324 A Manual of Veterinary Physiology. 



and that the strain and wear and tear is considerable we 

 know from practical experience. 



The only joint I propose dealing with in detail is one the 

 action of which I believe to be universally misunderstood. 

 I refer to the perfect hinge-joint formed by the articulation 

 of the tibia with the astragalus. 



Hock Joint. — Solipeds appear to stand alone in having 

 the ridges of the astragalus obliquely placed, instead of 

 vertically as in ruminants. The ridges are oblique in the 

 horse, and some considerable difference in the action of the 

 limb is the result. It is usual to speak of the screw action 

 of the hock produced by the oblique ridges of the astrag- 

 alus. This screw action I believe to be an entire mis- 

 conception ; the ridges on the astragalus do act as a screw, 

 but not on the hock. The effect is on the stifle, and 

 produces that remarkable stifle action particularly well 

 seen in trotters. If the ridges on the astragalus turned 

 the hock outwards, every horse would travel as if he were 

 'cow-hocked.' I hold that the leg below the astragalus 

 is carried directly forwards ; when, however, it comes to 

 the ground, and the body passes over it, it is not un- 

 common in some horses to observe a considerable twist 

 outward* of the hock-joint, the toe being turned in ; this 

 is due to the ascent of the tibia on the astragalus turning 

 in the stifle, the result of the leg being fully extended. 



The cuneiform bones of the hock have a movement on 

 each other and the astragalus, which is always in the one 

 direction, viz., obliquely outwards. 



The object of the stifle being turned outwards during 

 the flexion of the leg is to clear the abdominal wall, and 

 the reason why solipeds have oblique ridges on the astrag- 

 alus, and ruminants vertical ones, is that the ribs of the 

 latter class are short, and do not come near the pelvis (as 

 in the horse), and therefore the abdominal wall is not in 

 the way. 



A spring or automatic flexion action in the hock has 

 been described, such as may be readily observed in the dead 

 leg, when if the hock be flexed slightly it cither flics back, 



