(=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.