Fis-. ii.— I.ever of tho Third Order 



:U CONFORMATION AND ITS DEFECTS 



In levers of the third kind the fulcrum is again at one end, but the 

 power now occupies a position between it and the weight. A common 

 example of this kind is seen in a irdiv of fire-tongs when holding a lump of 



coal. The coal here repre- 



sents the weight, the hand 

 holding the tongs the power, 

 and the joint the fulcrum 

 (fig- 22). 



The two arms of a lever 

 are distinguished respectively as the power-arm and the weight-arm. The 

 former is represented by the distance between the point at which the power 

 acts and the fulcrum, and the latter by the distance between the fulcrum 

 and the line through which the weight acts. 



In the case of the biceps muscle acting upon the radius (fig. 25), the 

 power-arm is the distance between the centre of the elbow-joint, which is 

 the fulcrum, and the point of insertion of the muscle. The weight-arm is 

 the distance from the fulcrum to the knee. 



Of tlie three varieties of levers referred to above, those of the first class 

 are levers of speed, and for the most part are engaged in movements of 

 extension. Those of the third order are also levers of velocity, but are 

 more especially concerned in movements of flexion. 



Levers of the second class are much less frequently met with than those 

 of the first and third, and are essentially levers of force. 



In the animal machine an example of a lever of the first order will be 

 found in the extension of the cannon on the hock tvhen the foot is off 

 the ground. Here the muscle representing the power (fig. 23) acts upon 

 the point of the hock, the fulcrum is the hock-joint, and the parts below 

 the weight. The same parts of the limb also afford an illustration of a 

 lever of the second class ivheji the foot is on the ground (fig. 24). In this 

 instance the point of the hock is the part on which the power acts, the 

 ground is the fulcrum, and the weight is at the hock-joint. 



The third order of lever is represented in the fore-arm (fig. 25), where the 

 fulcrum is at the elbow-joint, the power is the biceps muscle, which is inserted 

 just below the elbow in front, and the weight is the parts of the limb below. 



It will be seen by an examination of these illustrations, that in levers of 

 the first and third class, or levers of speed, the power-arm is shorter than 

 the weight-arm, while in those of the second order the length of the former 

 exceeds that of the latter. 



Bones which act as levers of the first class when the foot is off the 

 ground, become levers of the second kind when the foot is on the ground 

 the fulcrum and the weight having now become displaced. 



