VENTRAL DIVISION 253 



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 pro- 

 minence 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 pos- 

 terior 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 trans- 

 versus). — 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 car- 

 tilage; (2) a fibrous raphe 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 raphe. The tendon of inser- 

 tion 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 sur- 

 face 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 (con- 

 vex 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 



