A shock is transmitted through the molecules of the ivory until it reaches the end ball, which is not held back by another. Here the transmitted force is expended in molar motion, the ball leaping away from its fellows as if it had been hung alone and had been struck with the same force.
It is well known that by placing an elbow firmly against a man’s jaw and then sharply striking the closed fist with the other hand, open, a very heavy blow can be given; yet the forearm, through which the shock is transmitted, does not move.
Now ivory is very like human bone. Further, it has been demonstrated that the law illustrated by the above experiment is equally applicable to the movement of vertebrae. The pushing or thrusting movement may move a specific vertebra, but it is probable that the chief factor in so doing is the element of transmitted shock contained in the movement and delivered at the instant of release of the hand from the spine at the end of the movement.
On the other hand it is obvious that a pushing or thrusting movement may move several vertebrae in addition to the one directly in contact with the adjusting hand, in consequence of the way in which the spinal segments are closely bound together. If a steady strain is used, in which muscles and ligaments have time to act, one of three results may occur: (a) the specific adjustment; (b) the movement of several vertebrae at one time, which does not constitute an adjustment; (c) the giving way of the spine at its weakest point, which may be some distance from the point of contact with the adjusting hand, the ligaments and muscles having communicated and diffused the strain throughout a large area. In the latter contingency the result is usually a new subluxation or the increase of an old one, instead of an adjustment.
The Rapid Movement
Thus Speed becomes an important factor in correct adjustment.
A good illustration of the value of speed may be taken from a pile of stakes bound together by a cord. If a man with a hammer desires to remove the center stake of the group, and attempts to do so with a slow pushing movement, the result is a change of position of many stakes, which adhere to the center stake and to each other. If, on the contrary, he strikes a sharp, quick blow with his hammer, meeting squarely the center of balance of the one stake, it will fly straight from its position leaving the others unmoved. This is exactly what we desire to accomplish with an adjustment. By the speed of the movement we expect to move one vertebra before adhesion or the contraction of muscles or inelasticity of ligaments can diffuse the force.
Close Contact
In order to accomplish the transmitted shock it would seem wisest, at first thought, to draw back the hand and strike the vertebra sharply. On the contrary, it has been found advisable to place the hand carefully in close and immediate contact with the vertebra to be adjusted. Nature herself shows us the way in the delicate shock-transmitting mechanism of the tympanum.
Also the hand of the adjuster will cover much more than merely the spinous or transverse process which is used as a lever and to which it is desired to transmit the shock, unless carefully placed so that only a small portion is in contact; by such a contact diffusion of the shock is prevented and its efficiency within a limited area is increased. A carpenter wishing to countersink a nail places in contact with the nail head a small instrument called a countersink, which he then strikes sharply with a hammer. The contact hand of the adjuster represents the countersink and is used by the two arms as a passive instrument for transmitting shock.