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).

Treatment.—­In the ordinary way the ‘treatment’ of side-bone is a thing but rarely mentioned.  The explanation lies, of course, in the fact that side-bones are so rarely the cause of lameness.  When lameness does occur with a side-bone, and we have reason to believe that the said side-bone is the cause of the lameness, it is well before talking of treatment to question ourselves thus:  ‘In what way does the side-bone cause lameness?’ The now generally-accepted answer to that query is the explanation put forward several years ago by Colonel Fred Smith—­namely, that the pain, and therefore the lameness, was due to the compression of the sensitive laminae between the ossified and enlarged cartilage and the non-yielding and often contracted wall of the quarters.  That, in fact, constitutes the basis upon which Smith’s operation for side-bone (that of grooving the wall of the quarters) is founded.

Before describing the operation, however, we may say that we are now able to understand that older operators who claimed success for other methods of treatment, were to a very great extent justified in so doing.