Chapter III: The Muscles of the Arm

This group consists of five muscles which are grouped around the humerus. They arise from the scapula and the humerus, and are inserted into the forearm. They act on the elbow joint and the fascia of the forearm.

FIG. 188.—BICEPS AND BRACHIALIS MUSCLES OF HORSE. (After
Ellenberger-Baum, Anat. für Künstler.)
]

1. =Biceps brachii= (Coraco-radialis; flexor brachii).—This is a strong, somewhat rounded muscle, which lies on the anterior surface of the humerus.

_Origin._—The tuberosity of the scapula.

_Insertion._—(1) The bicipital tuberosity of the radius; (2) the internal lateral ligament of the elbow joint; (3) the fascia of the forearm and the tendon of the extensor carpi radialis.

_Action._—To flex the elbow joint, to fix the shoulder, elbow, and carpus in standing, and to tense the fascia of the forearm.

_Structure._—The muscle is inclosed in a double sheath of fascia, which is attached to the lips of the bicipital groove and the deltoid ridge of the humerus. The tendon of origin is moulded on the bicipital groove; it is very strong and dense and is partly cartilaginous. Its play over the groove is facilitated by the large =bicipital bursa= (Bursa intertubercularis). The synovial membrane covers not only the deep face of the tendon, but extends somewhat over the edges to the superficial face. A well-marked tendinous intersection runs through the belly of the muscle and divides distally into two portions. Of these, the short, thick one is inserted into the bicipital tuberosity and detaches fibers to the internal lateral ligament. The long tendon (Lacertus fibrosus) is thinner, blends with the fascia of the forearm, and ends by fusing with the tendon of the extensor carpi radialis.

_Relations._—Externally, the mastoido-humeralis and brachialis muscles; internally, the posterior deep pectoral and the superficial pectoral muscles; in front, the anterior deep pectoral muscle; behind, the humerus, the coraco-brachialis muscle, the anterior circumflex and anterior radial vessels, and the musculo-cutaneous nerve.

_Blood-supply._—Branches of the brachial and anterior radial arteries.

_Nerve-supply._—Musculo-cutaneous nerve.

2. =Brachialis= (Humeralis obliquus s. externus; brachialis anticus).—This muscle occupies the musculo-spiral groove of the humerus.

_Origin._—The proximal third of the posterior surface of the humerus.

_Insertion._—The inner surface of the neck of the radius (under cover of the lateral ligament) and the arciform ligament.

_Action._—To flex the elbow joint.

_Structure._—The peculiar spiral course of this muscle gave rise to the name often applied to it—humeralis obliquus. Beginning on the posterior surface of the shaft, close to the head of the humerus, it winds over the lateral surface, crosses the biceps very obliquely, and finally reaches the inner side of the forearm by passing between the biceps and the extensor carpi. It is entirely fleshy, with the exception of its relatively slender tendon of insertion.

_Relations._—Externally the skin and fascia, the teres minor, deltoid, triceps (lateral head), biceps, and mastoido-humeralis muscles. The anterior radial artery crosses the deep face of the muscle in its distal third, and the radial nerve accompanies the muscle in the distal half of the musculo-spiral groove.

_Blood-supply._—Brachial artery.

_Nerve-supply._—Radial nerve.

3. =Tensor fasciæ antibrachii= (Fig. 187)[58] (Scapulo-ulnaris; long extensor of the forearm; accessory anconeus of the latissimus dorsi).—This is a thin muscle which lies on the inner surface of the triceps.

_Origin._—The tendon of insertion of the latissimus dorsi and the posterior border of the scapula.

_Insertion._—(1) The fascia of the forearm; (2) a small eminence on the posterior border of the olecranon.

_Action._—(1) To tense the fascia of the forearm and to extend the elbow joint.

_Structure._—The origin consists of a very thin aponeurosis which blends with those of the caput longum and the latissimus dorsi. The muscular portion is quite thin in its anterior part, somewhat thicker behind, and is narrower than the aponeurotic origin. It is succeeded by an aponeurotic insertion, which ends chiefly by blending with the fascia of the forearm a little below the elbow. There is, however, a small but constant tendinous attachment to the olecranon.

_Relations._—Externally, the panniculus, triceps (long and internal heads), the inner and middle flexors of the carpus, and the ulnar vessels and nerve; internally, the latissimus dorsi, serratus magnus, and posterior pectoral muscles.

_Blood-supply._—Subscapular, ulnar, and deep brachial arteries.

_Nerve-supply._—Radial nerve.

4. =Triceps brachii= (Figs. 178, 179, 187) (Brachial triceps; triceps extensor cubiti).—This, together with the preceding muscle, constitutes the large muscular mass which fills the angle between the posterior border of the scapula and the humerus. It is clearly divisible into three heads, as described below.

