22 8 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



Between these there is always a joint. When the muscle 

 contracts, its two attachments are brought nearer to- 

 gether, and the joint bends. Remember, again, that a 

 muscle can only pull, not push, and therefore motion 

 in two directions can only be effected by two opposing 

 muscles. Again, it must be borne in mind that in ani- 

 mal mechanism power is nearly always sacrificed to veloc- 

 ity, because swiftness is more valuable in the struggle for 

 life than slow dead strength. Therefore, of the 

 different orders of levers, the second is rarely, 

 almost never used, and in the first order the 

 fulcrum is so placed that power is sacrificed to 

 velocity. 



Examples of the first order are very numer- 

 ous. The action of the triceps on the point of 

 the elbow in straightening the arm and of the 

 gastrocnemius on the heel bone in bringing 

 down the toes (Fig. 148) 



~f — p 1- 3 are excellent examples. 



Fig. 146.— Diagram showing 1 Examples of the third or- 

 rkxingThe arm" 16 ""^ *" © der Ere e q ual ly numerous. 



We again take two : 1. Ac- 

 tion of biceps, pulling on its insertion, and flexing the 

 elbow (Fig. 146). 2. The action of the deltoid on its 

 insertion, in raising the arm (Fig. 147). 



The prodigious force of muscular contraction may 

 be easily calculated from these examples. Take the 

 case of the biceps in bending the arm at the elbow. I 

 suppose any one with average muscular vigor can hold 

 fifty pounds in the hand with the elbow joint at right 

 angles, as in Fig. 146. In such case, taking the distance 

 of the insertion of the tendon,/ 5 , from the fulcrum,/, as 

 one inch, and from the fulcrum to the weight, IV, one foot, 

 the pull of the muscle on the insertion at P necessary 

 to hold up the weight would be 50 X 12 = 600 pounds. 



