146 NAVICULARTHRITIS. 



but in the hock joint. Articular spavin to all intents and purposes 

 consists in the same pathological lesion as does navicularthritis : 

 a fact that will serve to cast additional light on the etiology of both 

 diseases. Still further light is derived from the superadded fact of 

 the knde joint being occasionally affected with the same disease. 

 In fine, there is no joint of the limbs, nor hardly any synovial 

 structure in them, but what is liable to acquire, under fitting 

 circumstances, a like disease. 



It will not appear strange that the navicular joints of the hind 

 limbs should exhibit no such disease as so frequently invades the 

 same joints in the fore limbs, when we come to consider the differ- 

 ence of function, in progression, performed by the fore and hind 

 extremities. While the former are little more than props of sup- 

 port, and for that reason have their bones ranged in the form of 

 upright columns, the latter have their shafts obliquely placed, 

 thereby constituting, one with the other, so many obtuse angles, to 

 the end that by forming powerful levers, and affording every ad- 

 vantage for action to the muscles attached to them, they may be 

 fitted for the grand purpose of propulsion of the body onward. 

 Any injury sustained in action by the upright column — the fore 

 limb — will originate in jar or concussion, aggravated by the moving 

 weight superimposed upon it; whereas, any injury that may accrue 

 to the hind limb will arise from the stress imposed upon the seve- 

 ral levers and angles at the moment progression is being effectu- 

 ated, the principal axis of which movement being the hock joint, 

 that, as might be expected, will be the part to feel any inordinate 

 pressure or force of action. The navicular joint fails in the fore 

 limb, then, simply from the circumstance of being the nethermost 

 joint of the column — the last to receive the shock from above, the 

 first from below; and the hock in the hind limb is the joint expected 

 to fail, because it is not only situated so as to receive the brunt of 

 the shock, which in the fore limb descends down the column, but 

 has likewise to sustain the weight of the body and its burthen, 

 at the time force is employed in their impulsion onward, even 

 while in state of motion. Joints appear to sustain more harm 

 from shock or concussion, caused either by imposing great weight 

 upon them while in action or by high or sudden descent of move- 



