The Muscles of the Forelimb

The muscles of the forelimb comprise the girdle musculature, passing between the trunk and the limb, and the intrinsic musculature.

Girdle Muscles

The girdle muscles join the forelimb to the trunk, forming a connection known as a synsarcosis that substitutes for a conventional joint.

FIG.

For the purpose of description, the girdle muscles can be considered in two layers.

The Superficial Layer

The superficial layer of girdle muscles consists of a cranial group (trapezius, omotransversarius, and brachiocephalicus) that split from a single primordium in the embryo and are supplied mostly by the accessory nerve, except the caudal part of the brachiocephalius that is of deltoid origin and is supplied by the axillary nerve. The caudal group of muscles consists of latissimus dorsi and the ventral group is comprised of pectoral muscles.

The trapezius (Fig. 2.55/5 and 5') is thin. It takes origin from the mid-dorsal raphe and supraspinous ligament, extending from about the level of the second cervical to that of the ninth thoracic vertebra, and converges to insert on the spine of the scapula. It consists of two fleshy parts, cervical and thoracic, usually separated by an intermediate aponeurosis. The fibers of the cervical part run caudoventrally to attach along the greater part of the length of the scapular spine; those of the thoracic part run cranioventrally to a more confined insertion on the tuberous thickening of the spine. The trapezius may raise the scapula against the trunk and swing the ventral angle of the bone cranially, thus advancing the limb.

The omotransversarius (Fig. 2.55/3) is a narrow muscle that extends between the transverse processes of the atlas (and possibly also the succeeding vertebrae) and the acromion and adjacent part of the scapula. It assists in advancement of the limb.

The brachiocephalicus (Fig. 2.55/2 and 2') is more complex, being formed by the union of two elements that are separated by the much reduced clavicle. The caudal part (cleidobrachialis) passes between the clavicle and the humerus and is a component of the deltoideus muscle.

FIG. 2.55 Superficial muscles of the shoulder and arm. 1, Sternocephalicus; 2 and 2', brachiocephalicus: Cleidocervicalis and cleidobrachialis; 3, omotransversarius; 4, superficial cervical lymph node; 5 and 5', cervical and thoracic parts of trapezius; 6, deltoideus; 7, latissimus dorsi; 8 and 8', long and lateral heads of triceps; 9, pectoralis profundus (ascendens); 10, accessory axillary lymph node.

The latissimus dorsi (Fig. 2.55/7) has a very broad origin from the thoracolumbar fascia and converges to an insertion on the teres tuberosity of the humerus. The most cranial fibers cover the caudal angle of the scapula and strap it against the chest. The muscle retracts the free limb and may also flex the shoulder joint. On the other hand, when the limb is advanced and the foot firmly planted on the ground, the latissimus may draw the trunk forward.

Two superficial pectoral muscles (see Fig. 2.41/5 and 6) arise, one behind the other, from the cranial part of the sternum. The cranial muscle (pectoralis descendens) terminates on the crest of the humerus, distal to the deltoid tuberosity. The caudal muscle (pectoralis transversus) descends over the medial aspect of the arm and in the larger species continues distally over the elbow joint, covering the median artery and nerve, to insert into the medial fascia of the forearm. Both muscles adduct the limb through the sideways shift of the trunk toward a previously abducted limb. The muscle may also assist protraction or retraction, depending on the initial position of the limb relative to the trunk. They are supplied by local branches (cranial pectoral nerves) from the brachial plexus.

The Deep Layer

The deep layer of girdle muscles comprises the rhomboideus dorsally, the serratus ventralis medially, and the pectoralis profundus ventrally.

The rhomboideus (Fig. 2.54/8) arises from median connective tissue structures extending from the poll to the withers and lies deep to the trapezius. In addition to cervical and thoracic parts, it has a capital part in carnivores. All attach to the dorsal border and adjacent area on the medial surface of the scapula. Most of the muscle draws the dorsal part of the scapula cranially, thereby retracting the limb. The muscle may also raise the limb and hold it firmly against the trunk. It is supplied from the brachial plexus in the dog, but in some species it is also supplied by dorsal branches of local spinal nerves, unusual for a limb muscle.

