Small cavities and notches in an otherwise perfect material will greatly reduce the strength of the material, particularly its tensile strength. They will do this by producing stress concentrations around themselves. It is shown that bone is permeated with such stress-concentrators in the form of canaliculi, osteocyte lacunae, blood-vessels, and muscle insertions. However, in mammalian bone the orientation of the lacunae and blood-vessels is such as to reduce, though not abolish, the concentrating powers of these elements in relation to the dangerous stresses acting in the bone. The smoothness of the diaphysis shaft and the raising of muscle insertions on to cristae will have similar advantageous effects. The larger the defect, the greater the tendency to be orientated favourably.

The reasons for the presence of osteocytes and their associated blood-supply are not known, and a comparison with the acellular bone of some teleosts is not helpful. It is suggested that the lacunae and blood-channels in bone may to some extent increase the robustness of bone by delaying the spread of minute cracks, which must inevitably be present in bone, though they are difficult to demonstrate. The orientation of the lacunae is favourable for this effect, and they are sufficiently close-packed in the bone to ensure that any minute crack will run up against many lacunae before it has travelled ioo fj.. An examination of artificially produced cracks in cattle and human bone shows that the cracks do stop very frequently on entering lacunae and blood-vessels. The lacunae are more likely to prevent the spread of fatigue cracks than of impact cracks. The crack, running up against a lacuna, could be a stimulus for local reconstruction, so that it would be repaired.