GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 81 



the functions of respiration, mastication, etc. The coordination of axial 

 and 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 If 



the muscles as sources of power for imparting move- * A *{ f ) 



ment to the levers with the object of overcoming ? 



resistance. F | . . 



In mechanics levers of three kinds or orders are A W 

 recognized according to the relative positions of the 

 fulcrum or axis of motion, the applied power, and w"~ p A 



the weight to be moved. (See Fig. 39 ) FlG 39 ._ THE THREE OR _ 



In levers of the first order the fulcrum, F, lies DERS OF LEVERS. 

 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 attached to the tuberosity 

 of the ischium. The opposite movement is equally one of the first 

 order, but the relative positions of P and W are reversed. 



2. The head in its movement backward and forward on the atlas. 



In levers of the second order the weight lies between the power and the 

 fulcrum. As illustration of this form of lever may be mentioned: 



1. The depression of the lower jaw, in which movement 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 is a gain in the extent and rapid- 

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

 6 



