THE ACTION OF LOCOMOTOR MECHANISMS. 417 



of the bone serving as an axis of rotation. The pivot-joint allow- of rotation 

 only; the atlanto-axial and radio-ulnar joint.- may be placed in this class. 



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 I era- of the first class, in which 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, upou which the rotation occurs, is the fulcrum. The 

 balancing of the head upou 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-joiut, aud 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 iu 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. 1 



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 bit ween the tendon of the muscle and the bone; for 

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

 vantage, ibr a large part of the force is expended in pulling the ulnar and 

 radius against the humerus, and is Lost Ibr movement, but as the elbow is 

 flexed the force is directed more and more nearly at right angle- t.. the bones 

 of the forearm, and there is a gain in leverage, which is of course again 

 decreased as flexion is completed. This gain in Leverage which accompanies 

 the shortening of the muscles i^ the more important, since the power of the 

 muscle is greatest when the muscle has its normal length, ami continually 

 lessens as the muscle shortens in contraction. There are a number of special 

 arrangements which help to increase the leverage of the muscles by lessening 

 the obliquity of attachment — viz. the enlarged heads of the bones, and in some 



1 Certain observers would class this movement as an example of a lever of the first class 



(Ewald: I'llii'/rr's Archiv, 189<J, Bd. lxiv. S. 53). 

 Vol. If.— 27 



