2. Muscles of the Brachium or Upper Arm.

M. clavobrachialis and the pectoralis group (Fig. 65, page 109) lie partly in this region; they have been described.

M. coracobrachialis (Fig. 77, f; Fig. 79, c).—A very short muscle covering the inner (medial) surface of the capsule of the shoulder-joint. It extends from the coracoid process to the proximal end of the humerus.

Origin (Fig. 78, h).—By a round tendon from the tip of the coracoid process.

Insertion (Fig. 82, e) usually by fleshy fibres on an area five to eight millimeters long and about half as wide, which lies on the medial side of the humerus parallel with the ridge which runs from the dorsal end of the lesser tuberosity to the shaft and close to it. The proximal end of the area is about one centimeter from the proximal end of the head of the humerus.

Relations.—Medial surface with the biceps (Fig. 77, g), pectoralis minor (Fig. 65, o), and teres major (Fig. 77, c). Lateral surface with the capsule of the shoulder-joint. Dorsal border with the subscapularis (Fig. 77, a) and the long portion of the caput mediale of the triceps (Fig. 77, j).

Action.—Adducts the humerus.

The part of the coracobrachialis just described is known as the short head. A long head is sometimes found. It is a conical bundle of fibres of varying size, which arises from the tendon of origin of the short head. It passes distad into a long and extremely slender tendon, the insertion of which varies greatly in different individuals. It is commonly on the humerus in the region of the supracondyloid foramen.

M. epitrochlearis or extensor antibrachii longus (Fig. 65, r).—A thin, flat muscle on the inner or medial side of the brachium, from the lateral surface of the latissimus dorsi (Fig. 65, q) to the olecranon process of the ulna.

Origin from the lateral or outer surface of the ventral border of the latissimus dorsi (Fig. 65, q) near the insertion of the cutaneus maximus. Fibres are often attached to the teres major and the pectoralis minor.

Insertion by a flat tendon which is closely connected with that of the pectoantibrachialis (Fig. 65, m) and is continuous with the general antibrachial fascia, into the caudal border of the cutaneous (dorsal) surface of the olecranon process of the ulna.

Relations.—Outer (medial) surface with the integument, the latissimus dorsi (Fig. 65, q), and the cutaneus maximus. Inner (lateral) surface with the biceps (Fig. 65, t), caput longum (Fig. 65, s), and caput mediale (Fig. 77, j, k) of the triceps.

Action.—Extends the antibrachium and tends to supinate the hand by rotating the ulna.

M. biceps brachii (Fig. 77, g; Fig. 65, t).—A thick, fusiform muscle lying on the front (ventral) surface of the humerus.

Origin (Fig. 76, c) by a strong, round tendon from the bicipital tubercle of the glenoid angle of the scapula, at its coracoid margin. (In man there is a second head—coracoid head—from the coracoid process.) The tendon passes through the capsule of the joint and then along the bicipital groove, which is converted into a canal by a strong ligament.

Insertion by a rounded tendon (Fig. 87, b) on the bicipital tuberosity of the radius (Fig. 86, k).

Relations.—The tendon of origin passes through the capsule of the joint, which is covered by a part of the pectoralis minor. The muscle is then spanned for the middle part of its course by the bicipital arch (Fig. 65, t′), a description of which is given below. Farther distad it is covered by the clavobrachial (Fig. 65, k) and epitrochlearis (Fig. 65, r). At its distal end (Fig. 79, l′) the muscle passes between the pronator teres on the radial side and the conjoined tendon of the brachialis and clavobrachial (Fig. 79, i′) on the other. The biceps lies throughout most of its length on the ventral surface of the humerus, and touches along the medial border of its inner surface the coracobrachialis (Fig. 77, f), the teres major, (Fig. 77, c), the intermediate portion of the caput mediale of the triceps (Fig. 77, k), and the short portion (Fig. 77, l) of the same. Its lateral border touches the pectoralis minor (Fig. 65, o) and the brachialis (Fig. 79, i).

Action.—Flexes the forearm, and tends to supinate the hand.

