• Origin: Cartilage of the fourth through the ninth ribs and the adjacent surface of the sternum; the xiphoid cartilage at the rear end of the sternum; the surface of the front end of the abdomen.
• Insertion: Inner and outer front corners of the upper end of the humerus (and the adjacent tendon of origin of the coracobrachialis in the horse).
• Action: Pulls the limb toward the midline of the body; pulls the limb rearward. If the limb is advanced and set firmly on the ground, it pulls the body forward during walking or running.
• Structure: The pectoralis ascendens begins from a large area of attachment on the lower portion of the rib cage and the abdomen and converges on the upper end of the humerus. The muscle inserts primarily onto the inner front corner of the upper end of the humerus. Tendinous fibers continue over the biceps to insert onto the outer front corner. The pectoralis ascendens is roughly triangular, beginning as a thin and wide muscle that lies on the chest, and thickening as it ascends. The muscle is seen on the lower portion of the side of the chest, to the rear of the elbow and lower portion of the triceps. Its upper edge is directed to a point just below the shoulder joint. The front portion is covered by the superficial pectorals.
• Origin: Surface of the sternum, beginning at the level of the second rib; a midline tendinous partition overlying this attachment; the surface of the front end of the abdomen.
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