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A TEXTBOOK OF PHYSIOLOGY 



applied to the bones in such a way that the latter is the case. In 

 order that the muscles may be packed within the skin and the body 

 made as compact as possible, the. power is applied at the insertion of 

 the muscles close to the joints or fulcra. All three orders of levera 

 are exemplified in the body. Belonging to the first order is the move- 

 ment by which the head, jointed to the top of the spine, is nodded 

 backwards and forwards by the neck muscles (Fig. 262, 1.). Another 

 example is the straightening movement of the forearm by the action 

 of the triceps muscle. The power is applied at the insertion of the 

 muscle into the ulna just above the elbow -joint, which is the fulcrum, 

 and the resistance (the weight of the forearm) lies be3'oiid. Range 

 and rapidity of movement of the hand are here gained at the expense 

 of power. 



The second order of lever is seen in the movement by which the 

 calf muscles raise the body on tiptoe (Fig. 262, II.). The power is 

 applied at the back of the heel, the fulcrum is at the toes, and the 

 weight of the body falls on the foot at the ankle-joint. Here power is 

 gained at the expense of range of movement. 



I. II. III. 





W P F P 



W 



Yi-r. 202. DIAGRAM OF THREE KINDS OF LEVER ACTION. 



F, Fulcrum ;P, power; W, weight. 

 I. The head is tilted back by neck muscles. 



II. The toes rest on the ground, and the body is raised by the calf muscles. 

 III. The forearm is bent up by the biceps muscle. 



Examples of the third order of lever are numerous. In the bending 

 of the forearm on the upper arm (Fig. 262, III.), the power is applied 

 by the biceps muscle to the radius just below the elbow-joint, the 

 fulcrum is at the elbow, and the resistance is the weight of the fore- 

 arm and hand. The bending and the straightening of the leg at the 

 knee-joint are other examples. In all these movements rapidity and 

 range of movement are obtained at the expense of power. 



We usually employ that combination of levers which require the 

 least muscular effort. It is easier to carry a weight with the arm 

 hanging fully extended, when it is slung to the shoulder by the bones 

 and tendons, and the muscles have only to maintain the grip of the 

 fingers, than it is to carry it with the arm bent, and much greater 

 muscular effort. Man is constantly devising methods to save the 

 expenditure of muscular effort. Thus a drayman pulls a beer -barrel 

 up an inclined plane, which bears a large part of the weight. 



