OSSIFIC DISEASE OF JOlJSTiJ. 51 



a sliming hardness and soliditv of" slructure remarkably different 

 from what is seen in caries or ulceration ; and from this circum- 

 stance has it acquired the denomiriation of porcellaineous de- 

 posit. For my own part, however, I do not look upon it as any 

 " deposit" at all; rather, that there \\diS heew ahsorption: at the 

 same time, no doubt, there has been a transmutation of the sub- 

 stance remaining into firmer and harder material. 



Were we asked what occasioned this absorption, we should 

 answer, pressure combined with friction ; and add, that the hock 

 joint is the especial seat of these excavations, owing to the cir- 

 cumstance of its being the joint most of all exerted in progression. 

 We look upon their presence as fortuitous ; for they are not to be 

 seen in all hock joints, and in some that are bone-spavined, even, 

 are absent : nor can we^ for our own part, see any use they serve, 

 or any inconvenience they give, by their presence. 



Ossific Disease of Joints. 



The epiphyses of bones, entering into the formation of joints, 

 are frequently found encrusted with adventitious deposits of osse- 

 ous matter, much augmenting their volume, and altogether, in 

 many cases, altering their original shape ; and these crustace- 

 ous bony deposits have their origin, commonly, in inflammation, 

 chronic or acute, of the periosteal coverings of the epiphyses : we 

 say " commonly," because it would appear that such changes of 

 structure, and additions to it, do now and then take place without 

 any precursory or accompanying inflammatory action, at least with- 

 out any that is discoverable through outward signs. There is a 

 very remarkable propensity in the horse's economy to what is called 

 ossific action : an injury of any kind — a blow or kick, contusion or 

 wound — to the periosteum, exciting inflammation in it, is almost 

 certain to be followed by exostosis. Inflammation — or even in- 

 creased arterial action — excited in the same tissue by other 

 causes, will give rise to similar conversion of it into bone. The 

 same observation will apply to the elastic fibrous tissue uniting the 

 small supplementary bones of the limbs to the shafts of the large 

 straight bones with which they are articulated, the union being 



