Part 29

_Structure._—The muscle is not rounded in the horse and ox, but flat and triangular. Its origin from the posterior border of the scapula is by means of an aponeurosis which also gives origin to fibers of the infraspinatus and triceps. A =bursa= is commonly found between the terminal part of the muscle and the capsule of the shoulder joint, and is often continuous with that of the infraspinatus.

_Relations._—Superficially, the deltoid and infraspinatus muscles; deeply, the scapula, the shoulder joint, and the triceps muscle.

_Blood-supply._—Subscapular artery (circumflex branches).

_Nerve-supply._—Axillary nerve.

B. INTERNAL GROUP

1. =Subscapularis.=—This muscle occupies the subscapular fossa, beyond which, however, it extends both before and behind.

_Origin._—The subscapular fossa.

_Insertion._—The posterior eminence of the internal tuberosity of the humerus.

_Action._—To adduct the humerus.

[Illustration:

FIG. 187.—MUSCLES OF THORACIC LIMB OF HORSE, INTERNAL VIEW.

_3_, _4_, Rhomboideus; _5_, latissimus dorsi; _8_, posterior deep pectoral; _9_, anterior deep pectoral; _11_, supraspinatus; _15_, subscapularis; _16_, teres major; _20_, long head of triceps; _22a_, _22b_, tensor fasciæ antibrachii; _23_, internal head of triceps. (After Ellenberger, in Leisering’s Atlas.) ]

_Structure._—The muscle is flat and triangular, with the base upward. The latter is thin and interdigitates with the scapular attachments of the serratus. Below this the belly thickens and becomes narrower. It is covered by an aponeurosis, and contains a considerable amount of tendinous tissue. The tendon of insertion is crossed by the tendon of origin of the coraco-brachialis; it is intimately adherent to the capsule of the shoulder joint, and may be regarded as replacing the internal ligament of the latter. A small bursa may occur here.

_Relations._—Superficially, the scapula and shoulder joint, the supraspinatus, triceps, and teres major muscles; deeply, the serratus magnus muscle, the brachial vessels, and the chief branches of the brachial plexus. The subscapular vessels run along or near the posterior edge of the muscle.

_Blood-supply._—Subscapular artery.

_Nerve-supply._—Subscapular nerves (from the brachial plexus).

2. =Teres major= (Teres internus; adductor of the arm).—This muscle is flat, widest about its middle, and lies chiefly on the deep face of the triceps.

_Origin._—The dorsal angle and the adjacent part of the posterior border of the scapula.

_Insertion._—The tubercle on the inner surface of the shaft of the humerus, in common with the latissimus dorsi.

_Action._—To flex the shoulder joint and adduct the arm.

_Structure._—It is for the most part fleshy, but the origin consists of an aponeurosis which blends with that of the tensor fasciæ antibrachii. The insertion is by a flat tendon which fuses with that of the latissimus dorsi.

_Relations._—Superficially, the triceps, infraspinatus, and deltoid muscles; deeply, the serratus magnus muscle. The subscapular vessels lie in a groove between the anterior edge of this muscle and the posterior border of the subscapularis; near the shoulder joint the posterior circumflex artery and the axillary nerve emerge between the two muscles. The deep face of the muscle is crossed by the thoracic branches of the brachial plexus, and by the branch of the subscapular artery which supplies the latissimus dorsi.

_Blood-supply._—Subscapular artery.

_Nerve-supply._—Axillary nerve.

3. =Coraco-brachialis= (Coraco-humeralis).—This muscle lies on the inner surface of the shoulder joint and the arm.

_Origin._—The coracoid process of the scapula.

_Insertion._—(1) A small area above the internal tubercle of the humerus; (2) the middle third of the anterior surface of the humerus.

_Action._—To adduct the arm and to flex the shoulder joint.

_Structure._—The long tendon of origin emerges between the subscapularis and the inner branch of the supraspinatus. It passes over the terminal part of the subscapularis and is provided with a =synovial sheath=. The muscular part spreads out and divides into two portions. The smaller and shorter portion is inserted into the proximal third of the shaft of the humerus, close to the origin of the lateral head of the triceps; the larger and longer portion is inserted into the middle third of the humerus, in front of the internal tubercle and the inner head of the triceps.

_Relations._—Externally, the subscapularis muscle and the humerus; internally, the deep pectoral and brachialis muscles. The anterior circumflex artery and the nerve to the biceps usually emerge between the two insertions, and the brachial vessels lie along the posterior border of the muscle.

_Blood-supply._—Anterior circumflex artery.

_Nerve-supply._—Musculo-cutaneous nerve.

4. =Capsularis= (Scapulo-humeralis posticus s. gracilis).—This is a very small muscle, which lies on the back of the capsule of the shoulder joint.

_Origin._—The scapula, close to the rim of the glenoid cavity.

_Insertion._—The posterior surface of the shaft of the humerus, a short distance below the head.

_Action._—It may perhaps tense the capsule of the shoulder joint and prevent its being pinched during flexion.

