i he cage and the pelvic bones are connected by a portion of the spine called the lumbar region. Muscular power acts on these masses as levers and allows the body to move forwards and backwards or turn. The pelvis can be compared to a wheel with only two spokes; the hub is the hip joint and the spokes are the legs which swing back and forth as in walking or running. When force is applied to the long end of a lever, the power is increased. When speed is desired, the lever is shortened.
The muscular power of the human body can only pull upon and bend the levers at the joints, when the masses of the back and pelvis are bent backward or forward, or to the side. The movement of the back is limited to the extent that the bony structure of the spine allows. Each segment of the spine is a lever, upon which the masses of the rib cage and the pelvis bend or turn. From the rear, the torso presents a great wedge with its apex directed downward. The base of the wedge is at the shoulders. This wedge is driven in between the two buttresses of the hips. In movement these two masses turn or bend.
Shoulder blades are embedded rather than attached to the back. They move from their attachment at the summit of the blade to the collarbone and are raised, lowered, or twisted by muscular force. The movements of the collar bones and the shoulder blades are free except where the collarbone joins the sternum in front. These bones curve around the cone shaped thorax, and are known as the thoracic girdle.
This girdle, except at its attachment at the sternum, may be raised or lowered; thrown forward or twisted round the static rib cage without interfering in any way with the act of expiration or inspiration. There is a space between the borders of the shoulder blades at the back and in front and between the two ends of the collar bones. The muscles that raise the shoulders away from the rib cage, when set in motion, work against each other with perfect balance.
The deltoid muscle resembles a delta in shape. It arises from the outer third of the clavicle and the convex border of the acromion and runs the entire length of the spine of the shoulder blade. All three portions are directed downward. The middle portion is vertical and the inner and outer descend obliquely, to be inserted by a short tendon into the outer surface of the humerus. Nature allows these three portions to work in harmony. The deltoid, when all three portions are working, pulls the arm up vertically. The portions that pull diagonally from the collarbone, and from the crest of the shoulder blade, carry it forwards and backwards.
The pectoralis major muscle twists upon itself when the arm is down. When the arm is extended or raised above the head, its fibres are parallel. When drawing a pectoral seven points should be noted: (1) where the tendon leaves the arm (2) its attachment on the collarbone (3) where it meets at its step-down from clavicle to sternum (4) its descent down the sternum (5) its attachment to the seventh rib (6) where it crosses till it leaves the sixth rib (7) the location of the second and third ribs that are just below the pre-sternum.
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