Causes. - To commence with, we may remark that, although met with sometimes in very early life, side-bones are seldom, if ever, congenital, and that more often than not they may be looked for in animals of three years old, or older, seldom earlier. They appear, in fact, only when the animal is shod and commences work.
This at once suggests two of the principal factors in their causation - namely, concussion and loss of normal function. Directly the horse is put to work he has for a great part of his time to travel upon roadways - either macadamized roads or town sets - where everything is calculated to bring concussion about. In addition to that he has the lateral cartilage itself thrown largely out of action by shoeing. We explained in Chapter III (General Physiological And Anatomical Observations). (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.
Side-bones, therefore, are hereditary. We think, however, the statement needs qualifying. It is in this way: side-bones occur only at a certain, usually well-defined, time after birth, and we might say are never congenital. They occur only after the animal has been put to work, and are more or less plainly due to mechanical causes - namely, the ill effects of shoeing and concussion. The cause of their appearance, in short, is more plainly extrinsic than intrinsic, and side-bone in the horse is, as Professor McCall puts it, about as much due to heredity as is corn on the human foot.
Between these two opinions - that they are plainly hereditary, and that they just as plainly are not - it is well to strike a middle course. They are, we will say, hereditary in this way: So long as a cart animal is bred, to put it vulgarly, 'top-heavy' (that is, with a body out of reasonable proportion to the feet that have it to support), so long will the foot be subjected to a greater concussion, and so long will side-bones in such animals commence to make their appearance at about middle life.
In addition to the causes we have now mentioned, side-bones are often the result of other diseases of the foot. They thus occur as a sequel to sub-horny quittor, to suppurating corn, to complicated quarter sand-crack, or to the inflammation of the parts occasioned by a prick. They also arise in many instances from the effect of a prick or injury to the coronet. Among the latter we may mention treads from other animals, and treads inflicted by the animal himself with the calkin of an opposite shoe, or the repeated injury occasioned by the shafts being carelessly allowed to drop on to the foot. In severe cases of laminitis, too, the cartilages are nearly always affected. In this instance the inflammatory phenomena in the os pedis no doubt give rise to an abnormal activity of bone-forming cells. The cartilage is invaded, and the side-bone formed (see Fig. 118).