Should the periostitis be complicated by the formation of pus, then the vessels entering and supplying the bone are, in the suppurative area, destroyed. With their destruction it may happen that we get also death of a portion of the osseous tissue. This, however, when the suppuration is abundant, cannot commonly occur, as the bloodvessels within the bone--those of the medulla--commence to supply blood to the affected part. In cases of trouble with the bones of the foot, these last few remarks have a special significance. Here we have three bones whose medullary cavity is extremely small--almost nil, in fact--which explains in some measure how easy it is when suppuration exists to get necrosis and exfoliation of, say, portions of the os pedis. Necrosis and sloughing of the periosteum itself may also happen, but as the extreme vascularity of the membrane is a fairly strong safeguard against that it is of only rare occurrence. In connection with the deep layer of the periosteum, and forming part of it, are found numerous bone-forming cells (_osteoblasts_). These, under ordinary conditions, are relatively quiescent. Under the slightest irritation or stimulation, however, their bone-forming functions are stirred into abnormal activity, thus explaining how easy it is (especially with bones so open to receive slight injuries as are those of the foot) to get ossific deposits, the starting-point of which we are quite unable to account for. With this brief introduction we will now describe such pathological changes as occur in the separate structures, and which we are likely to encounter in the various diseases of the foot. While so doing, we shall draw attention to such diseases as we have previously described in which the pathological conditions we are considering may be met with. 1. PERIOSTITIS. This we shall consider under _(a)_ Simple Acute Periostitis, _(b)_ Suppurative Periostitis, _(c)_ Osteoplastic Periostitis. _(a) Simple Acute Periostitis_.--This is the periostitis that follows on the infliction of a slight injury to the membrane--an injury without an actual wound and free from infective material. It is one, therefore, which we always judge as existing in those cases where we have distinct evidence or history of injury, but in which the injury has not been severe enough to lead to fracture or to the infliction of an actual wound. Such cases may be those of lamenesses persisting after violent blows upon the foot--cases where the animal has been kicking against the stable fittings, or where the foot has been partially passed over by the wheel of a waggon. It may be, too, that in a case of 'nail-bound' a great deal of the pain and lameness is due to a simple periostitis caused by pressure of the bulged inner-layer of horn upon the sensitive structures.