116 PHYSIOLOGY OP FARM ANIMALS [CH. 



the fetlock. Thus the hock cannot be extended without the 

 fetlock being flexed. The flexor metatarsi bends the hock. 



The joints below the hock are extended by muscles which 

 take origin near the stifle, run down the front of the limb, are 

 contmued as tendons down the front part of the cannon bone, 

 and finally are inserted on the pastern and coffin bones. They 

 are flexed by a muscle originating at the back of the upper 

 portion of the tibia behind which it runs down to the hock joint 

 where it is continued as a tendon (flexor perforans), and terminates, 

 as in the fore limb, at the bottom of the coffin bone. 



The Mechanics of Locomotion. The principal kinds of 

 movements which the limbs may be made to undergo by the 

 contraction and expansion of their muscles may be classified 

 naturally under four heads : (1) Flexion or bending, (2) Extension 

 or straightening out, (3) Abduction or drawing away from the 

 middle line, and (4) Adduction or bringing to the middle line. 

 To these may be added (5) Rotation, when a limb is made to 

 turn on its own axis, and (6) Circumduction, when it is made to 

 describe a conical surface by rotation around an imaginary 

 axis. 



Now in a large number of these movements a joint is involved, 

 and the bone of the part which moves acts as a lever, and turns 

 about that portion of itself which forms part of the joint con- 

 cerned and acts as a relatively fixed point or fulcrum. We may 

 now consider the three kinds of levers, and then proceed to give 

 examples of these levers as shown by the movements of certain 

 of the limb muscles which have been already described. 



In the first kind of lever the fulcrum is between the weight 

 and the power. It is the lever of power and in the body it is the 

 I'jver of extension. Thus in extending the hind leg, the centre 

 of the hock joint is the fulcrum, the gastrocnemius is the power, 

 the distance from the summit of the calcaneum to the centre of 

 the hock jomt is the power arm, the leg below the hock is the 

 weight and the length of the metatarsus is the weight arm. 



I A I 



Weight or Fulcrum Power 



resistance 



In the second kind of lever the fulcrum is at one end and is 

 nearer to the weight than to the power. This lever is not common 



