THE ACTION OF LOCOMOTOR MECHANISMS. 857 



the shape of the joint-surfaces cause flexion to be produced by the combined 

 action of sliding, rolling, and rotation movements. In complete extension 

 the lateral ligaments and the posterior and anterior crucial ligaments are 

 put on the stretch, and there is a locking of the joint, no rotation being 

 possible; in complete flexion, on the other hand, the posterior crucial 

 ligament is tight, but the others are sufficiently loose to allow of a consider- 

 able amount of pronatiou and supination. In the saddle-joint there is a 

 double axis of rotation e. g. the articulation of the trapezius with the first 

 metacarpal bone permits of rotation about an axis extending from before back- 

 ward, and another, at nearly right angles to this, extending from side to side. 



The ball-and-socket joint, of which the shoulder- and hip-joints are exam- 

 ples, permits of the greatest variety of movements, any diameter of the head 

 of the bone serving as an axis of rotation. 



Method of Action of Muscles upon the Bones. The bones can be 

 looked upon as levers actuated by the forces which are applied at the points 

 of attachment of the muscles. All three forms of levers are represented in 

 the body ; indeed, they may be illustrated in the same joint, as the elbow. 



An example of a lever of the first class, in \vhich the fulcrum is between 

 the power and the resistance, is to be found in the extension of the forearm in 

 such an act as driving a nail : the inertia of the hammer, hand, and forearm 

 offers the resistance, the triceps muscle acting upon the olecranon gives the 

 power, and the trochlea, upon which the rotation occurs, is the fulcrum. The 

 balancing of the head upon the atlas is another example : the front part of the 

 head and face is the resistance, the occipito-atlantoid joint the fulcrum, and 

 the muscles of the neck the power. 



In the case of a lever of the second order, the resistance is between the ful- 

 crum and the power; for example, when the weight of the body is being 

 raised from the floor by the hands : the fulcrum is where the hand rests on the 

 floor, the weight is applied at the elbow-joint, and the power is the pull of the 

 triceps on the olecranon. The raising of the body on the toes is another ex- 

 ample : the fulcrum is at the place where the toes are in contact with the 

 floor, the resistance is the weight of the body transmitted through the tibia to 

 the astragalus, and the power is applied at the point of attachment of the 

 tendo Achillis to the os calcis. 



The raising of a weight in the hand by flexion of the forearm through 

 contraction of the biceps gives an example of a lever of the third order, in 

 which the power is applied between the fulcrum and the weight. This form 

 of lever, because of the great length of the resistance arm, as compared with 

 the power arm, is favorable to extensive and rapid movements, and is the 

 most usual form of lever in the body. 



The power is applied to best advantage when it is exerted at right angles 

 to the direction of a lever, as in the case of the muscles of mastication and of 

 the calf of the leg. If the traction be exerted obliquely, the effect is the less 

 the more acute the angle between the tendon of the muscle and the bone ; for 

 example, when the arm is extended the flexor muscles work to great disad- 



