306 PARTICULAR LAMENESSES. 



surface of an astragalus, the observer will see that the screw- 

 like motion is subject to three deviations. First, the direction 

 is from within outwards for a short distance ; then it seems to 

 run perpendicularly ; and, finally, the lines of the prominences 

 are seen to curve towards the inner side and on to the inner 

 third of the cuneiform magnum, the inner prominence of the 

 astragalus terminating abruptly, and leaving a hollow space 

 for the end of the internal prominence (malleolus) of the tibia. 

 This abrupt termination allows the final act of flexion in the 

 hock to be performed with gTeat velocity and suddenness. One 

 can see it in a fast-going, well-actioned horse, giving sharpness 

 and grace to the hock action ; but whilst it allows this advantage, 

 it is apt to be a source of mischief, by acting directly upon the 

 cuneiform bones, and proving a source of concussion to them 

 during sharp flexion. This, in addition to the concussion 

 received when the foot comes to the ground, is sufficient to 

 cause inflammation of the cuneiform bones, and spavin. 



The relative obliquity and position of the true hock-joint, 

 and the direction of its motion with relation to the production 

 of disease of the lower rows of bones, are subjects well worthy 

 of consideration, and open a fair field of investigation to the 

 veterinary pathologist. Pertinent to this matter, one might 

 naturally ask, Why should there be three rudimentary screws 

 on the surface of the astragalus ? The explanation is based 

 upon the fact that the animal must be able to lock the joint 

 whilst asleep on its legs. It has been already explained that 

 the ligaments and the tendinous structure of the muscles of the 

 fore arm enable the fore legs to remain unbent during sleep. 

 In addition to tendinous muscles and ligaments, we find that 

 the posterior extremities (and they are much more liable to 

 bend under the animal's weight, as may be witnessed in cases 

 of debility) are furnished with a lock in the true hock-joint, — 

 the lock being the deviation of the screw lines, which enables 

 it to sleep without falling. The remarkable manner in which 

 flexion of the true hock-joint is performed has been remarked 

 by Dr. Graves of Dublin; and a valuable paper by Professor 

 Dick on the subject, and on the conformation of the knee and 

 elbow joints, may be consulted with advantage. It is printed 

 along with his memoirs. 



Amongst the variety of hocks that a veterinary surgeon 



