LOCOMOTOR SYSTEM 591 



from being driven through the withers by the serratus. It is 

 essential for the humerus that the shoulder- joint should be fixed 

 and the elbow- joint locked. The large muscular mass of the 

 triceps, acting from the humerus and scapula on the ulna, is 

 mainly engaged in effecting this (Fig. 185, 1). The more powerfully 

 the triceps contracts, the more firmly is the beak -of the ulna 

 forced into the olecranon fossa of the humerus. It is this 

 mechanism which keeps the elbow- joint locked. But, bearing 

 in mind what has previously been said about the absence of 

 exclusiveness in muscles, it is evident that the flexors of the 

 fore-leg act passively on the elbow and keep it firmly extended 

 (Figs 185, 5 and 6), while the biceps brachii (flexor brachii), 

 through its connection with the main extensor muscle of the 

 limb, helps to keep the scapula in position and the shoulder-joint 

 from opening in front (Fig. 185, 3). This is a good example of 

 co-operative antagonism. Here we see the flexor of the arm, 

 extensor of the leg, and extensor of the elbow, all of which are 

 antagonists when actively contracting, working together with 

 one common object in view. The tendon running throughout 

 the length of the biceps is of invaluable assistance in keeping the 

 shoulder- joint fixed while the limb is in the standing position ; 

 it acts, in fact, as a ligament from scapula to radius (Fig. 185, 2). 



The shoulder and elbow being fixed, the next joint, the knee, 

 is far more easily controlled. From the knee to the foot it is 

 only necessary to prevent the joints opening in front in order to 

 maintain the limb rigid and upright, and to ensure this each 

 segment of the limb is furnished with an extensor tendon (Fig. 186). 

 There is a large one to the knee, and another to the pastern and 

 foot, both of which are in turn reinforced for extra strength. 

 The muscles which manipulate this locking apparatus run from 

 the humerus, cross the front of the elbow-joint, and are dis- 

 tributed segment by segment until the foot is reached. Their 

 efficacy in locking the fore-limb from the elbow to the foot 

 depends upon the elbow-joint itself being firmly extended and 

 locked. The large knee extensor receives powerful assistance 

 from the biceps brachii in the form of a ligament which runs from 

 the biceps to the extensor (Fig. 185, where 2 and 3 meet). 



The position thus created is as follows : The biceps during the 

 standing attitude, though not out of action, is not engaged in 

 flexing the elbow. It is a flexor muscle which, we have just 

 seen, is passively resisting the triceps, but it does not hesitate to 

 help an extensor, an opponent, during the time it is not other- 

 wise actively employed on its own special duty. This is a good 

 example of the non-party feeling shown by muscles, and their 

 desire for general rather than special utility. 



The weight from the elbow to the foot is carried on the flexor 



