84 TEXT-BOOK OF PHYSIOLOGY 



appendicular muscles enables the individual to assume certain postures, 

 such as standing, sitting, and lying; to engage in various acts of locomotion, 

 as walking, running, dancing, swimming. 



Levers. The function or special mode of action of individual muscles 

 can be understood only when the bones with which they are connected are 

 regarded as levers whose fulcra or fixed points lie in the joints where the 

 movement takes place, and the muscles as sources of power for imparting 

 movement to the levers with the object of overcoming resistance. 



In mechanics levers of three kinds or orders are recognized according 

 to the relative positions of the fulcrum or axis of motion, the applied 

 power, and the weight to be moved. (See Fig. 38.) 



In levers of the first order the fulcrum, F, lies between the weight or 

 resistance, W, and the power or moving force, P. The distance P F is 

 known as the power arm and the distance W F as the weight arm. As 

 examples of this form of lever found in the human body may be mentioned : 



1. The elevation of the trunk from the flexed position. The axis of move- 



ment, the fulcrum, lies in the hip- joint; the weight, that of the trunk, 



acting as if concentrated at the center of gravity, which lies close to the 



tenth dorsal vertebra; the power, the muscles at- 



$ tached to the tuberosity of the ischium. The 



T? * fa) opposite movement is equally one of the first 



? order, but the relative positions of P and W are 



F | reversed. 



A W P ' 2. The head in its movement backward and forward 



_ | p on the atlas. 



W~~ p A^ I* 1 levers of the second order the weight lies be- 



FIG. 38. THE THREE tween the power and the fulcrum. As illustration of 

 ORDERS OF LEVERS, this form of lever may be mentioned: 



i. The depression of the lower jaw, in which move- 

 ment the fulcrum is the temporomaxillary articulation; the resistance, 

 the tension of the elevator muscles; the power, the contraction of the 

 depressor muscles. 



2. The raising of the body on the toes, in which movement the fulcrum is 



the toes, the weight that of the body acting through the ankle, the 

 power the gastrocnemius muscle applied to the heel bone. 

 In levers of the third order the power is applied at a point lying between 



the fulcrum and the weight. As example of this form of lever may be 



mentioned : 



1. The flexion of the forearm, in which the fulcrum is the elbow-joint, the 



power the biceps and brachialis anticus muscles applied at their points 

 of insertion, the weight that of the forearm and hand. 



2. The extension of the leg on the thigh. 



When levers are employed in mechanic operations, the object aimed at 

 is the overcoming of a great resistance by the application of a small force 

 acting through a great distance, so as to obtain mechanic advantage. In 

 the mechanism of the human body the reverse generally obtains, viz., the 

 overcoming of a small resistance by the application of a large force acting 

 through a short distance. As a result there fs a gain in the extent and rapid- 

 ity of the movement of the lever. The power, however, owing to its point 