The serratus ventralis (Fig. 2.54/6) is a large fan-shaped muscle that takes an extensive origin via separate digitations from the fourth cervical vertebra to the tenth rib. The fibers run dorsally to terminate on a well-defined area on the medial aspect of the scapula and scapular cartilage.

The pectoralis profundus (Fig. 2.55/9) may be regarded as having cranial and caudal parts. The cranial part, well-formed only in the horse and pig, probably corresponds to the subclavius of other mammals and is now so named officially. Both parts (or muscles) arise from the ventral aspect of the length of the sternum and adjacent cartilages, and the most caudal fibers extend beyond it onto the abdominal floor. In the horse and pig the subclavius passes dorsally along the leading edge of the scapula, attaching to the supraspinatus (see Fig. 23.5A/2). The larger caudal part, also known as the pectoralis ascendens, inserts on the lesser tubercle of the humerus. Both play a role, secondary to that of the serratus ventralis, in slinging the trunk between the forelimbs. They may also act as retractors of the forelimb when this is free. When the limb is advanced and fixed, they draw the trunk forward, toward the limb. The nerves are local branches (caudal pectoral nerves) of the brachial plexus.

Intrinsic Muscles of the Forelimb

The intrinsic muscles are conveniently grouped according to their common location, actions, and innervations.

Muscles Acting Primarily on the Shoulder Joint

The muscles acting on the shoulder joint are arranged in lateral, medial, and caudal groups.

The lateral group comprises the supraspinatus and infraspinatus, which arise from and fill the corresponding fossae of the scapula. The supraspinatus (Fig. 2.56/3) terminates on the summits of both tubercles of the humerus. The infraspinatus inserts by a tendon that splits into a shorter deep part, which attaches to the summit, and a longer superficial part, which attaches to the lateral face of the (caudal part of the) greater tubercle; a bursa between the bone and the longer tendon may be the seat of a painful inflammation. Both muscles brace the joint laterally. The supraspinatus tendon passes cranial to the axis of rotation and may therefore also extend the shoulder. It is sometimes asserted that the infraspinatus tendon passes cranial or caudal to the axis of rotation depending on the actual position of the joint and may then further extend the already extended joint or further flex the already flexed joint; clearly, it is unlikely to be very effective in either role. Both muscles are supplied by the suprascapular nerve from the brachial plexus.

The medial group consists of the subscapularis and coracobrachialis. The subscapularis (Fig. 2.56/9) arises over much of the deep surface of the scapula and inserts on the medial tubercle of the humerus, distal to the axis of the shoulder joint. It braces the medial aspect of the joint. It is also a potential adductor of the arm and, like the infraspinatus, has an equivocal relationship to flexion and extension of the shoulder. It is supplied by the subscapular nerve from the brachial plexus. The small coracobrachialis (Fig. 2.56/10) extends between the medial aspect of the supraglenoid tubercle and the proximal part of the shaft of the humerus. It is a fixator of the shoulder with the same equivocal relationship to shoulder flexion and extension. It is supplied by the proximal branch of the musculocutaneous nerve from the brachial plexus.

The caudal or flexor group comprises the deltoideus, teres major, and teres minor. The deltoideus has one head of origin in the horse and two in species possessing an acromion (Fig. 2.56/4 and 4'). The constant head arises from the caudal border and spine of the scapula; the inconstant second head arises from the acromion. Both insert on the deltoid tuberosity of the humerus. The teres major (Fig. 2.56/2) arises from the dorsal part of the caudal margin of the scapula and terminates on the teres tuberosity, midway down the humerus. The relatively insignificant teres minor lies over the caudolateral aspect of the joint between the deltoideus and infraspinatus. These three muscles are clearly primarily flexor; the deltoideus may also be an abductor and an outward rotator of the arm. The group is supplied by the axillary nerve from the brachial plexus.

In contrast to the well-defined group of flexors, it seems that no muscles are clearly established as primarily extensors of the shoulder. The potential candidates, brachiocephalicus, biceps brachii, supraspinatus, and pectoralis ascendens, have other, apparently more important, roles.

