Part 78

Relations.—Superficially, the skin, fascia, and panniculus; deeply, the posterior division, the deep pectoral, and the biceps. The cephalic vein lies in the groove between this muscle and the mastoido-humeralis.

(=b=) =Posterior superficial pectoral= (portio sternocostalis; pectoralis transversus).—This is a wide muscular sheet which extends from the ventral edge of the sternum to the fascia on the inner surface of the forearm.

Origin.—(1) The ventral edge of the sternum as far back as the sixth cartilage; (2) a fibrous raphé common to the two muscles.

Insertion.—(1) The fascia on the proximal third of the forearm; (2) the curved line of the humerus with the preceding muscle.

Action.—To adduct the limb and to tense the fascia of the forearm.

Structure.—It is thin and pale, and mixed with a good deal of fibrous tissue. The right and left muscles fuse at a median fibrous raphé. The tendon of insertion unites with the fascia on the inner side of the forearm for the most part; only a small part in front, about an inch in width, is attached to the humerus.

Relations.—Superficially, the skin, fascia, and the preceding muscle; deeply, the deep pectoral, the biceps, and the brachialis; at the elbow, the posterior radial vessels, the median nerve, and the inner and middle flexors of the carpus.

3. =Deep pectoral= (M. pectoralis profundus).—This muscle is much thicker and more extensive in the horse than the superficial pectoral. It consists of two distinct portions.

(=a=) =Anterior deep pectoral= (portio prescapularis; pectoralis parvus).—This division is prismatic and extends from the anterior part of the lateral surface of the sternum to the cervical angle of the scapula.

Origin.—The anterior half of the lateral surface of the sternum and the cartilages of the first four ribs.

Insertion.—The aponeurosis which covers the supraspinatus at its dorsal end, and the scapular fascia.

Action.—To adduct and retract the limb; when the limb is advanced and fixed, to draw the trunk forward.

Structure.—The muscle is almost entirely fleshy. It describes a curve (convex anteriorly), passing at first forward, then upward over the front of the shoulder, a little to its inner side, and finally upward and backward along the anterior border of the supraspinatus. It is loosely attached to the latter muscle, and terminates in a pointed end which becomes more firmly attached near the cervical angle of the scapula.

Relations.—Superficially, the skin and fascia, the panniculus, superficial pectoral, trapezius, and mastoido-humeralis muscles, the cephalic vein, and the inferior cervical artery; deeply, the posterior deep pectoral, biceps, supraspinatus, omo-hyoideus, and serratus magnus muscles, the brachial vessels, and the branches of the brachial plexus of nerves.

(=b=) =Posterior deep pectoral= (portio humeralis s. ascendens; pectoralis magnus).—This is much the largest of the pectoral group in the horse. It is somewhat triangular or fan-shaped.

Origin.—(1) The abdominal tunic; (2) the xiphoid cartilage and ventral aspect of the sternum; (3) the cartilages of the fourth to the ninth ribs.

Insertion.—(1) The internal tuberosity of the humerus; (2) the external lip of the bicipital groove; (3) the tendon of origin of the coraco-brachialis.

Action.—To adduct and retract the limb; if the limb is advanced and fixed, to draw the trunk forward.

Structure.—This muscle is also almost entirely fleshy. Its posterior part is wide and thin, but as the muscle is traced forward, it becomes narrower and much thicker. It passes forward and slightly upward in a gentle curve to its insertion. The humeral insertion is just below that of the inner division of the supraspinatus. Part of the fibers are inserted by means of a tendinous band which binds down the tendon of the biceps and is attached to the external lip of the bicipital groove, and a small part is attached to the tendon of origin of the coraco-brachialis.

Relations.—Superficially, the skin, panniculus, and superficial pectoral; deeply, the abdominal tunic, the external oblique, the rectus abdominis et thoracis, the brachial vessels, and branches of the brachial plexus of nerves. The external thoracic vein lies along the outer or upper border.

Blood-supply.—Internal and external thoracic, inferior cervical, anterior circumflex, and intercostal arteries.

Nerve-supply.—Pectoral (or thoracic) nerves, from the brachial plexus.

4. =Serratus magnus= (M. serratus ventralis).—This is a large, fan-shaped muscle, situated on the lateral surface of the neck and thorax. It derives its name from the serrated ventral edge of its thoracic portion. It consists of a cervical and a thoracic portion.

(=a=) =Cervical part= (M. serratus cervicis; levator scapulæ hominis).

Origin.—The transverse processes of the last four or five cervical vertebræ.

Insertion.—The anterior triangular area on the costal surface of the scapula and the adjacent part of the cartilage.

(=b=) =Thoracic part= (M. serratus thoracis; serratus anterior hominis).

Origin.—The external surfaces of the first eight or nine ribs.