X] THE ORGANS OF LOCOMOTION 115 



and the elbow flexed. This action is due to the contraction of 

 a muscle (the flexor brachii) attached at one end to the front of 

 the shoulder blade and at the other end to the front of the radius 

 just below the elbow joint. 



The chief muscle that extends the ' knee " or wrist (the extensor 

 metacarpi magnus) has its origin in the front part of the humerus ; 

 it runs down the fore-arm, its tendon passing over the 'knee' and 

 being inserted on the head of the cannon bone. 



The three muscles that bend the knee take origin on the back 

 of the humerus just above the elbow and are inserted on the 

 splint bones (internus, medius and externus metacarpi flexor). 



The two muscles which extend the fetlock, pastern and coffin 

 joints run down the front of the fore-arm. One has its origin 

 on the head of the radius and is inserted on the front of the large 

 pastern bone (extensor metacarpi). The other starts on the 

 humerus above the elbow and passes to the front part of the 

 coffln bone (extensor pedis). 



The muscles that flex the fetlock, pastern, and coffin joints, 

 and also aid in bending the knee, take their origin just above the 

 elbow joint at the back of the humerus, and proceed down the 

 posterior side of the fore-arm. A little above the 'knee' they 

 become joined to their tendons. These are attached one to the 

 base of the cofiin bone (the flexor pedis perforans tendon), and 

 the other to the small pastern or coronet (the flexor pedis per- 

 foratus tendon). 



The Muscles of the Hind limb. The hip is extended by the 

 croup (gluteal) muscles, and also by some muscles which lie at 

 the back of the femur. It is flexed bj^ muscles which have their 

 origin on the under surface of the lumbar vertebrae and are 

 inserted on to the femur. 



The stifle (true knee) is extended by a muscle (triceps ex- 

 tensor) which has its origin on the under surface of the pelvis 

 joint in front of the hip joint, and is inserted on the patella or 

 knee cap. It is flexed by a mviscle attached to the portion of the 

 pelvis behind the hip joint, and to the tibia. 



The hock is extended by the gastrocnemius which has its 

 origin on the lower end of the femur, and is inserted by tendons 

 on the point of the hock. One of the tendons (the one passing 

 underneath) terminates at the hock, but the other (the flexor 

 pedis perforatus) passes to the small pastern, and is the flexor of 



