74 



THE HORSE IN MOTION. 



action of the knee depends. The other is the humeralis externus of 

 Percivall. It originates behind and below the head of the humerus, 

 and, winding around that bone, fills the furrow of torsion (Plate II., 34). 

 It is inserted into the anterior heads of the radius and ulna, and acts 

 as an assistant to the flexor brachii : the two muscles originate from 

 opposite sides, but act as a unit lifting the forearm. It has greater 

 power than its associate, being larger and more muscular, and from 

 its spiral course its fibres are longer ; it is capable, therefore, of giving 

 higher action than its' associate. 



The triceps of the arm is a powerful muscle which plays a very 

 important part in the mechanism of the anterior extremity. As its 

 name implies, it is a three-headed muscle, if we choose to consider it 

 one muscle, and it is an extensor; but the correspondence in name 

 with the triceps extensor of the thigh should not lead us to confound 

 its mechanism with that of the latter. The triceps of the arm 

 (Plate IV., K) fills the angular space between the point of the elbow 

 (olecranon process) and the lower border of the scapula. The infra- 

 spinatus, i s, covers the origins of the three heads, but their com- 

 mon insertion at the short end of the ulna, as their lever, is clearly 

 shown in a strong tendon. The two upper heads are attached to the 

 lower border of the scapula, and when these divisions contract they 

 tend to close the angle between these bones ; but the third, or lower 

 head, is not attached to the scapula, but to the posterior face of the 

 humerus. This branch, sometimes called the short extensor, being 

 independent of the scapula, may act in extending the arm when the 

 angle formed by the latter bone and the humerus is so small that 

 the limit of contraction of the other two branches is reached, as is the 

 case in every instance before the fore foot leaves the ground in run- 

 ning. The triceps is a powerful combination of muscles. Its length, 

 varies with the distance from the joint at the shoulder, being seven- 

 teen inches at its greatest and eight at the least distance. Its weight 

 which is eight pounds, does not give a full conception of its power, 

 for its action is nearly direct. 



The anconeus is a small muscle attached to the capsular ligament 

 of the elbow joint, and is inserted into the olecranon process of the 



