The erect torso presents in profile the long curve of the front, broken by depressions at the border of the breast muscle and at the umbilicus or navel into three lesser curves, almost equal in length. The back presents the sharp anterior curve of the waist, opposite the umbilicus, bending into the long posterior curve of the chest, and the shorter curve of the buttocks. The curve of the chest is broken by the almost vertical shoulder blade and the slight bulge of the latissimus below it.
In profile the torso presents three masses: that of the chest, that of the waist, and that of the pelvis and abdomen. The first and last are comparatively unchanging.
Above, the mass of the chest is bounded by the line of the collar bones; below, by a line following the cartilages of the ribs, being perpendicular to the long diameter of the chest.
This mass is widened by the expansion of the chest in breathing, and the shoulder moves freely over it, carrying the shoulder blade, collarbone, and muscles.
The chest is marked by the ridge of costal cartilages that forms its border, sloping up and forward, and by the ribs themselves, sloping down and forward, and by the "digitations" (finger marks) of the serratus magnus (big saw-toothed) muscle, little triangles in a row from the corner of the breast muscle, paralleling the cartilages of the ribs, disappearing under the latissimus.
Below, the mass of the pelvis and abdomen slopes up and forward. It is marked by the iliac crest and hip. In front it may be flattened by contraction of the abdominal muscles. Over its surface the hip moves freely, changing the tilt of the pelvis.
Between these the central mass contains the waist vertebrae, and is very changeable. Practically all of the movement of flexion and extension for the whole spine occurs here, and much of the side-bending.
This mass is marked by a buttress of lateral muscles, slightly overhanging the pelvic brim and bearing inward against the side above. It changes greatly in different positions of the trunk.
The Torso, side view:
External Oblique: From eight lower ribs to iliac crest and ligament to pubis. Action: Flexes thorax.
I. bones: (1) Scapula, (shoulder blade), a large flat bone triangular in shape. It articulates with the collar bone at the summit of the shoulder. (2) The serratus magnus muscles follow the ribs. See Muscles No. II. (3) The thorax or rib cage is the cavity enclosed by the ribs, attached to the spine behind and to the sternum in front. The upper ribs are quite short and grow longer till they reach the seventh rib, which is the longest and the last to fasten to the breastbone. The upper seven ribs are named the true ribs.
II. muscles: (I) The latissimus dorsi muscle covers the region of the loins to be inserted into the upper part of the arm at the lower border of the bicipital groove. It is a superficial muscle, a thin sheathing that finds attachments at the small of the back and at the crest of the ilium near the lumbar and last dorsal vertebrae. (2) The serratus magnus muscle is seen only at its lower parts as prominent digitations that show on the side of the thorax or rib cage below the armpit. A large portion of this muscle is covered by the pectoralis major and the latissimus dorsi muscles. (3) The external oblique is attached aboye to the lower eight ribs, where they interlock with the serratus magnus. From here they are carried downward to be attached to the iliac crest.
III. The serratus magnus draws the shoulder blade forward and raises the ribs. The latissimus dorsi draws the arm backward and inward. Its upper border curves backward at the level with the sixth or seventh dorsal vertebrae, as it passes over the lower angle of the shoulder blade.
The serratus magnus muscle forms the inner wall of the armpit. Its insertion to the ribs above are not seen, while those below, three or four in number are plainly visible in the region between the great pectoral and the latissimus dorsi.
IV. In profile, the torso in front is marked by the ridge of the costal cartilage that forms its border. Sloping up and forward (with the ribs themselves sloping down and forward) the digitations of the serratus magnus meet the external oblique.
In its attachment to the crest of the ilium, the external oblique forms a thick oblique roll, its base marking the iliac furrow. When one side of this muscle contracts, it gives the trunk a movement of rotation to the right or left side. When both sides pull, the oblique muscles draw the ribs downward, thus bending the body forward.
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