296 FASCLE AND MUSCLES OF THE HORSE 
terior division, the deep pectoral, and the biceps. The cephalic vein lies in the 
groove between this muscle and the brachiocephalicus. 
(b) Posterior superficial pectoral muscle (pars transversa).1—This is a wide 
muscular sheet which extends from the ventral edge of the sternum to the medial 
surface of the elbow. 
Origin.—(1) The ventral edge of the sternum as far back as the sixth car- 
tilage; (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 inser- 
tion unites with the fascia on the medial 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 median vessels 
and nerve, and the medial and middle flexors of the carpus. 
3. Deep pectoral muscle (M. pectoralis profundus)—This muscle is much 
thicker and more extensive in the horse than the superficial pectoral. It consists 
of two distinct parts. 
(a) Anterior deep pectoral muscle (pars scapularis).-—This 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 car- 
tilages 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 (con- 
vex anteriorly), passing at first forward, then upward over the front of the shoulder, 
a little to its medial side, and finally inclines somewhat 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 cer- 
vical angle of the scapula. 
Relations.—Superficially, the skin and fascia, the cutaneus, superficial pec- 
toral, trapezius, and brachiocephalicus muscles, the cephalic vein, and the in- 
ferior cervical artery; deeply, the posterior deep pectoral, biceps, supraspinatus, 
omo-hyoideus, and serratus ventralis muscles, the brachial vessels, and the branches 
of the brachial plexus of nerves. 
(b) Posterior deep pectoral muscle (pars humeralis s. ascendens).*—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 posterior part of the medial tuberosity of the humerus; (2) 
the anterior part of the lateral tuberosity of the humerus; (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 almost entirely fleshy. Its posterior part is wide 
1 Also termed the pectoralis transversus or pars sternocostalis. 
2 Also termed the pectoralis parvus or pars preescapularis. 
3 Also known as the pectoralis magnus. 
