APPAEATUS OP MOTION IN GENEEAL. 133 



Now in animal bodies, muscles are generally inserted very 

 obliquely, and thus very unfavourably in respect of their in- 

 tensity of contraction. The enlargement of the extremities 

 of the bones, as compared with the 

 shafts, serves to counterbalance 

 this obliquity in a certain degree, m 

 giving to the joints at the same 

 time more security. The tendons 

 (i) of the muscles (m) situated < 

 above the articulations, are in- 

 serted generally immediately be- 

 neath the enlarged extremity of ri s- 84 

 the bone, and thus reach the 

 moveable bone (o) in a direction more approaching the per- 

 pendicular, as may readily be understood by comparing Figure 

 85 with 84. 



267. The distance which separates the point of attach- 

 ment of the muscle from the point of support on which the 

 bone moves, and of the opposite extremity of the lever which 

 this organ represents, influences also in the most powerful 

 manner the effects produced by its contraction. 



The bones represent levers, which move on a fixed point, 

 called the point of support. The force which puts the lever 

 in action is the power ', and that which opposes its displace- 

 ment is the resistance. Finally, the name of arm of 

 the lever of the power, and arm of the lever of the resist- 

 ance, is given to the distance separating the point of 

 support from that to which is applied one or other of these 

 forces. 



Now the length of these arms of the lever has a powerful 

 influence over the force required to produce an equilibrium 

 to a given resistance. Observe the mechanism of the balance 

 called steelyard (Fig. 86). The beam is divided into two 

 unequal parts by the point of sup- 

 port a. At the extremity of one 

 of the branches (r) which is very 

 short, is placed the resistance, 

 that is, the object to be weighed ; r j 

 and on the other (p) slides any 

 weight which produces the equili- O 

 brium to a weight or resistance, ^S 86 



always the more considerable that 

 it is further removed from the point of support, and that we 



