LOCOMOTOR SYSTEM 587 



extensor of the hip, and to secure this the latter joint is fixed by 

 a contraction of its flexor muscles. In such cases the nervous 

 mechanism is identical, not reciprocal, for the flexors of the hip 

 contract in order to prevent extension of this joint at the same 

 moment as the semitendinosus contracts and flexes the stifle. 



The condition revealed by this experiment is that flexor and 

 extensor muscles, true antagonists, may contract together, 

 co-operate, in order to fix a joint, so that another joint may be 

 more effectively acted upon. Similarly, two extensor muscles 

 may be actively employed some distance from each other in 

 fixing the joint of a limb ; they are not true antagonists, they are 

 both extensors, and they act at the same moment : they are 

 spoken of as ' partial antagonists.' The best example of this is 

 afforded by the gastrocnemius muscle, which always acts as an 

 extensor, whether during standing or in active movements, yet 

 its origin from the femur is such that it might flex the stifle. It 

 never flexes the stifle, for the reason that the muscles of the 

 patella keep the stifle extended at the time the gastrocnemius 

 is acting by keeping the hock extended. The patella and gastroc- 

 nemius muscles are partial antagonists : they contract together and 

 relax together, and by so doing ensure that one joint is kept fixed 

 in order that a muscle crossing it may act better on another joint. 

 The innervation such muscles receive is identical (see p. 456). 



When muscles have a muscular origin and a tendinous inser- 

 tion, the direction in which their contraction occurs is from 

 muscle to tendon ; but when muscles are fleshy throughout, 

 such as the long muscles found in the dorsal region, they may 

 act from either end — for instance, in the group referred to they 

 act from the neck end in kicking, and the croup end in rearing 

 (Figs 214, 215). 



Tendons and Ligaments. — The chief function of ligaments is 

 to hold bones in position. The greater the necessity for joint 

 freedom, the fewer and simpler the ligaments. The shoulder- 

 joint, for instance, has no special lateral ligaments. The range 

 of motion is too considerable for these, so that the tendinous 

 insertion of the muscles takes their place. There is very much 

 less motion in the hip, and the ligaments are centrally placed. 

 With the other joints they are laterally situated, but from knee 

 and hock to the foot a ligament seems to have been put in 

 wherever space not otherwise occupied existed. Ligaments are 

 also employed for holding floating bones like the patella and 

 sesamoid in position. They knit together the various segments 

 of the spine, and one of peculiar structure supports the head 

 and neck. Ligaments may also run to tendons to enable the 

 muscle to be partly cut off during the standing attitude. Two 

 to the flexors of the lower limb Of the horse undoubtedly assist 



