Chapter I: THE MUSCLES OF THE SHOULDER GIRDLE (Figs. 177, 178, 179, 186)

This group consists of those muscles which connect the thoracic limb with the head, neck, and trunk. The group naturally falls into two divisions—dorsal and ventral.[55]

A. DORSAL DIVISION

This division consists of two layers which overlie the proper muscles of the neck and back.

FIRST LAYER

1. =Trapezius.=—This is a flat, triangular muscle, the base of the triangle corresponding with the spine. It is divided by an aponeurotic portion into two divisions:

(=a=) =Trapezius cervicalis.=—_Origin._—The funicular portion of the ligamentum nuchæ, from the second cervical to the third thoracic vertebra.

_Insertion._—The spine of the scapula and the fascia of the shoulder and arm.

(=b=) =Trapezius thoracalis s. dorsalis.=—_Origin._—The supraspinous ligament, from the third to the tenth thoracic vertebra.

_Insertion._—The tubercle of the spine of the scapula.

_Action._—Acting as a whole, to elevate the shoulder; the cervical portion draws the scapula forward and upward and the thoracic portion draws it backward and upward.

_Structure._—The muscle arises by a short, thin aponeurosis, from which the fibers of the flat fleshy portion converge to the spine of the scapula and the aponeurosis which separates the two portions. The cervical fascia joins the ventral edge of the cervical portion to the mastoido-humeralis, or the two muscles may unite here.

_Relations._—Superficially, the skin and fascia; deeply, the rhomboideus, latissimus dorsi, supraspinatus, infraspinatus, deltoid, splenius, serratus magnus, and anterior deep pectoral muscles, and the cartilage of the scapula.

_Blood-supply._—Deep cervical and intercostal arteries.

_Nerve-supply._—Spinal accessory nerve.

SECOND LAYER

This consists of two muscles—the rhomboideus and the latissimus dorsi.

2. =Rhomboideus.=—This consists of two portions:

(=a=) =Rhomboideus cervicalis s. cervicis.=—_Origin._—The funicular portion of the ligamentum nuchæ, from the second cervical to the second thoracic vertebra.

_Insertion._—The internal surface of the cartilage of the scapula.

(=b=) =Rhomboideus thoracalis s. dorsalis.=—_Origin._—The spinous processes of the second to the seventh thoracic vertebra by means of the dorso-scapular ligament.

_Insertion._—The inner surface of the cartilage of the scapula.

_Action._—To draw the scapula upward and forward. When the limb is fixed the cervical portion will elevate the neck.

_Structure._—The cervical portion is narrow, pointed at its anterior extremity, and lies along the funicular part of the ligamentum nuchæ, to which it is attached by short tendon bundles. The fibers are directed for the most part longitudinally. The thoracic portion is quadrilateral in shape, and its fibers are nearly vertical. Its deep face is intimately attached to the dorso-scapular ligament.

_Relations._—Superficially, the skin and fascia (over a small area), the trapezius, and the cartilage of the scapula; deeply, the dorso-scapular ligament, the splenius, complexus, longissimus dorsi, and serratus anticus.

_Blood-supply._—Dorsal and superior cervical arteries.

_Nerve-supply._—Sixth cervical nerve.

3. =Latissimus Dorsi.=—This is a wide muscle which has the form of a right-angled triangle. It lies for the most part under the skin and panniculus, on the lateral wall of the thorax, from the spine to the arm.

_Origin._—The lumbo-dorsal fascia—and by this means from the lumbar and thoracic spines as far forward as the highest point of the withers.

_Insertion._—The internal tubercle of the humerus, in common with the teres major.

_Action._—To draw the humerus upward and backward and flex the shoulder joint. If the limb is advanced and fixed, it draws the trunk forward.

_Structure._—The muscle arises by a wide aponeurosis, which fuses with that of the serratus posticus and with the lumbo-dorsal fascia. The muscular portion is at first rather thin, but by the convergence of its fibers becomes thicker as it approaches the arm. The anterior fibers pass almost vertically downward over the dorsal angle of the scapula and its cartilage. The posterior fibers are directed downward and forward. The thick belly formed by the convergence of these passes under the triceps to end on the flat tendon of insertion, which is common to this muscle and the teres major.

_Blood-supply._—Subscapular, intercostal, and lumbar arteries.

_Nerve-supply._—Brachial plexus (eighth cervical and dorsal roots).

B. VENTRAL DIVISION

1. =Mastoido-humeralis= (M. brachiocephalicus; levator humeri).—This muscle extends along the side of the neck from the head to the arm. It is incompletely divisible into two portions.

_Origin._—(1) The mastoid process of the petrous temporal bone and the occipital crest; (2) the wing of the atlas and the transverse processes of the second, third, and fourth cervical vertebræ.

_Insertion._—The deltoid tuberosity and the curved rough line which extends from this to the distal extremity of the humerus.

_Action._—When the head and neck are fixed, to draw the limb forward, extending the shoulder joint. When the limb is fixed, to extend the head and neck, if the muscles act together; acting separately, to incline the head and neck to the same side.

