1. The cause of cyst formation lies in the major overloading of certain regions caused by the unevenness of the articular surface produced by the arthritic process.

2. Photoelasticity investigations disclose an overburdening of the bone beneath the surface of the joint, which in turn markedly resembles the shapes of actual cysts.

3. The results of this investigation fit organically into the logical sequence of processes in accordance with our theory.

4. This theory is further borne out by the successful formation of cyst phenomena in healthy joints by overloading (Trias 1961).

5. Our theory has no immediate practical bearing on the treatment of arthritic cysts. However, it uncovers certain interesting aspects of the behaviour of bone tissues under mechanical overloading above the limits of biological, though below those of mechanical bearing capacity. Conclusions arising therefrom might in many respects be useful in research into the physiology of bone.

1 . Twenty patients with fibrous dysplasia, confirmed histologically, are reported and discussed in regard to classification, etiology, pathogenesis and treatment. The various fibrous or fibrocystic lesions of bone are characterised briefly for purpose of contrast, and the position of fibrous dysplasia in this heterogeneous collection is suggested.

2. The classification of fibrous dysplasia based on the degree of skeletal involvement is used, and the diagnostic, therapeutic and prognostic implications of this classification emphasised. The authors endorse the opinion that fibrous dysplasia is a developmental defect. Clinical, histological and radiographic evidence is presented to point to the distinct evolution of the monostotic lesions, to which a positive and aggressive approach is recommended.

A minority of cases of osteopetrosis show in addition syndactyly and distal phalangeal anomalies. The seven cases that have been reported with this combination have had an unusual degree of density and thickening of the skull vault, face and mandible, and of the shafts of the metacarpals, metatarsals, proximal phalanges and clavicles. It is submitted that these features constitute a distinct morphological variety of osteopetrosis.

Two cases are described in which a traction lesion of the brachial plexus was complicated by sensory loss and anhidrosis in the second, third, and fourth cervical dermatomes. Both patients recovered spontaneously, though in one the recovery of muscle power in the limb was incomplete. It is believed that both were examples of a traction lesion of the cervical plexus. No similar case appears to have been recorded.