1. Bones consist essentially of bundles of collagenous fibres united by a cementing substance in which the inorganic material lies in the form of minute plate-like crystals.

2. During weight bearing and muscle action bones as a whole are deformed to a variable extent. Periods of deformation are followed by periods of relaxed pressure during which the bones tend to return to their normal form.

3. These variations in deformation and elastic recoil set up alternating pressures and tensions within the bones along the bone cyrstal encrusted fibres which make up the trabeculae, lamellae and Haversian systems, and these alternating phases of compression and tension stimulate the activity of osteoblasts so that bone formation predominates over bone resorption.

4. These alterations of pressure and tension are intermittent and reciprocal in nature and do not, as postulated by the trajectorial theory, involve different trabeculae, nor is it necessary to consider whether tension or pressure is the more important phase in determining bone deposition.

5. The pressure exerted by cysts, tumours, erupting teeth, etc., is of a quite different nature, as is the response to trauma or callus formation in the healing of fractures. These processes are essentially vascular phenomena involving localised areas of bony tissue and not bones as mechanical units.