The Locomotor Apparatus. 321 



5 feet, and the short-arm 1 foot, a power of 1 lb. at the long- 

 arm will support a weight of 5 lbs. on the short-arm ; but 

 it is to be noted that as a lever increases in power it loses 

 in speed. 



In the second lever, which is a rare one in the body, the 

 weight is placed between the fulcrum and the power, as in 

 a wheelbarrow, the wheel being the fulcrum. 



When the leg is fixed on the ground the muscles of the 

 olecranon act as a lever of the second order, the ground 

 being the fulcrum, the triceps the power, and the body 

 through the elbow-joint the weight. The same applies to 

 the gastrocs when the weight is on the limb and the foot 

 on the ground. 



In the third order of lever the power is between the ful- 

 crum and weight. It is the lever of speed, and what it 

 gains in speed it loses in power. It is also the lever of 

 flexion, and examples abound in the body ; the flexor 

 brachii is one, the weight is the leg below the elbow, the 

 power the muscle at its insertion into the radius, and the 

 fulcrum the elbow-joint above. This is a wasteful lever, 

 but an essential one in the limbs. The nearer the power is 

 to the fulcrum, the greater the flexion obtained with the 

 least amount of muscular force, and the same remark will 

 apply to the first lever. Other examples are the masseter 

 muscles, flexor metatarsi, the psoas magnus, the muscles 

 which flex the head, etc. 



In nearly all cases the fulcrum of the lever is formed by 

 a joint, which is fixed or rendered rigid by other muscles 

 during its action. Antagonistic muscles are not relaxed 

 during the contraction of their opponents, nor are they 

 contracted. They may help the antagonistic muscles to 

 return to their position of rest. 



Groups of muscles act together ; this co-operation admits 

 of certain points being fixed, or increases the effect, or 

 gives the movement a special direction (Foster). 



The reason why the third lever is more frequent than 

 the others, is due to the fact that the chief movements of 

 the limbs are directed to moving comparatively light weights 



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