gely out of action by shoeing. We
explained 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 explanati