Muscles Acting Primarily on the Elbow Joint

There are extensor and flexor groups for the elbow joint. The extensor group, which largely fills the angle between the scapula and humerus, consists of the triceps brachii, tensor fasciae antebrachii, and anconeus. The large and powerful triceps brachii (Fig. 2.56/6, 6 ', and 6'') possesses three (four in the dog) heads of origin. The long head, which arises from the caudal margin of the scapula, is potentially also a flexor of the shoulder. The lateral, medial, and (in the dog) accessory heads arise from the shaft of the humerus and act on the elbow. The heads combine to make a stout tendon that inserts on the summit of the olecranon, where it is protected on its deep aspect—against the bone— by the tricipital bursa. A second, subcutaneous bursa often lies between the tendon and the skin.

FIG. 2.56 Intrinsic muscles of the left shoulder and arm of the dog. (A) Lateral and (B) medial views. 1, Rhomboideus; 2, teres major; 3, supraspinatus; 4 and 4', scapular and acromial parts of deltoideus; 5, latissimus dorsi; 6, 6', and 6", long, lateral, and medial heads of triceps; 7, brachiocephalicus; 8, brachialis;

9, subscapularis; 10, coracobrachialis; 11, tensor fasciae antebrachii; 12, biceps.

The tensor fasciae antebrachii (Fig. 2.56/11) is a thin sheet, partly muscular, partly aponeurotic, that lies over the medial aspect of the long head of the triceps, extending from the scapula to the olecranon. The anconeus is much smaller and arises from the distal part of the humerus to insert on the lateral part of the olecranon; it is directly related to the elbow joint capsule and may have the additional function of tensing the capsule so that it is not pinched between the humerus and ulna. All parts of the extensor group are supplied by the radial nerve from the brachial plexus.

All of the extensors previously described are supplied by the radial nerve.

The flexor group comprises the biceps brachii and brachialis. The biarticular biceps brachii (Fig. 2.56/12) arises from the supraglenoid tubercle of the scapula and runs through the intertubercular groove of the humerus before continuing distally to insert on the medial tuberosity of the proximal extremity of the radius and on the adjacent part of the ulna. It is thus also a potential extensor of the shoulder. The brachialis (Fig. 2.56/8) arises from the proximocaudal part of the humerus and winds laterally in the spiral groove of this bone before inserting next to the biceps. Both are supplied by the musculocutaneous nerve.

Pronator and Supinator Muscles of the Forearm

Generalized mammals possess forearm muscles that have supination or pronation as a prime function, but these muscles tend to become vestigial or to disappear when the capacity for the movements is reduced or lost. Among domestic species significant movement is possible only in the dog and cat, in which there are two supinator muscles and two pronators. The brachioradialis or long supinator is a thin fleshy ribbon that extends from the lateral epicondyle of the humerus to the distal medial part of the forearm within the superficial fascia. It is quite prominent in the cat but is slight, often lost, in the dog. The short supinator muscle is more consistently developed. It is a small fusiform muscle, placed deep to the extensor muscles and passing obliquely over the flexor aspect of the elbow from the lateral humeral epicondyle to the upper quarter of the medial border of the radius. The supinator muscles are supplied by the radial nerve.

FIG. 2.57 Muscles of the left forearm of the dog. (A) Lateral and (B) medial views. 1, Extensor carpi radialis; 2, common digital extensor; 3, lateral digital extensor; 4, ulnaris lateralis; 5, flexor carpi ulnaris; 6, extensor carpi obliquus; 7, extensor retinaculum; 8, carpal pad; 9, biceps; 10, superficial digital flexor; 11, flexor carpi radialis; 12, pronator teres; 13, radius; 14, deep digital flexor; 15, flexor retinaculum.

The pronator teres (Fig. 2.57/12) arises from the medial epicondyle of the humerus and converges on the insertion of the supinator on the radius. It is functional only in the dog and cat. The pronator quadratus is found only in carnivores. It passes from the shaft of the ulna to that of the radius, bridging the medial aspect of the interosseous space of the forearm. The pronator muscles are supplied by the median nerve.

The rotation from the neutral position that may be produced by these muscles is most free when the elbow is flexed. The movements are limited to about 40 degrees of pronation and about 45 degrees of supination in the dog, although the cat has a somewhat larger range.

Muscles Acting Primarily on the Carpal and Digital Joints

Muscles that act primarily on the carpal and digital joints are simply classified as flexor or extensor, although the action of one muscle is equivocal.