The bicipital arch (Fig. 65, t′) is a tendinous arch formed over the biceps muscle. Its outer or lateral pillar is formed by the tendon of the pectoralis minor (o), to which may be added part of the latissimus (q) tendon. Its inner or medial pillar is formed by the conjoined tendon of the teres major and latissimus dorsi (Fig. 79, d′), while the xiphihumeralis (Fig. 65, p), cutaneus maximus (Fig. 62, b), and epitrochlearis (Fig. 65, r) are connected with one or the other pillar or with the muscles composing them. The caudal portion of the deep layer of the pectoralis major (Fig. 65, l) may be connected with the inner pillar.

M. brachialis (Fig. 79, i; Fig. 75, i).—From the lateral surface of the humerus to the ulna.

Origin (Fig. 83, g, g′) from a long V-shaped line two to four millimeters wide on the lateral surface of the humerus. The apex of the V is just distad of the teres minor tubercle (d). Its dorsal limb (g′) extends along the lateral supracondyloid ridge to a point opposite the proximal margin of the supracondyloid foramen; its ventral limb (g) extends in the direction of the deltoid ridge to the middle of the bone. No muscle-fibres take origin between the two limbs. The fibres converge and end in a flat tendon which joins the tendon of the clavobrachial (Fig. 79, k).

Insertion (Fig. 87, c).—The dorsal portion of the depressed rough area on the lateral surface of the ulna just distad of the semilunar notch.

Relations.—Outer surface with the acromiodeltoid (Fig. 75, f), the caput laterale of the triceps (Fig. 75, h), the brachioradialis (Fig. 75, k), and the proximal end of the extensor carpi radialis longus (Fig. 75, l). Inner surface with the humerus. Medial edge with the pectoralis major (Fig. 65, l) and the biceps (Fig. 77, g).

Action.—Flexor of the antibrachium.

M. triceps brachii.—The triceps muscle of the cat, like that of man, is divisible (as its name indicates) into three main portions. These are a lateral portion (caput laterale), an intermediate or long portion (caput longum), and a medial portion (caput mediale). The first two correspond to the similarly named heads in man; the medial portion falls into a number of subdivisions whose homologues are uncertain.

Caput laterale (anconeus lateralis) (Fig. 75, h; Fig. 68, l).—The lateral portion is a flat muscle (most of it subcutaneous) on the lateral side of the brachium, connecting the proximal end of the humerus with the olecranon process of the ulna.

Origin (Fig. 81, h′).—By a flat tendon from the proximal portion of the deltoid ridge and the distal border of the teres minor tubercle.

Insertion (Fig. 86, c) by a thin, flat tendon into the lateral border of the dorsal surface of the ulna between a point opposite the distal margin of the semilunar notch and the proximal end of the olecranon.

Relations.—Lateral surface with the integument, and at the proximal end with the spinodeltoid and acromiodeltoid (Fig. 75, e and f). Medial surface with the brachialis (Fig. 75, i), the caput longum (g), the caput mediale, the brachioradialis (k), the anconeus (Fig. 80, l), and the origins of the extensor carpi radialis longus (Fig. 75, l), the extensor communis digitorum (m), and the extensor digitorum lateralis (n).

Caput longum (anconeus longus) (Fig. 75, g; Fig. 68, k; Fig. 77, i).—A prismatic, fusiform mass connecting the scapula with the olecranon.

Origin (Fig. 78, i) by a thick, flat tendon from a triangular area one or two centimeters long at the glenoid end of the scapula on the glenoid border. The base of the area is about one millimeter from the margin of the glenoid fossa.

Insertion (Fig. 86, b).—The muscle ends in a thick tendon which passes over the bifurcated ventral end of the olecranon and is inserted upon the rounded tuberosity which forms the dorsal angle of the olecranon.

Relations.—Lateral surface with the caput laterale (Fig. 75, h), a portion of the caput mediale (Fig. 77, j and k), the spinodeltoid (Fig. 75, e), the infraspinatus (Fig. 75, c), and the integument. Medial surface with the epitrochlearis (Fig. 65, r) and the conjoined portions of the latissimus dorsi and teres major (Fig. 79, d′). Ventral border with the caput mediale (Fig. 77, j and k).

