in Chapter III. (p. 66) that the chief function of the cartilage was to take concussion received by the plantar cushion and direct the greater part of it outwards and backwards.  Now, with the animal shod, the plantar cushion does not itself, as normally it should, receive concussion.  By the shoeing the frog is lifted from the ground, and the plantar cushion, together with the cartilage, taken largely out of active work.  In other words, the normal outward and inward movements of the cartilage are enormously reduced.

It is fair, we think, to take it that the mere fact of the 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 commences 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 safely 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 compression of the parts within, with a certain amount of irritation and the exact low type of inflammatory phenomena calculated 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 comparative 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 saying that, some several years ago, the Council of the Royal College of Veterinary Surgeons, at the request of the Royal Commission on Horse Breeding, drew up a list of those diseases ’which by heredity rendered stallions so affected unfit as breeding sires,’ and that in that list was included side-bone.