_Structure._—As already mentioned, the muscle is capable of incomplete division into two parts, the line of division being indicated by the emergence of superficial branches of the ventral divisions of the cervical nerves. The mastoid portion (M. cleido-mastoideus) partly overlaps the other portion (M. cleido-transversarius), which lies dorsal to it. The former is attached to the mastoid process and the occipital bone by a broad tendon which fuses with that of the splenius and trachelo-mastoideus; it is also attached to the tendon of insertion of the sterno-cephalicus by aponeurosis. The dorsal portion is attached to the transverse processes by four fleshy digitations. The belly of the muscle is adherent superficially to the cervical fascia and the panniculus, and deeply to the subscapulo-hyoideus. In front of the shoulder its deep face is marked by a tendinous intersection of variable development.[56] Here the muscle becomes wider, covers the shoulder joint, passes between the brachialis and biceps, and is inserted by means of a wide tendon which it shares with the superficial pectoral muscle.

_Relations._—Superficially, the skin, cervical fascia, the parotid gland, the panniculus, brachialis, and branches of the cervical nerves; deeply, the splenius, trachelo-mastoideus, rectus capitis anterior major, omo-hyoideus, serratus magnus, anterior deep pectoral and biceps muscles, the inferior cervical artery, the prescapular lymph glands, and branches of the cervical nerves. The ventral edge of the muscle forms the dorsal boundary of the jugular furrow. The dorsal border may be in contact with the cervical trapezius, or separated from it by a variable interval.

_Blood-supply._—Inferior cervical, carotid, and vertebral arteries.

_Nerve-supply._—Spinal accessory and cervical nerves.

The =pectoral fascia= is a thin membrane covering the surface of the pectoral muscles, to which it is, for the most part, pretty intimately attached. It detaches a layer which passes between the superficial and deep pectorals. At the posterior edge of the triceps another layer is given off, which passes on the outer surface of this muscle to blend with the scapular fascia; the deeper layer becomes continuous with the subscapular and cervical fasciæ.

The =pectoral muscles= form a large fleshy mass which occupies the space between the ventral part of the chest-wall and the shoulder and arm. They are clearly divisible into a superficial and a deep layer. The superficial layer may be subdivided into two portions by careful dissection; the deep layer is clearly made up of two muscles.

2. =Superficial pectoral= (M. pectoralis superficialis).

(=a=) =Anterior superficial pectoral= (portio clavicularis s. descendens; pectoralis anticus).—This is a short, thick, somewhat rounded muscle, which extends between the anterior part of the sternum and the front of the arm. It forms a distinct prominence on the front of the breast, which is easily recognized in the living animal.

_Origin._—The cariniform cartilage of the sternum.

_Insertion._—(1) The curved line of the humerus with the mastoido-humeralis; (2) the fascia of the arm.

_Action._—To adduct and advance the limb.

_Structure._—The belly of the muscle is convex on its superficial face, but deeply it is flattened where it overlaps the posterior superficial pectoral. Here the two muscles are usually pretty intimately attached to each other, and care must be exercised in making the separation. The tendon of insertion blends with that of the mastoido-humeralis and with the fascia of the arm. At the middle line of the breast a furrow occurs between the two muscles; laterally, another furrow, containing the cephalic vein, lies between the muscle and the mastoido-humeralis.

_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.

_Insertion._—The posterior triangular area on the costal surface of the scapula and the adjacent part of the cartilage.

_Action._—The two muscles form a sort of sling in which the trunk is suspended. Contracting together, they raise the thorax; contracting singly, the weight is shifted to the limb on the side of the muscle acting. The two parts can contract separately and are antagonistic in their action on the scapula. The cervical part draws the base of the scapula toward the neck, while the thoracic part has the opposite action; these effects concur in the backward and forward swing of the limb respectively. With the limb fixed, the cervical part extends (raises) the neck or inclines it laterally. The thoracic part may act as a muscle of forced inspiration.

_Structure._—In the domesticated animals there is no such clear division of the muscle as is found in man and the apes. On account of the difference in action, however, it seems desirable to distinguish the two portions. The cervical part is thick and almost entirely fleshy. The thoracic part has on its superficial face a thick, tendinous layer which may sustain the weight of the trunk when the muscle substance relaxes. The ventral edge presents distinct digitations, the last four of which alternate with those of the obliquus externus abdominis, and are covered by the abdominal tunic. The fourth, fifth, and sixth digitations extend nearly to the distal ends of the ribs. The last digitation is small and may be absent. Exceptionally additional digitations may be attached to the tenth or eleventh rib or to the fascia over the intercostal muscles. The fibers converge to the insertion, which is thick and is intersected by elastic lamellæ derived from the dorso-scapular ligament.

_Relations._—Superficially, the mastoido-humeralis, trapezius, deep pectoral, subscapularis, teres major, latissimus dorsi, panniculus carnosus, the abdominal tunic, the brachial vessels, and the long thoracic nerve; deeply, the splenius, complexus, longissimus, transversalis costarum, the ribs and external intercostal muscles, and branches of the superior cervical and dorsal arteries.

_Blood-supply._—Superior cervical, dorsal, vertebral, and intercostal arteries.

_Nerve-supply._—Brachial plexus.