(=a=) =Long head= (Caput longum tricipitis; anconeus longus; caput magnum).—This, the largest and longest of the three heads, is a powerful, thick, triangular muscle, which extends from the posterior border of the scapula to the olecranon.

_Origin._—The posterior border of the scapula.

_Insertion._—The outer and posterior part of the summit of the olecranon.

_Action._—(1) To extend the elbow joint; (2) to flex the shoulder joint.

_Structure._—The muscle arises by a wide, strong aponeurosis from the posterior border of the scapula. From this the bundles of the fleshy portion converge to the short, strong tendon of insertion. A careful examination will show that the muscle is penetrated by a tendinous intersection from which many fibers take origin obliquely. The superficial face is covered by an aponeurosis which is specially developed at its distal part. A small =bursa= occurs under the tendon of insertion.

_Relations._—Externally, the panniculus, deltoid, infraspinatus, teres minor, and the external head; internally, the tensor fasciæ antibrachii, teres major, latissimus dorsi, and posterior deep pectoral muscles, and the subscapular vessels; in front, the brachialis, and the inner head, the deep brachial and posterior circumflex vessels, and the axillary and radial nerves; behind, the skin and fascia.

_Blood-supply._—Subscapular and deep brachial arteries.

_Nerve-supply._—Radial nerve.

(=b=) =External head= (Caput laterale tricipitis s. anconeus lateralis s. externus; caput medium).—This is a strong, quadrilateral muscle, which lies on the outer surface of the arm. Its proximal third is covered by the deltoid and teres minor muscles; the remainder only by the thin panniculus and the skin.

_Origin._—The deltoid tuberosity and the curved rough line which extends from it to the neck of the humerus.

_Insertion._—(1) A small prominent area on the outer surface of the olecranon; (2) the tendon of the long head.

_Action._—To extend the elbow joint.

_Structure._—The origin consists of short tendinous fibers. The belly is thick, and is composed of parallel bundles which are directed obliquely downward and backward. They are inserted partly into the tendon of the long head and partly into the olecranon below and in front of that tendon.

_Relations._—Externally, the deltoid, teres minor, and panniculus muscles; internally, the long and inner heads and the brachialis muscle. Branches of the circumflex vessels and axillary nerve emerge between the posterior edge of the muscle and the long head. The deep face of the muscle is related to the branches of the deep brachial artery and of the radial nerve.

_Blood-supply._—Posterior circumflex and deep brachial arteries.

_Nerve-supply._—Radial nerve.

(=c=) =Internal head= (Fig. 193) (Caput mediale tricipitis; anconeus medialis s. internus; caput parvum).—This is much the smallest of the three heads. It is situated on the inner surface of the arm, and extends from the middle third of the humerus to the olecranon.

_Origin._—The middle third of the inner surface of the shaft of the humerus, behind and below the internal tubercle.

_Insertion._—The inner and fore part of the summit of the olecranon, between the insertion of the long head and the origin of the ulnar head of the flexor perforans.

_Action._—To extend the elbow joint.

_Structure._—The muscle is fleshy except at its insertion, where it has a flat tendon, under which a small bursa may be found.

_Relations._—Externally, the humerus, brachialis, anconeus, and the external head; internally, the posterior deep pectoral, coraco-brachialis, teres major, latissimus dorsi, and tensor fasciæ antibrachii muscles, the brachial and deep brachial vessels, and the median and ulnar nerves; behind, the long head, branches of the deep brachial vessels, and the radial nerve.

_Blood-supply._—Deep brachial and ulnar arteries.

_Nerve-supply._—Radial nerve.

5. =Anconeus= (M. anconeus parvus s. subanconeus).—This is a small fleshy muscle which covers the olecranon fossa and is covered by the triceps. It is somewhat difficult to separate from the outer head.

_Origin._—The distal third of the posterior surface of the humerus.

_Insertion._—The outer surface of the olecranon.

_Action._—To extend the elbow joint, and to raise the capsule of the joint and prevent its being pinched during extension.

_Structure._—It is almost entirely fleshy, and is adherent by its deep face to the joint capsule.

_Relations._—Superficially, the triceps muscle; deeply, the humerus and the elbow joint.

_Blood-supply._—Deep brachial artery.

_Nerve-supply._—Radial nerve.

FIG. 189.—DIGIT OF HORSE, POSTERIOR VIEW.

_14_, Deep flexor tendon; _15_, superficial flexor tendon; _16_,
posterior annular ligament of fetlock; _17_, proximal annular or
vaginal ligament of digit; _11_, lateral cartilage; _24_, plantar
cushion. (After Ellenberger-Baum, Anat. für Künstler.)
]