_Differential Diagnosis_.--Navicular disease may be mistaken for ordinary contracted foot. It will be remembered, however, that in the early stages of navicular disease contraction is absent, and that it is only when the disease in the bursa is of long standing that contraction comes on. With ordinary contracted foot, too, careful paring and suitable shoeing soon sees a diminution in the degree of lameness, and a return to the normal in shape (see Treatment of Contracted Foot, p. 125). With navicular disease, however, such shoeing as is beneficial in the treatment of contracted foot (notably the various methods of giving to the frog counter-pressure with ground) soon brings on an aggravation of the lameness. It is, perhaps, even more likely to be confounded with contraction when we have with the contraction a state of atrophy and thrush of the frog. With a frog in this condition pressure will give rise to pain, and navicular disease be erroneously judged to be present. In such a case we must rely wholly upon either extreme flexion or extreme extension of the joint to guide us, when, if contraction _only_ is the offending condition, no symptom of pain will be shown. Navicular disease may also be confused with rheumatic affections, with sprain of the posterior ligaments of the first interphalangeal articulation, and with sesamoid lameness. Mistakes are sometimes made, too, especially with a hasty observer, in confounding it with shoulder lameness. In rheumatism the constant changing of the seat of pain, the sometimes elevated temperature, and the appearance of symptoms of heat, tenderness, and swelling in the affected area should guide one to a right conclusion. In sprain of the posterior ligaments of the coronet and in sesamoid lameness, nothing but a careful examination and manipulation of the parts will ward off error, for in each of these cases there is 'pointing' and resting of the limb, and considerable disinclination to put weight firmly upon it. If at the same time manipulation gives distinct evidence of pain, all doubt may be set at rest.