and deeper group, formed by small muscles belonging to the fingers, and placed deep in the hand beneath the thick palmar fascia, which, occupying the hollow of the palm, fills up the space between the thenar and hypothenar eminences (Fig. 79).

1st. The thenar eminence (2–5, Fig. 79) is of a long ovoid form, with the large superior extremity corresponding to the carpus, and the smaller inferior extremity corresponding to the base of the first phalanx of the thumb. It is formed by six muscles, namely, the abductor pollicis (24, Fig. 77; 2, Fig. 79), which proceeds from the trapezium and annular ligament to the outer side of the first phalanx of the thumb; the opponens pollicis (3, Fig. 79), with a similar origin, inserted into the entire length of the outer border of the first metacarpal bone, so that its contraction draws the whole thumb (phalanges and metacarpal bone) towards the palm of the hand, and thus opposes it to the other fingers; the flexor brevis pollicis (superficial part) (4, Fig. 79), which proceeds from the annular ligament to the base of the first phalanx; the adductor obliquus pollicis (5, Fig. 79), arising from the carpus and heads of the metacarpal bones, and inserted into the inner side of the first phalanx of the thumb; the adductor transversus pollicis (6, Fig. 79), a muscle remarkable for its arrangement, for it arises from the middle of the palm of the hand, from the anterior surface of the third metacarpal bone, and forming a comparatively broad fleshy mass, extends outwards to be inserted along with the adductor obliquus into the inner side of the base of the first phalanx of the thumb; and finally a small muscle deeply placed in the space between the first and second metacarpal bones, and known as the deep part of the flexor brevis pollicis (or the interosseus primus volaris).

2nd. The hypothenar eminence is long and elliptical in form, but smaller than the thenar eminence; it is covered in part by a small muscle which is not shown by any external prominence, but only by the folds which it marks in the skin during its contraction; this is the palmaris brevis, formed of transverse fibres, which, arising from the inner border of the palmar fascia, is inserted into the deep surface of the skin on the inner border of the hand. The contraction of its fibres draws the skin of the part upwards and forms an irregular vertical furrow, so that the prominence of the skin of the upper part of the hypothenar eminence becomes more clearly marked. The hypothenar eminence itself is formed by three small muscles vertically arranged over the fifth metacarpal bone:—1, the abductor minimi digiti (28, Fig. 77; 10, Fig. 79), passing from the pisiform bone to the inner side of the first phalanx of the little finger; 2, the flexor brevis minimi digiti (27, Fig. 77; 11, Fig. 79), passing from the prominence of the unciform bone to the same phalanx; and lastly, 3, the opponens minimi digiti, arising from the unciform bone and inserted into the entire length of the fifth metacarpal bone, so that its contraction slightly draws outwards the whole of the little finger, and opposes it to a certain degree to the thumb.

3rd. The muscles of the middle region of the palm of the hand are in two series. One set is arranged between the tendons of the flexor profundus digitorum muscle (see anterior deep muscles of the forearm, page 237) as small fusiform muscles, which have been compared to the form of a worm—hence their name of lumbricales (7, Fig. 79). The others are arranged in the spaces between the metacarpal bones, and are known as the interosseous muscles.

The lumbricales muscles, as Figure 77 shows (23, 23), are four in number—one for each of the four fingers. Their upper ends are attached to the deep flexor tendons; and from this origin they descend to reach the outer or radial border of the first phalanx of each finger. Sweeping round the knuckle on its outer side, each tendon is inserted into the metacarpo-phalangeal capsule, the outer side of the base of the first phalanx, and into the expansion of the extensor tendon on the dorsum of the first phalanx of each finger. In consequence of this mode of insertion each muscle is a flexor of the metacarpo-phalangeal joint, and in addition, because of its connection with the extensor tendon, it is at the same time an extensor of the inter-phalangeal joints.

The interosseous muscles are seven in number, divided into two series, dorsal and palmar. The dorsal muscles, four in number, are found one in each interosseous space between the metacarpal bones. The three palmar interosseous muscles occupy the three inner spaces. They arise from the sides of the metacarpal bones (the dorsal muscles by double origins, the palmar muscles by single heads from the inner three metacarpal bones); and they are inserted in the same way as the lumbrical muscles into the fingers, so as to act as flexors of the metacarpo-phalangeal, and extensors of the inter-phalangeal joints. The several muscles pass in relation to the sides of the knuckles in a way that need not be detailed here, so as to occupy positions which enable them to act so as to separate or draw together the fingers. The dorsal interossei muscles serve to separate the fingers from each other, while the palmar serve to bring them together; or, to be more precise, the dorsal muscles abduct the fingers from the middle line of the middle finger, while the palmar muscles adduct the fingers on which they act (third, fourth, and fifth) towards the axis of the middle finger.