Caput mediale.—The medial head consists of three portions (Fig. 79, g, h, j).

(1) The long portion (anconeus posterior) (Fig. 77, j; Fig. 79, g; Fig. 80, f).

Origin (Fig. 83, e) from a triangular area on the dorsal surface of the humeral shaft. The base of the area is against the articular head, and its apex about one-sixth the length of the humerus from the head. It is between the coracobrachialis and the lateral head of the triceps.

Insertion (Fig. 86, a).—The muscle ends in a long, slender tendon which passes through the furrow on the ventral angle of the olecranon and is inserted into an oblique ridge which forms the dorsal limit of the furrow. The tendon is separated from the floor of the furrow by a synovial bursa.

(2) The intermediate portion (anconeus internus) (Fig. 77, k; Fig. 79, h; Fig. 80, j).

Origin (Fig. 82, g) by fleshy fibres from a triangular area proximad of the middle of the dorsomedial surface of the humerus. The length of the area equals about one-fourth the length of the bone. Its apex points proximad and is almost continuous with the apex of the area of origin of the long portion.

Insertion (Fig. 87, i) by short tendon-fibres into the medial border of the ventral and proximal surfaces of the olecranon. It is connected with the tendon of the long portion.

(3) Short portion (Fig. 79, j; Fig. 77, l).

Origin (Fig. 82, h) from the outer surface of the bony bar which encloses the supracondyloid foramen of the humerus, as far as the medial epicondyle.

Insertion (Fig. 87, e) by fleshy fibres into the medial border of the olecranon distad of the tuberosity for the insertion of the caput longum.

Relations of the caput mediale.—Lateral surface with the caput laterale (Fig. 75, h), the humerus, and the anconeus (Fig. 80, l). Dorsal surface with the caput longum (Fig. 77, i), the epitrochlearis (Fig. 65, r), and the teres major (Fig. 79, d). Ventral surface with the biceps (Fig. 77, g).

Action of the Triceps.—The entire triceps group forms a powerful extensor of the forearm. The short portion of the medial head tends also to rotate the arm outward, so as to supinate the hand.

M. anconeus (Fig. 80, l).—A triangular muscle on the outside of the elbow-joint from the humerus to the ulna.

Origin (Fig. 83, i).—An irregularly triangular area at the distal end of the dorsal surface of the humerus. The area of origin is limited laterally by the sharp lateral supracondyloid ridge. It sometimes extends onto the lateral epicondyle.

Insertion (Fig. 86, d).—The lateral surface of the ulna from the distal margin of the semilunar notch to the proximal end of the olecranon. The muscle covers the capsule of the joint and is closely adherent to it.

Relations.—Outer (dorsal) surface with the caput laterale (Fig. 75, h) and the intermediate portion (Fig. 80, j) of the caput mediale. Inner surface with the bones and the capsule of the joint.

Action.—Keeps the capsule tense and probably rotates the ulna slightly so as to pronate the hand.

3. Muscles of the Antibrachium or Forearm.

Fascia of the Forearm.

—The muscles of the forearm are not in contact with the skin, but are covered by the strong antibrachial fascia. This consists of two layers. The superficial one is a continuation of the general subcutaneous fascia of the arm; it covers the forearm as a continuous sheet which becomes thinner distad, and is finally lost near the wrist. The deep layer forms a dense, tendinous sheet which is closely applied to the muscles. It begins partly as a continuation of the general fascia of the arm, partly from the tendons of the triceps and epitrochlearis muscles. The sheet is attached to the dorsal surface of the ulna, between the extensor carpi ulnaris and the first head of the flexor profundus digitorum. It surrounds the forearm as a continuous sheath. On the lateral surface it dips between the extensor muscles, becoming closely attached to them and forming a partial sheath for each; it is attached with special firmness to their tendons. On the medial side of the arm the fascia passes smoothly over the flexors without dipping between them; it is continuous, however, with the border of the pronator teres, and distad of this muscle it is firmly attached to the radius.

