Fracture of the navicular bone, if in any way diagnosed with certainty, offers us an almost hopeless case, for it appears to be a commonly reported fact that attempts at reunion are rare. This, in all probability, is due to the pressure put upon it every now and again, when the animal's weight presses the bone between the os coronæ and the os pedis above and the perforans tendon below. Even should reunion take place, the resulting callus, interfering as it does with the movements of the perforans, leaves us a case of incurable lameness. When the fracture is complicated by the formation of pus, as in the case of prick, then the case, with the attendant purulent synovitis and arthritis, is even more hopeless still. Diagnosis of fracture of either of the bones of the foot is, as we have said before, extremely difficult. It so happens, therefore, in those cases caused by violent blows, that anything approaching an accurate opinion cannot be given until some months after the injury. After some time we are met with unmistakable changes in the form of the foot, and are able to assume that the persisting lameness is due to pressure of a reparative callus within the hoof. In such cases the only treatment of any use is that of neurectomy. CHAPTER XII DISEASES OF THE JOINTS[A] [Footnote A: Properly speaking, we have in the foot of the horse but _one_ joint--namely, the corono-pedal articulation. Although not a joint in the strict sense of the word, we, nevertheless, intend here to consider the navicular bursa as such. In this apparatus, although we have no articular cartilage proper, and no apposition of bone to bone, we still have a large synovial cavity, and in close proximity to it bone. We may, in fact, and do get in it exactly similar changes to those termed 'synovitis' and 'arthritis' elsewhere. Therefore, we include the changes occurring in it in this chapter, and hence the plural use of the word to which this note refers.] A. SYNOVITIS. _Definition_.--By the term 'synovitis' is indicated an inflammation of the synovial membrane. It may be either (_a_) _Simple_ or _Acute_, or it may be (_b_) _Purulent_ or _Suppurative_. In the simple form there is little or no tendency for the affection to implicate the other structures of the joint, whereas in the suppurative form the joint capsule, the ligaments, and the bones soon come to participate in the diseased processes, giving us a condition which we shall afterwards describe as acute arthritis. (_a_) SIMPLE SYNOVITIS. 1. _Acute--(Causes)_.--Simple or acute synovitis is nearly always brought about by injury to the joint--by blows or bruises, or by sprains of the ligaments. At other times it occurs without ascertainable cause, and is then put down to the influence of cold, or to poisonous materials (as, for example, that of rheumatism) circulating in the blood-stream.