The whole of the anterior and fully one-half of the posterior aspect of the neck of the femur is covered by synovial membrane, so that lesions not only of the epiphysis and epiphysial junction, but also of the neck of the bone, are capable of spreading directly to the synovial membrane and to the cavity of the joint. Conversely, disease in the synovial membrane may spread to the bone in relation to it. Infective material may escape from the joint into the surrounding tissues through any weak point in the capsule, particularly through the bursa which intervenes between the capsule and the ilio-psoas, and which in one out of every ten subjects communicates with the joint. TUBERCULOUS DISEASE Tuberculous disease of the hip, morbus coxæ, or "hip-joint disease," is especially common in the poorer classes. It is a frequent cause of prolonged invalidism, and of permanent deformity, and is attended with a considerable mortality. It is essentially a disease of early life, rarely commencing after puberty, and almost never after maturity. #Pathological Anatomy.#--Bone lesions bulk more largely in hip disease than they do in disease of other joints--five cases originating in bone to one in synovial membrane being the usual estimate. The upper end of the femur and the acetabulum are affected with about equal frequency. In addition to primary tuberculous lesions, secondary changes result from the inflamed and softened bones pressing against one another subsequent to the destruction of their articular cartilages. The head of the femur undergoes absorption from above downwards, becoming flattened and truncated, or disappearing altogether. In the acetabulum the absorption takes place in an upward and backward direction, whereby the socket becomes enlarged and elongated towards the dorsum ilii. To this progressive enlargement of the socket Volkmann gave the suggestive name of "wandering acetabulum" (Fig. 108). The displacement of the femur resulting from these secondary changes is one of the causes of real shortening of the limb. [Illustration: FIG. 108.--Advanced Tuberculous Disease of Acetabulum with caries and perforation into pelvis. (Anatomical Museum, University of Edinburgh.)] #Clinical Features.#--It is customary to describe three stages in the progress of hip disease, but this is arbitrary and only adopted for convenience of description.