There are just over 600 voluntary muscles in the human body, but for our purposes here it is necessary to discuss only
• the larger superficial muscle groups which affect the shape of the body and are responsible for the movement of the limbs, and
• the rather more complex muscles which effect facial movement
All these are referred to as skeletal muscles. Most skeletal muscles are attached at both ends to bones (via tendons) and act rather like tension springs, in that they are able to contract; in doing so they cause the bones to pivot against one another like a lever on a fulcrum. The muscles that give expression to the face link bone to skin.
A muscle is made up of thousands of fibres, each of which is controlled by a nerve ending. These nerve endings respond to signals from the brain by releasing minute amounts of acetylcho-
Biceps Trumps, e, which causes the muscle fibres to contract along their length, that they become shorter and thicker, j The diagram on this page shows the action of the biceps uscle on the front of the arm. When it shortens, the arm is bent. ~o straighten the arm again another muscle - the triceps - must ntract at the back of the arm while the biceps relaxes. All uscles that are responsible for the movements of the skeleton re paired in this way so that for every muscle pulling in one irection there is another sited so that it can pull in the opposite direction.
Generally a whole complex of muscles is brought into play for each movement of the body; bending the arm involves many ore muscles than just the biceps and triceps. One set of muscles provides the primary moving force, while the muscles opposing it relax and lengthen in unison with it. Meanwhile, others immobilize joints not needed in the action and still others adjust to stabilize the body's equilibrium.
The muscle pattern is the same in the male and female figures, but differential fat deposits on breast and hip cause sex-specific differences in body-shape (see page 50).
The upper mass of the torso consists of the bones and muscles of the shoulder girdle and rib-cage. The lower mass consists of the rigid bones of the pelvis. All flexing movements of the torso are made possible by the structure of the spine which, although each individual vertebra can move and rotate on the next by only a small amount, can twist and bend throughout its length. This allows motion at the waist in every direction: forward, backward and sideways, and rotation to left and right. In the standing position the spine describes a double-S curve.
When you bend forwards your spine generally describes a gentle curve. However, this curve is usually more pronounced in one section of the spine, commonly the lumbar region at the base of the back, although in some individuals it is higher up. When you bend backwards, the greatest flexibility once again lies in the lumbar region, and some individuals can show remarkable backward articulation around the fourth lumbar vertebra. This spinal flexibility decreases with age and varies a great deal from one person to the next.
The abdominal muscles form a corrugated sheet down the front of the torso for bending the body forwards, while the sacrospinalis (erector spinae) muscles have the opposite function in bending the body backwards.
The very wide range of movement of the arms and shoulders is made possible by the arrangement of the clavicles and scapulae in what is referred to as the shoulder girdle. The scapulae are not held static, but are able to move laterally against the back of the rib-cage.
The deltoids or shoulder muscles add width to the shoulders. Along with the pectorals (the slabs of muscle on the chest) and the latissimus dorsi (which gives breadth to the back), their function is the movement of the upper arm.
The female breast is a gland superimposed on top of the pectorals and surrounded by fat, which gives it its distinctive
The articulation of the shoulder - upward, forward, backward and sideways.
The bone of the upper arm is the humerus. The bones of the forearm are the ulna and the radius. The bones of upper and forearm are connected at the elbow by a hinge-type joint.
Both ends of the ulna are evident just under the skin at the elbow and at the outer edge of the wrist. The radius, which like the ulna is hinged to the humerus at the elbow, is able to rotate around the ulna, giving an almost 360 degree rotation of the hand. This rotation is not, therefore, a function of either the elbow or wrist joints, but occurs within the forearm itself.
The main muscles of the upper arm are !he biceps on the front and the triceps at the rear. They control the bending of the arm at the elbow.
The wrist permits all movements except rotation - that is, forward-and-back and side-to-side. It consists of eight small bones, known as the carpal bones, arranged in two transverse rows.
The bones of the hand are the metacarpals and the phalanges. At the back of the hand the bone and muscle structure is just under the skin. On the palm there is a thick layer of protective tissue which acts as padding.
Whole books have been written on the subject of drawing hands. Being made up of such a large number of small bones, with attendant muscles, ligaments and tendons, the human hand is surprisingly flexible and versatile. To understand and draw it well you will need to do a lot of studies from life. A good way to start is by making drawings of your own hands using a mirror in the way we described on page 27.
Think of the palm as a flat square shape with a curved outer edge from which the four fingers radiate: to the basic square is added, on one side, a fleshy and very flexible wedge shape in which the thumb is rooted.
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