We will recall at the outset that the lesser pectoral muscle in man is completely covered by the great. In animals this is not the case; the lesser pectoral being very highly developed, projects beyond the great pectoral posteriorly, and occupies to a greater or less extent the inferior surface of the abdomen.
It also consists of two parts: one anterior, which we designate by the name of sterno-prescapular; the other, posterior, bearing that of sterno-humeral.[20]
[20] This division of the pectorals certainly complicates the nomenclature of these muscles; nevertheless, it introduces no insuperable difficulty from the mnemonic point of view. But where the study becomes less profitable, and comparison with the corresponding muscles in man more complicated, is in adopting the nomenclature of Bourgelat. Indeed, the great pectoral is designated by this author the ‘common muscle of the arm and forearm,’ while the lesser pectoral (or deep pectoral) is called the ‘great pectoral’ in its sterno-trochinian and ‘lesser pectoral’ in its sterno-prescapular portion. We do not consider it necessary to give the other theories relative to the homologies of these, notwithstanding the very real interest which they present from the purely anatomical point of view, as they have but few applications in the study of forms.
The sterno-prescapular muscle, being covered by the sterno-humeral, has little interest for us. It arises from the sternum, and is directed towards the angle formed by the junction of the scapula and humerus; then it is reflected upwards and backwards, to terminate on the anterior margin of the shoulder by insertion into the aponeurosis, which covers the supraspinatus muscle.
We can, especially in the horse after removal of the skin, recognise it, at the level of this region, in the interspace limited by the superficial muscles ([Fig. 70], 26).
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.