258 THE EXTERIOR OF THE HORSE. 



He must stand in such a manner that each of the four members sup- 

 ports its own share of the body-'^eight and lies in its natural axis. 

 If this little detail is neglected, the eye will be deceived as to the true 

 dimensions of the region and cannot compare the one side with the 

 other, as we shall see in a future chapter. 



Movements. — The hock is the seat of only two extensive move- 

 ments, that of extension and that of flexion. The plane of these move- 

 ments, on account of the obliquity of the astragaloid trochlea, is 

 slightly oblique outward, a disposition which, coinciding with a certain 

 obliquity of the femur, permits the member to be extended without" 

 being restrained by its contact with the abdominal parietes. 



There are, besides these principal movements, others, very limited, 

 consisting of simple gliding of the contiguous bones, whose eifect is to 

 attenuate the locomotory reactions, and which are, in the case of the 

 hock, an apparatus of dispersion analogous to that of the knee. 



When leaving the state of repose, the foot is always more abducted 

 in flexion than in extension, and both are executed with the most per- 

 fect regularity. When, however, the articulations of the member are 

 diseased, whether those of the leg or those of the hock, as deduced 

 from the observations of Rigot and from our own, flexion is sudden, 

 jerking, exaggerated, and sometimes so extensive that the anterior face 

 of the fetlock almost touches the parietes of the abdomen. This phe-' 

 nomenon constitutes string-halt or dry spavin; but the hock offers no 

 trace of external deformity. We will again revert to this on the 

 subject of the gaits. 



Finally, when, at the moment of contact, the points of the calcanei 

 are turned outward by a sort of rotation, of which the foot is the cen- 

 tre, the horse is said to have rotating hocks. (See Gaits.) 



The Hock as a Centre of Amortissement and Impul- 

 sion. — This region is one of the most energetic centres of impul- 

 sion in the posterior member. It is by its aid that the tibio-tarsal 

 angle can suddenly open in order to project the body forward at the 

 end of the phase of contact. 



The quantity of movement of the body, after being first dissemi- 

 nated upon the bones and the ligaments of the coxo-femoral articulation, 

 and secondly upon those of the femoro-tibial union, is then transmitted 

 to the tibio-tarsal articulation, where, a certain portion having already 

 been destroyed, it is again dispersed upon the tarsal bones and their 

 ligaments. The combined action of the weight of the body and its 

 velocity results in a diminution of the tibio-tarsal angle as well as of 

 the other angles in the superior part of the member. In the hock^ as 