_Structure._—It is fleshy and usually about the breadth of a finger. It may, however, consist of only a few bundles of fibers; sometimes it is double. Its attachment to the joint capsule is slight. It passes through the brachialis muscle to reach its insertion.

_Relations._—Superficially, the teres minor and triceps muscles; deeply, the teres major and subscapularis muscles, and the capsule of the joint.

_Blood-supply._—Posterior circumflex artery.

_Nerve-supply._—Axillary nerve.

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.

[Illustration:

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.

[Illustration:

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.) ]

IV. FASCIÆ AND MUSCLES OF THE FOREARM AND MANUS

The forearm is covered on three sides by the muscles of this group, leaving the inner surface of the radius for the most part subcutaneous. The extensors of the carpus and digit lie on the anterior (dorsal) and external part of the region, while the flexors occupy the posterior (volar) surface.

The =fascia= of the =forearm= (Fascia antibrachii) forms a very strong and complete investment for all the muscles of the region. The =superficial fascia= is thin, and disappears at the carpus by fusing with the deep fascia. The =deep fascia= is very strong and tendinous in character. It furnishes insertion at its upper and inner part to the tensor fasciæ antibrachii and posterior superficial pectoral muscles; at its upper anterior and outer part, to aponeuroses from the mastoido-humeralis and biceps. It is attached at the elbow by its deep face to the outer tuberosities of the humerus and radius, to the ulna, and to the lateral ligaments. On the inner surface of the forearm it blends with the periosteum on the subcutaneous surface of the radius. It is closely adherent to the surface of the extensor muscles, but is rather loosely attached to the flexors. From its deep face are detached =intermuscular septa=, which form sheaths for the muscles and are attached to the underlying bones. The principal septa are: (_a_) One which passes between the common extensor (in front) and the lateral extensor and flexor carpi externus (behind); (_b_) one between the common extensor and the extensor carpi radialis; (_c_) one between the inner and middle flexors of the carpus.

The =carpal fascia= (Fascia carpi) is a direct continuation of that of the forearm. It is attached chiefly to the tuberosities at the distal end of the radius, to the accessory carpal (pisiform) bone, and to the lateral ligaments. In front it forms the so-called =anterior annular ligament= (Ligamentum carpi dorsale), bridging over the grooves and binding down the extensor tendons and their synovial sheaths. Behind it is greatly thickened and forms the =posterior annular= or =transverse ligament= of the carpus (Ligamentum carpi transversum). This stretches across from the accessory carpal bone to the internal lateral ligament and the proximal extremity of the inner metacarpal bone. It thus completes the carpal canal, in which lie the flexor tendons, their synovial sheath, and the principal vessels and nerves of the region.

The =fascia= of the =metacarpus= and =digit= (Fascia metacarpea et digitalis) is, in general, thinner than the preceding. It is attached to the tendons, ligaments, and the exposed bony prominences—especially to the small metacarpal bones. On the flexion surface of the fetlock joint it is much thickened by fibers passing transversely from one sesamoid bone to the other, forming an =annular ligament= which binds down the flexor tendons in the sesamoid groove or canal. Distal to this is a second thick quadrilateral sheet (Ligamentum vaginale) which covers and is adherent to the tendon of the flexor perforatus. It is attached on either side by two bands to the borders of the first phalanx, thus firmly binding down the flexor tendons. A little further down a crescentic =fibro-elastic sheet= covers the terminal expansion of the deep flexor tendon. It is attached on either side by a strong band to the side of the first phalanx about its middle.

A. EXTENSOR DIVISION

1. =Extensor carpi radialis= (M. extensor carpi radialis s. radialis dorsalis; extensor metacarpi magnus; anterior extensor of the metacarpus).—This is the largest muscle of the extensor division, and lies on the anterior (dorsal) surface of the radius.

_Origin._—(1) The external condyloid crest of the humerus; (2) the coronoid fossa.

_Insertion._—The tuberosity on the anterior (dorsal) surface of the proximal extremity of the large (third) metacarpal bone.

_Action._—To extend and fix the carpal joint and to flex the elbow joint.

_Structure._—The belly of the muscle is rounded, and runs out to a point at the distal third of the forearm. The tendon, which runs nearly the whole length of the fleshy portion, appears on the surface of the latter about its middle; here the muscle shows a distinctly pennate arrangement. The tendon passes through the middle groove at the distal extremity of the radius and over the capsule of the carpal joint, bound down by the anterior annular ligament and invested with a =synovial sheath=. The latter begins three to four inches (ca. 8 to 10 cm.) above the carpus and extends almost to the insertion of the tendon. In the lower half of the forearm the deep fascia blends with the tendon, and here the latter is joined by the long tendon of the biceps.

_Relations._—Superficially, the skin, fascia, and the oblique extensor; deeply, the capsule of the elbow joint, the biceps tendon, the radius, the carpal joint capsule, the anterior radial artery, and the radial nerve; externally, the anterior or common extensor; internally, at the elbow, the brachialis and biceps.