At the wrist the fascia becomes attached to the longitudinal ridges on the dorsal surface of the head of the radius, bridging thus the intervening grooves and holding the tendons in place between the ridges. The fascia then continues to form the dorsal fascia of the hand. On the ventral side of the wrist the fascia becomes thickened to form a strong transverse ligament (Fig. 77, 1), which is attached at its radial and ulnar ends to the carpus, and which holds in place the tendons of the flexor muscles. Distad of the transverse ligament the fascia spreads out in the palm of the hand, and becomes united with the fibrous pad which is situated here. On the fingers the fascia becomes attached to the phalanges, forming tendinous sheaths, the so-called vaginal ligaments, for the flexor muscles. Near the base and the head of the first phalanx strong, narrow, ring-like bands, the annular ligaments (Fig. 88, 2), are formed in these sheaths; these surround and bind down the tendons of the flexor muscles.

A. Muscles on the Ulnar and Dorsal Side of the Forearm (extensors and supinators)

(Fig. 75, p. 158).

M. brachioradialis or supinator longus (Fig. 75, k; Fig. 77, n).—This is a ribbon-like muscle lying on the lateral side of the brachium and the ventrolateral border of the antibrachium just beneath the superficial fascia. It connects the humerus and the radius.

Origin.—By a thin tendon from about the middle fifth of the dorsal border of the humerus, or sometimes proximad of the middle. The muscle curves over the outer surface of the brachialis (Fig. 75, i), to which it is closely applied, and runs along the lateral border of the antibrachium to its insertion.

Insertion by a short tendon into the ridge which forms the ventral limit of the groove on the outer surface of the styloid process of the radius and upon the adjacent ligaments.

Relations.—Outer surface with the integument, and at the proximal end with the caput laterale of the triceps (Fig. 75, h). Inner surface with the brachialis (i), the extensor carpi radialis longus (l) and brevis, and the extensor communis digitorum (m).

Action.—Supinator of the hand.

M. extensor carpi radialis longus (Fig. 75, l; Fig. 77, o).—This lies along the radial side of the forearm, and connects the humerus and the second metacarpal.

Origin (Fig. 81, k) from the middle portion of the lateral supracondyloid ridge between the origin of the brachialis (j) and the anconeus. About the middle of the forearm the muscle ends in a slender tendon (Fig. 84, e) which passes through the groove on the radial side of the dorsal surface of the distal end of the radius.

Insertion (Fig. 84, c).—The tendon passes across the carpus and is inserted onto the dorsal surface of the base of the second metacarpal on the radial side just distad of the groove for the radial artery.

Relations.—Outer surface at the proximal end with the caput laterale of the triceps (Fig. 75, h), farther distad with the integument and the brachioradialis (k); the distal tendon with the extensor pollicis brevis (Fig. 84, d). Radial surface with the brachialis (Fig. 75, i) and the integument. Ulnar surface with the extensor carpi radialis brevis (Fig. 77, p), and near the proximal end with the extensor communis digitorum (Fig. 75, m).

Action.—Extensor of the hand.

M. extensor carpi radialis brevis (Fig. 77, p).—A slender muscle from the humerus to the third metacarpal.

Origin (Fig. 81, l) from the distal part of the lateral supracondyloid ridge distad of the extensor longus. Near the carpus the muscle ends in a slender tendon which passes through the groove with the extensor longus tendon. The tendon then diverges from the extensor longus and has its

Insertion (Fig. 84, f) into the radial side of the dorsal surface of the base of the third metacarpal.

Relations.—Outer surface with the extensor longus (Fig. 77, o), the integument, and distad with the extensor pollicis brevis (Fig. 84, d). Radial surface with the extensor longus. Ulnar surface with the extensor communis digitorum (Fig. 75, m) and the extensor brevis pollicis. Inner surface with the pronator teres (Fig. 77, q), supinator (Fig. 85, b), and extensor brevis pollicis (Fig. 85, a).

Action.—Extensor of the hand.

M. extensor digitorum communis (Fig. 75, m).—A slender muscle from the lateral supracondyloid ridge of the humerus to the phalanges of the second, third, fourth, and fifth digits.

