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- FIG. 293. SUPERIOR EX- 



TREMITY OF THE HORSE 

 IN EXTENSION. (Colin.) 



AD, BD, and CD. lines of action 

 of the triceps extensor brachii, scapulo- 

 ulnaris, and aconeus muscles. E F, 

 flexor brachii. G II, line of action of 

 flexor pedis muscles. 



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.) 



AB, line of action of gluteus 

 medius. C D, line of action of triceps 

 extensor. E F, line of action of gas- 

 trocnemius. Gil, 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 



