With regard to alteration in gait, we may say at once that side-bones in heavy animals are not often the cause of lameness. In fact, where the foot is well developed, when neither the foot as a whole nor the phalangeal bones give evidence of disease, and where the pasterns are fairly oblique and well formed, this alteration of the cartilages may be looked upon as of no serious import at all. Neither is the side-bone due to blows or other injuries likely to be productive of lameness--that is, always supposing, of course, that the foot in other respects is of good shape. If lameness is met with at all, then it is where we have a foot that is in other respects unsound, with badly contracted heels and upright 'stumpy' hoof, or where side-bones have occurred in a young animal, and have already reached a large size before the horse is put to labour. In this latter case, the added effects of concussion and the evil influences of shoeing are sufficient to turn the scale. Directly the animal, previously sound, is asked to work, lameness is the result. It follows, therefore, that side-bone in the feet of young animals is of far more serious import than when occurring in older horses. In a nag animal they constitute a positive unsoundness, and lameness in this case is more often than not an accompanying symptom. _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. (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.