From a piece of lath and a few inches of copper or other flexible wire, a working model of the solid portions of the body may be constructed. Cut three pieces from the lath to represent the three solid masses of the body: the head, chest and hips. Approximately, the proportions of the three blocks, reduced from the skeleton, should be—Head, 1 inch by s/s of an inch; torso, IV2 inches by IV4 inches; hips, 1 inch by 1 lA inches.
Drive two parallel holes perpendicularly through the center of the thickness of each of these blocks, as closely together as practicable. Wire the blocks together by running a strand of flexible wire through each of these holes, allowing about half an inch between the blocks, and twist the wires together.
The wire in a rough way represents the spine or backbone. The spine is composed of a chain of firm, flexible joints, discs of bone, with shock-absorbing cartilages between them. There are twenty-four bones in the spine, each bending a little to give the required flexibility to the body, but turning and twisting mostly in the free spaces between the head and chest, and between the chest and hips. The spine is the bond of union between the different parts of the body.
The portion of this wire between the head and chest blocks represents the neck. On the neck the head has the power to bend backward and forward, upward and downward, and to turn. The head rests upon the uppermost vertebra of the spine, to which it is united by a hinged joint. Upon this joint it moves backward and forward as far as the muscles and ligaments permit. The bone beneath this hinged joint has a projection or point resembling a tooth. This enters a socket or hole in the bone above, and forms a pivot or axle upon which the upper bone and the head, which it supports, turn.
So, when we nod, we use the hinged joint, and when we turn our heads, we use the pivot or axle.
The wire between the two lower blocks represents that portion of the spine which connects the cage or chest above with the basin or pelvis below. This portion of the spine is called the lumbar region. It rests upon the pelvis or basin into which it is mortised. Its form is semicircular: concave from the front. On this portion of the spine, the lumbar, depends the rotary movement between the hips and the torso. As the spine passes upward, becoming part of the cage or chest, the ribs are joined to it.
The masses of head, chest and pelvis, represented by the three blocks, are in themselves unmoving. Think of these blocks in their relation to each other and forget, at first, any connecting portions other than the slender wire of the spine.
In the little tin soldier at "attention" we have an example of the symmetrical balance of these blocks one directly over the other. But this balance never exists when the body is in action, seldom, indeed, when it is in repose. The blocks in their relation to each other are limited to the three possible planes of movement. They may bend forward and back in the sagittal plane, twist in the horizontal plane or tilt in the transverse plane. As a rule, all three movements are present and they may be closely approximated by turning and twisting the three blocks in the little model of lath and wire.
The limitation to the movement of the spine limits the movements of the three masses or blocks. Such movement as the spine allows the muscles also allow.
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