Origin (Fig. 81, m).—The distal surface of the lateral supracondyloid ridge dorsad of the origin of the extensor carpi radialis brevis.

At the junction of the middle and distal thirds of the radius the muscle passes into a large, flat tendon (Fig. 75, m′) which passes through the groove on the middle of the dorsal surface of the distal end of the radius. The groove is converted into a canal by a strong transverse ligament. The tendon divides near the groove into four portions (Fig. 84, a) which diverge and pass to the four ulnar digits.

Insertions.—Each division of the tendon as it passes over the dorsal surface of the first phalanx has its border connected by strong fascia to the dorsal surface of the phalanx. It finally passes through the groove at the distal end of the first phalanx and is inserted mostly into the base of the second phalanx; a portion continues distad to be inserted into the third phalanx.

Relations.—Outer surface with the integument and at the origin with the caput laterale (Fig. 75, h) of the triceps. Radial surface with the extensor longus (l) and extensor brevis. Ulnar surface with the extensor digitorum lateralis (n). Inner surface with the supinator (Fig. 85, b) and the extensor pollicis brevis (Fig. 85, a).

Action.—Extensor of the four ulnar digits.

M. extensor digitorum lateralis (Fig. 75, n). (This corresponds in position to the extensor digiti quinti proprius of man.)

Origin (Fig. 81, n) from the lateral supracondyloid ridge of the humerus distad of the origin of the extensor communis.

The muscle passes along the ulnar border and dorsal surface of the forearm between the extensor communis (Fig. 75, m) and the extensor carpi ulnaris (o). Near the carpus it ends in a tendon (n′), which divides immediately into three or four (Fig. 84, b). These pass through the groove on the ulnar side of the dorsal surface of the radius and diverge to the digits lying beneath the extensor communis tendon.

Insertion (Fig. 84, b).—The three tendons on the ulnar side join the ulnar sides of the tendons of the extensor communis (a) at their insertions. The radial of the four tendons (not always present) joins the tendon of the extensor indicis (c) and may also give a branch to the radial side of the base of the first phalanx of the third digit.

Relations.—Outer surface with the integument. Radial border with the extensor communis digitorum (m); ulnar border with the extensor carpi ulnaris (o). Inner surface with the supinator (Fig. 85, b) and the extensor brevis pollicis (Fig. 85, a); distad with the extensor indicis (Fig. 85, c).

Action.—Extensor of the four ulnar digits.

M. extensor carpi ulnaris (Fig. 75, o).

Origin.—(1) By a short, broad tendon from the distal portion of the lateral epicondyle of the humerus (Fig. 81, o) distad of the origin of the extensor lateralis, and (2) by a smaller tendon from the ulna at the dorsal tip of the semilunar notch (Fig. 86, e).

The muscle passes along the ulnar side of the forearm dorsad of the extensor digitorum lateralis (n). It ends near the junction of the middle and distal thirds of the forearm in a large, flat tendon (o′) which is closely adherent to the ligaments of the wrist and which passes over the distal end of the ulna to its

Insertion into the tubercle on the ulnar side of the base of the fifth metacarpal (Fig. 84, g).

Relations.—Outer surface with the integument. Radial (or ventral) border with the extensor digitorum lateralis (Fig. 75, n); ulnar (or dorsal) border with the flexor profundus digitorum (q). Inner surface with the radius, the extensor indicis (Fig. 85, c), and the extensor pollicis brevis (Fig. 85, a).

Action.—Indicated by its name.

M. extensor indicis (proprius) (Fig. 85, c; Fig. 75, p) (includes extensor pollicis longus also).

Origin (Fig. 86, f) by short, fleshy fibres from the lateral surface of the ulna between the semilunar notch and the junction of the third and fourth (distal) fourths.

The muscle-fibres pass obliquely distad and toward the radial side and join the tendon (Fig. 85, c′), which runs nearly the whole length of the radial surface.

Insertion (Fig. 84, c).—The tendon passes through the groove on the ulnar side of the dorsal surface of the base of the radius and divides into two. Both of these usually go to the base of the second phalanx of the second digit, but one may pass to the pollex, or there may be three divisions going to the first three digits.