DISEASES OF THE LATEEAL CAETILAGES 369 



lateral cartilage persisting as cartilage is due in large 

 measure to its constant movement. Directly, therefore, 

 it is placed in a state of comparative idleness, then it com- 

 mences to ossify, more particularly if there should at the 

 same time be a tendency to a low type of inflammation of 

 the parts. 



Does this latter exist ? We may safel}' say that it does. 

 It is in this way : The secondary effect of loss of ground- 

 pressure upon the frog and plantar cushion is to bring 

 about contraction of the heels. With this we get compres- 

 sion of the parts within, with a certain amount of irritation 

 and the exact low type of inflammatory phenomena calcu- 

 lated to assist in the bone-forming process. 



The fact that concussion acts as a cause explains in great 

 measure how it is that side-bones are more frequent in cart 

 animals than in nags, and also why they should be more 

 common in the fore-feet than in the hind. Taking, in both 

 animals, a rough calculation as to the weight of body 

 carried by feet of a certain size, we notice at once that the 

 cart animal has proportionately more weight to carry than 

 has the nag. Concussion to the foot is therefore greater. 

 The greater part of the body-weight is borne by the fore- 

 limbs. Concussion is therefore greater to the fore-feet than 

 to the hind. 



This, however, does not explain altogether the compara- 

 tive immunity of the nag animal from this defect. He, too, 

 must also be subject to the effects of concussion, especially 

 when his higher and faster action is taken into account. 

 To our minds there is only one explanation to be offered 

 here. We point at once to the years of constant and 

 judicious breeding of the nag. Compare that with the 

 relatively few minutes that have been devoted to a more 

 careful selection of the cart animal, and we at once see a 

 possible explanation. That the explanation holds some 

 amount of truth is borne out by the fact that, since a greater 

 attention has been paid to the selection of our cart animals, 

 side-bone has grown a great deal less common. 



Is side-bone hereditary ? We can best answer that by 



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