[Up]    The Pectoralis Muscles    [Trunk mm.] [Hoof] 

The pectoral muscles are extrinsic muscles of the arm. The extrinsic muscles are those which have their origin on the axial skeleton and their insertion on the proximal portion of the appendicular skeleton of the forelimb (either the scapula or the humerus). You will understand why we are covering the pectoral muscles now when you butcher them in Lab 5. A&B make up the superficial pectoral muscles. A is the descending superficial pectoral muscle, as its fibers descend from the cranial sternebrae to the humerus. Part of B, the transverse superficial pectoral muscle, is covered by A. Note that the fibers of B run transversely across the ventral thorax. The last pectoral muscle, the deep pectoral muscle (C) lies deep to the superficial pectoral muscles cranially. It's fibers run in a caudoventral direction. Think about the direction of the muscle fibers from their attachment to the arm and relate it the action of each of these muscles. With extrinsic muscles, we must consider the action of the muscle when the limb is elevated as well as when it is bearing weight.