The Extensor Muscles of the Carpus and Digits

Extensor muscles of the carpus and digits include digital extensor muscles in addition to muscles whose action is confined to the carpus.

All carpal and digital extensors have an extensor action at the carpus, a craniolateral position in the forearm, a radial nerve supply, and, with one exception, an origin from the cranial aspect of the lateral epicondyle of the humerus.

The extensor carpi radialis (Fig. 2.57/1), the most medial member of the group, is situated directly cranial to the subcutaneous border of the radius. It inserts on the proximal extremity of the third (sometimes also second) metacarpal bone. The ulnaris lateralis (Fig. 2.57/4) (extensor carpi ulnaris) is the most lateral member and runs parallel to the ulnar flexor of the carpus on the outer aspect of the limb to insert on the accessory carpal and the upper end of the most lateral metacarpal bone. It may extend an already extended carpus but further flexes the joint when it is in a flexed position. This muscle may also deviate the paw laterally. Despite its equivocal character the ulnaris lateralis retains the extensor nerve supply. The extensor carpi obliquus (Fig. 2.57/6) (also known as the abductor pollicis longus) originates from the cranial surface of the radius and has an oblique mediodistal course to attach to the most medial metacarpal bone. It is an extensor of the carpus and potentially, in the dog and the cat, deviates the paw medially.

All species possess both a common and a lateral digital extensor muscle; however, the common one may be subdivided. Because the common digital extensor (Fig. 2.57/2) inserts on the extensor process of the distal phalanx of each functional digit, the number of branches corresponds to the number of digits. A subdivision of the common extensor, which is present in all species but the horse and cat, inserts on the most medial of the functional digits; it sends an oblique branch to the dewclaw in the dog. It is unofficially but usefully termed medial digital extensor. Running on the lateral edge of the common extensor is the lateral digital extensor (Fig. 2.57/3). The undivided tendon inserts on the dorsal surface of the proximal phalanx in the horse. The muscle also has one insertion tendon in ruminants, two in the pig, three in the dog, and four in the cat; in these species the insertion is in common with that of the branch of the common extensor to the distal phalanx of the most lateral one, two, three, or four functional digits. In the smaller species, separation of the digital divisions begins more proximally and is more complete.

The Flexor Muscles of the Carpus and Digits

The carpal flexor group includes digital flexor muscles in addition to muscles that act only at the carpus.

All carpal and digital flexors have a caudal position in the forearm; arise, in part at least, from the caudal aspect of the medial epicondyle of the humerus; and are supplied by the median or ulnar nerve or from both these nerves.

Some have additional, even principal, origins in the forearm and also act on the digital joints. The flexor carpi radialis (Fig. 2.57/11) is most medial and runs directly caudal to the subcutaneous border of the radius. It ends on the upper end of the second (sometimes third) metacarpal bone. The flexor carpi ulnaris (Fig. 2.57/5) is lateral and ends on the accessory carpal bone. Both muscles are solely carpal flexors.

The superficial digital flexor (Fig. 2.57/10) sits in the caudomedial part of the forearm and is not enclosed in a synovial sheath where it passes the carpus. Later it divides into a branch for each functional digit and inserts in the region of the proximal interphalangeal joint. Each branch of the superficial flexor tendon splits into two diverging slips that create an arch to let through the deep digital flexor tendon to its more distal insertions. The deep digital flexor (Fig. 2.57/14) lies more deeply in the forearm and passes the carpus through the carpal canal before dividing into one to four digital branches; each perforates the corresponding branch of the superficial flexor tendon and then continues to its insertion on the palmar aspect of a distal phalanx.

Short Digital Muscles

Interosseous muscles support the metacarpophalangeal joints. They show marked species differences in number, structure (they are mostly tendinous in the large species), and function. These muscles arise from the palmar aspect of the proximal ends of the metacarpal bones and find initial insertion on the sesamoid bones at the metacarpophalangeal joints; from here they are continued by distal sesamoidean ligaments that attach to the phalanges and by extensor branches that wind around to the dorsal aspect of the digit to join the extensor tendons. They are considered in detail later for the species in which they are important.

In the carnivores and pig a number of small digital muscles assist in the extension, flexion, abduction, or adduction of the abaxial digits—one, two, and five in the dog and the cat and two and five in the pig. It is unnecessary to describe them.