LOCOMOTOR SYSTEM 



611 



trochlea? of the femur appear above the patella, the flexor metatarsi 

 is placed in operation and flexes the hock, and as the stifle becomes 

 more and more flexed, the bulk of the whole joint increases, 

 and is turned outwards from the abdomen, in order to accom- 

 modate its increase in size. This turning outwards of the stifle, 

 a feature seen in any pace, but particularly well marked in the 

 trotting horse, is not due to any mechanism in the stifle, but to 

 the hock-joint. It will be shown presently that the ridges on 

 the astragalus, through their action 

 on the tibia, turn the stifle out- 

 wards. 



The Hock- Joint is not only of 

 physiological but of the greatest 

 clinical interest, owing to its being 

 the frequent seat of lameness. It 

 is a hinge-joint, and a hinge in 

 such a position on the limb must 

 be of great strength to resist the 

 tendency to close under the in- 

 fluence of the body weight. In 

 fact, the hock-joint, unlike the 

 knee, to which it corresponds, is 

 under the constant stress of ex- 

 tension, which is the reason why, 

 in spite of the automatic ap- 

 pliances with which it is furnished 

 for keeping it open, the joint is 

 always being alternately closed for 

 the purpose of rest. The long 

 lever at the back of the joint, F,G - I97 - Fle £° N b.° F the H "" > - 

 in the form of the calcis, is a 



lever of the first order ; it is the lever of power, and the 

 strain on these muscles will be realised when it is remembered 

 that the whole weight of the hind-quarters is supported by 

 the gastrocnemius and flexor perforatus through their inser- 

 tion into the calcis. The strain is largely removed by the in- 

 troduction of a tendo-ligament — viz., a cord running from end 

 to end within the muscle, and forming its two attachments, 

 by which means it is capable of relieving much of the strain 

 from the muscle. The existence of two such cords in the hock 

 is described at p. 596 (see Fig. 191, 2, j). 



The two movements of the hock are flexion and extension ; 

 the extension, which occurs during the attitude of rest, is dealt 

 with at p. 601. The extension of locomotion is carried on by 

 the same mechanism — viz., the gastrocnemius muscle, and the 

 tarsal tendon running from the biceps femoris muscle (biceps 



