In the dog and cat this portion of the muscle does not exist. The other division of the muscle, the sterno-trochinian, is more interesting. It arises from the abdominal aponeurosis and the posterior part of the sternum; hence it passes forward, turns under the superficial pectoral, and is inserted into the lesser tuberosity (trochin) of the humerus. In the pig, dog, and cat, it is inserted into the greater tuberosity (trochiter) of the bone of the arm. The superior border of this muscle is in relation with a superficial vein, which is distinctly visible in the horse--the subcutaneous thoracic vein, which in this animal is called the vein of the spur. The sterno-humeral muscle, in contracting, draws the shoulder and the whole anterior limb backwards. =Serratus Magnus= (Fig. 67, 2; Fig. 69, 8; Fig. 70, 9).--This muscle, which is situated on the lateral aspect of the thorax, is covered to a considerable extent by the shoulder, the posterior muscular mass of the arm, and by the great dorsal muscle. It arises by digitations from the external surface of the dorsal vertebræ; from the first eight in the horse, ox, and dog. [Illustration: FIG. 67.--MYOLOGY OF THE HORSE: INFERIOR ASPECT OF THE TRUNK. 1, Anterior extremity of the sternum; 2, point of the shoulder and inferior portion of the mastoido-humeral muscle; 3, pectoralis major (sterno-humeral); 4, pectoralis major (sterno-aponeurotic); 5, point of the elbow; 6, pectoralis minor (sterno-trochinian); 7, serratus magnus; 8, external oblique; 9, sheath of the rectus abdominis; 10, linea alba; 11, the umbilicus; 12, external oblique divided in order to expose the rectus abdominis; 13, rectus abdominis.] The muscular bundles, converging as they proceed, towards the scapula, pass under this bone, to be inserted into the superior portion of the subscapular fossa, near the spinal border. The inferior portion of its posterior digitations is visible in the ox and in the horse; these digitations are less visible in the pig. They are not seen at all in the dog (Fig. 68) or cat, for in these animals the great dorsal muscle covers them completely. The great serratus muscle, by the position which it occupies and the arrangement that it presents, forms with the corresponding muscle of the opposite side a sort of girth, which supports the thorax, and at the same time helps to fix the scapula against the latter. When it contracts, in taking its fixed point at the ribs, it draws the superior portion of the scapula downwards and backwards in such a way that this bone has its inferior angle directed forwards and upwards. If it takes its fixed point at the shoulder, it then acts on the ribs, raises them, and so becomes a muscle of inspiration.