PHYSIOLOGY OF MOVEMENT. 



727 



weight-arm. In the posterior extremity of the horse (Fig. 296) the 

 superficial gluteus muscle and the ischio-tibial muscles are levers of the 

 third class. 



Levers of the second class are more rarely met with. In the horse 



Fig. 292.— Anterior Ex- 

 tremity of the Horse 

 in Extension. (Colin.) 

 AD, BD, and CD. lines of action 

 of the tricep9 extensor brachii, scapulo- 

 ulnaris, and aconeus muscles. E F, 

 flexor brachii. G H, line of action of 

 flexor pedis muscles. 



Fig. 293.— Superior Ex- 

 tremity of Man. ( Colin. ) 



A C, line of action of the biceps 

 flexor. B, triceps extensor. 



Fig. 294.— Posterior Ex- 

 tremity of the Horse 

 in Extension. (Colin.) 

 A B, line of action of gluteus 

 medius. C D, line of action of triceps 

 extensor. E F, line of action of gas- 

 trocnemius. G E, line of action of 

 metatarsal flexor. 



the gastrocnemius acts through a lever of the second class when the foot 

 is in contact with the ground. Then the fulcrum is at the point of con- 

 tact of the foot with the ground, the power-arm is the distance from 

 the calcaneum to the ground, the weight-arm the distance from the 



