ORGANS OF MOTION AND LOCOMOTION.
Ile active agents of movement in the body are the muscles, which, by their contraction, cause the bones to move one on the other, the motion being permitted owing to the union of bones to one another being accomplished through time medium of joints. Of course if all the bones were rigidly connected with one another all movements would be impossible. Now, the muscles, bones, and joints are related to one another in different ways in order to make the movements more effective for different purposes. The bones form levers, the attached muscles supplying the power for moving theni, the joints being the fulcrum, or point of sup port, while the resistance is supplied by the weight of the limb, the weight to be lifted, or the force to be overcome. In mechanics there are three orders of levers described, according to the relative positions of power, fulcrum, and resistance ; and it is found that all the movements of bones on one another can be referred to one or other of the three orders of levers.
Fig. 74 shows a lever of the first order, where the fulcrum (F) is between the weight to be lifted (w) and the power (v). There are several exam ples of this kind of lever in the human body. The head supported on the atlas is a good example. The joint between atlas and skull (p. 61) is the fulcrum, the resistance is the weight due to the part of the head and face in front of the joint, which tend to produce falling down of the chin on the chest, and the power is behind, where the muscles from the neck are attached to the back of the skull. The effect of this particular arrangement is to keep the head steady, balanced on the back-bone, and it is easily seen how well adapted this kind of lever is for such a balancing purpose. It is therefore called the lever of stability. The back-bone is balanced on the haunch-bones, and the leg is balanced on the foot, by a similar arrangement, the hip-joint in the former case, and the ankle-joint in the latter, being fulcrum. This lever is also used in the body more directly to effect movement. When the forearm is straightened on the arm the elbow-joint is fulcrum, the power is supplied by the tri ceps muscle behind, and the resistance is the weight of the forearm in front of the fulcrum (Fig. 75).
In levers of the second order the weight is between the power and fulcrum (Fig. 76). It is not common in the body. Standing on tip toe, however, is an example. The fulcrum is afforded by the toes, in contact with the ground ; the power is the action of the muscles of the calf, and between these is the weight of the body transmitted down the bones of the leg to the foot (Fig. 77).
In levers the force exerted is always pro. portional to the distance between the fulcrum and the point of application of the force. Now, in levers of the second order the distance be tween the power and the fulcrum— called the power arm is greater than the distance between the weight and the fulcrum — the weight arm. So that a force act ing through the greater dis tance between the power and fulcrum would be able to overcome the same force acting through the smaller distance between the weight and fulcrum, just because, though the forces were the same, the power arm was longer than the resistance arm. In other words, a smaller power would he : able to overcome a larger resistance, because of its greater distance from the fulcrum. This lever is therefore called the lever of power. This lever has the disadvantage that the power must always move through a greater distance than the weight.
In levers of the third order the power is between the weight and the fulcrum (Fig. 78). It is common in the human body. In bending the forearm on the arm the power is sup plied by the bi ceps muscle at tached to the radius, the ful crum is the joint at one end of the lever, and the weight is at the other end—the weight of the forearm (Plate I), It will be remembered that at the elbow-joint an illustration of a lever of the first order also is afforded in the extension of the joint by the triceps muscle.
This is the lever of rapidity, for it will be observed that a small movement of the biceps will produce considerable movement of the hand. It is, however, a lever that loses power, for, unlike the last case, here time weight arm is long and the power arm short.