The first muscles usually presented for study to artists being the pectorals, it is their [homologues] that we will first describe here. We will afterwards describe the [abdominal region], then the muscles which occupy the [dorsal aspect] of the trunk. With regard to the lateral surfaces, they will be found, by this fact alone, almost completely studied, since the muscles of the two preceding (back and abdomen), spreading out, so to speak, over them, contribute to their formation. Nothing further will remain but to incorporate with them the [muscles of the shoulder]; but these will be studied in connection with the anterior limbs, from which they cannot be separated.
The neck, in man, may be considered in an isolated fashion, because, on account of its narrowness in proportion to the width of the shoulders, it is clearly differentiated from the trunk; for this reason we combine the study of it with that of the head. In animals, because of the absence or slight development of the clavicles, the neck is generally too much confounded with the region of the shoulders to make it legitimate to separate it from that region in too marked a fashion. It will, accordingly, be considered [next].
We will then undertake the study of the [muscles of the limbs], and end with the [myology of the head].
THE MUSCLES OF THE TRUNK
We shall divide them into muscles of the thorax, of the abdomen, and of the back.
Muscles of the Thorax
The Pectoralis Major ([Fig. 66], 1, 2; [Fig. 67], 3, 4; [Fig. 68], 7; [Fig. 69], 10; [Fig. 70], 11).—Further designated by the name of superficial pectoral, this muscle is described in treatises on veterinary anatomy as formed of two portions: an anterior one, called the sterno-humeral muscle; the other, situated below and behind the preceding, bearing the name of sterno-aponeurotic.
It occupies the region of the breast, and, as a whole, it takes origin from the median portion of the sternum, from which it is directed towards the arm and forearm.
The anterior portion (sterno-humeral muscle)—thick, forming an elevation under the skin, and really constituting the pectoral region—is directed downwards and outwards to be inserted into the anterior margin of the humerus—that is to say, to the ridge which limits in front the spiral groove of this bone.