A broad fan-shaped muscle situated on the side of the chest and partly concealed beneath the shoulder.

Attachments. - Below, to the outer surface of the first eight ribs, by a corresponding number of slips, which give it a serrated border from which it takes its name. Above, to the under surface of the upper extremity of the scapula or blade-bone.

Action. - When the horse occupies a standing position, this muscle, with its fellow on the opposite side, supports the trunk as if in a sling (fig. 44, Vol. I). When it acts from the chest it pulls down the posterior angle of the scapula and moves the point of the shoulder upward and forward. When the limb is fixed, it pulls the ribs upward and forward, and by enlarging the chest becomes a muscle of inspiration.