296 FASCIA AND MUSCLES OF THE HORSE 



terior division, the deep pectoral, and the biceps. The cephalic vein hes in the 

 groove between this muscle and the brachiocephalicus. 



(b) Posterior superficial pectoral muscle (pars transversa).^ — 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 raphe common to the two muscles. 



Insertion.— {!) 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 raphe. The tendon of inser- 

 tion unites ^^ith 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 flrst 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 hmb 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. 

 ' Also termed the pectoralis parvus or pars praescapularis. 

 ' Also known as the pectoraUs magnus. 



