More unchanging than either of the above is the mass of the abdomen. The central groove is here shallow and may lose itself below. The long wedge ends in the symphysis pubis.
The Trunk. Muscles of the Trunk, front view.
1 Pectoralis major.
3 Rectus abdominis.
4 Serratus magnus.
5 External oblique.
Pectoralis major, page 154 Deltoid, page 104.
From symphysis pubis to cartilages of ribs, from fifth to seventh.
From eight upper ribs to scapula - spinal edge, under surface.
Draws shoulder blade forward, raises ribs.
From eight lower ribs to iliac crest and ligament to pubis.
Skeleton Of The Trunk. Muscles Covering Upper Portion, Front View.
1 Pectoralis minor. 2 Pectoralis major.
From third, fourth and fifth ribs to coracoid process.
Depresses point of shoulder.
From inner half of clavicle, sternum, costal cartilages as far as sixth and seventh ribs to humerus.
Draws arm downward and forward.
The Trunk. Trunk, front view Armpit and Shoulder.
The erect torse 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 former, that of the chest, is broken by the almost vertical shoulder blade and the slight bulge of the latissimus below it.
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, collar bone, and muscles.
It 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, described later. 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 vertebra, 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 back presents numerous depressions and prominences. This is due not only to its bony structure, but to the crossing and recrossing of a number of thin layers of muscles. It should be borne in mind that the superficial or outside layers manifest themselves only when in action. For this reason, under all changes of position, the spine, the shoulder-blade with its acromion process, and the crest of the ilium, must be regarded as the landmarks of this region.
The spine is composed of twenty-four vertebrae. It extends the full length of the back, and its course is marked by a furrow. The vertebrae are known as the cervical, dorsal and lumbar. The cervical vertebras are seven in number, and the seventh is the most prominent in the whole of the spine. It is known as vertebra prominens. In the dorsal region the furrow is not so deep as below. Here there are twelve vertebrae. When the body is bent forward, the processes of the vertebrae in this section are plainly indicated.
The spinal furrow becomes deeper as it reaches the lumbar vertebras, where it is marked by dimples and depressions. It widens out, too, in this part of the body, and as it passes over the surface of the sacrum to the coccyx it becomes flattened. The average length of the spine is about two feet three inches.
The outer corner of the shoulder girdle is the acromion process, which is the high outer extremity of a ridge rising from the shoulder blade. The shoulder blade or scapula (spade) is a flat plaque of bone fitting snugly against the cage of the thorax, having a long inner vertical edge, parallel to the spine; a sharp lower point; a long outer edge pointing to the arm pit: and a short upper edge parallel with the slope of the shoulder. The ridge, or spine of the scapula, starts at the spinal edge, about a third of the way down, in a triangular thickening, and rises until it passes high over the outer upper corner, where the shoulder joint ties, then turns forward to join with the collar bone at the acromion. The prominent portions are this ridge and the spinal edge and the lower corner. The upper outer corner is thickened to form the socket for the head of the humerus, forming the shoulder joint proper.