_Blood-supply._—Anterior radial artery.

_Nerve-supply._—Radial nerve.

2. =Anterior= or =common digital extensor= (M. extensor digitalis communis; anterior extensor of the phalanges; extensor pedis).—This muscle lies external to the foregoing, which it resembles in general form, although less bulky.

_Origin._—(1) The front of the distal extremity of the humerus, just external to the coronoid fossa; (2) the external tuberosity on the proximal extremity of the radius, the external lateral ligament of the elbow, and the external border of the radius at the junction of its proximal and middle thirds; (3) the external surface of the shaft of the ulna; (4) the fascia of the forearm.

[Illustration:

FIG. 190.—MUSCLES OF LEFT THORACIC LIMB OF HORSE, FROM ELBOW DOWNWARD, ANTERIOR VIEW.

_a_, Extensor carpi radialis; _g′_, superficial pectoral muscle. (After Ellenberger-Baum, Anat. für Künstler.) ]

_Insertion._—(1) The extensor (or pyramidal) process of the third phalanx; (2) the anterior surface of the proximal extremities of the first and second phalanges.

_Action._—To extend the digital and carpal joints, and to flex the elbow joint.

_Structure._—The muscle is a compound one, representing the common extensor, together with vestiges of the proper extensors of the digits. Usually at least =two heads= may be distinguished. The principal or humeral head (Caput humerale) arises from the front of the extensor epicondyle of the humerus in common with the extensor carpi. Its belly is fusiform, and terminates in a point near the distal third of the radius. The tendon appears on the surface of the muscle about the middle of the belly, the arrangement being pennate. The tendon passes downward through the outer of the two large grooves on the front of the distal end of the radius, and over the capsule of the carpal joint. Passing down over the front of the metacarpus, it inclines gradually inward, reaching the middle line of the limb near the fetlock. A little below the middle of the first phalanx it is joined by the branches of the suspensory ligament, and the tendon thus becomes much wider. Two =synovial membranes= facilitate the play of the tendon. The proximal one is a =sheath= which begins about three inches (ca. 7 to 8 cm.) above the carpus, and terminates at the proximal end of the metacarpus. At the fetlock a =bursa= occurs between the tendon and the joint capsule, but otherwise the two are adherent. The smaller head, arising chiefly from the radius and ulna, is often divisible into two parts (Fig. 443). The larger of these (Caput radiale, muscle of Phillips)[59] arises from the outer tuberosity and outer border of the radius, and from the lateral ligament of the elbow joint. The flat belly is succeeded by a delicate tendon, which accompanies the principal tendon over the carpus (included in the same sheath), and then passes outward to fuse with the tendon of the lateral extensor. Usually a slip is detached which is inserted on the proximal extremity of the first phalanx, or ends in the fascia here. The smaller and deeper division (Caput ulnare, muscle of Thiernesse)[60] is usually somewhat difficult to isolate. It arises from the ulna close to the interosseous space. It has a small rounded belly and is provided with a delicate tendon which may fuse with the principal tendon or may be inserted into the capsule and the fascia in front of the fetlock joint.

_Relations._—The chief relations of the belly of the muscle are: superficially, the skin and fascia; deeply, the elbow joint, the radius and ulna, the extensor carpi obliquus, and the anterior radial vessels and radial nerve; in front and internally, the extensor carpi radialis; behind, the lateral extensor and the interosseous vessels.

_Blood-supply._—Radial and interosseous arteries.

_Nerve-supply._—Radial nerve.

3. =Lateral digital extensor= (M. extensor digitalis lateralis s. digiti quinti proprius; lateral extensor of the phalanges; extensor suffraginis).—This muscle is much smaller than the preceding, behind which it is situated.

_Origin._—The external tuberosity of the radius and the lateral ligament of the elbow joint, the shaft of the ulna, the outer border of the radius, and the intermuscular septum.

_Insertion._—An eminence on the front of the proximal extremity of the first phalanx.

_Action._—To extend the digit and carpus.

_Structure._—The muscle is pennate, and is inclosed in a sheath formed by the deep fascia, from which many fibers arise. The belly is thin and fusiform and terminates at the lower third of the forearm. From here the tendon (at first small and round) passes downward through the groove on the outer tuberosity of the distal end of the radius, then over the carpus, and, gradually inclining toward the front, but not reaching the middle line of the limb, it passes over the metacarpus and fetlock to its insertion. Two =synovial membranes= occur in connection with the tendon. A =sheath= envelops the tendon, beginning about three inches (ca. 6 to 8 cm.) above the carpus, and reaching to the proximal end of the metacarpus. At the fetlock a small bursa lies between the tendon and the joint capsule, but otherwise the tendon is adherent to the capsule. The tendon becomes flat and much larger below the carpus, having received the tendon of the radial head of the anterior extensor and a strong band from the accessory carpal bone.

[Illustration: