724 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



and adductors of the arm have their insertion almost perpendicular to 

 the hones on which they act. 



Although all three classes of levers are 

 met with, in general the lever of the first class 

 comes into play in movements of extension, and 

 that of the third class in movements of flexion. 



A lever may be defined as an inflexible bar 

 capable of being freely moved about a fixed point or 

 line, which is called the fulcrum. In the first class 

 of lever the fulcrum lies between the weight and the 

 power, and may be illustrated by a common crowbar 

 or a pair of scissors. In levers of the second class 

 the weight lies between the fulcrum and the power, 

 and may be illustrated by the wheelbarrow or nut- 

 cracker. In the third class of lever the power falls 

 between the fulcrum and the weight, and may be 

 illustrated by a pair of fire-tongs or sheep-shears 

 (Fig. 286.) 



(1) 



• 



F 





1 



w 



A 





p 



F 



• 





k 



A 



w 





p 



• 





fc 



F 



(2) 



. w p A 



Fig. 286.— Three Classes of Levers. 



(3) 



(Landois.) 



W, weight; F, fulcrum; P, powor; 1, in levers of first class the fulcrum 

 Is hetween the power and the weight; 2, in levers of second class the weight 

 falls between the power and fulcrum ; 3, in levers of third class the power is 

 applied between the fulcrum and weight. The index shows the direction in 

 which the power acts. 



Fig. 287.— Diagrams show- 

 ing the Mode of Ac- 

 tion of the Three 

 Orders of Levees, 

 Numbered from above 

 Downward, Illus- 

 trated by the Action 

 of the Elbow-Joint. 

 (Yeo.) 



In considering the development of power by the use of levers, the relation- 

 ship between the power- and the weight-arm has to be considered. The power- 



Fig. 288.— Partial Contraction of Biceps. (Perrier.) 



arm of the lever may be defined as the perpendicular distance from the line in 

 which the power acts to the fulcrum ; the weight-arm, the perpendicular distance 



