1. Congenital dislocation of the hip in identical twins is reported.

2. The heredity of congenital dislocation of hip is discussed. Studies in twins show that congenital dislocation of hip is probably a hereditary dysplasia of the acetabulum and upper end of the femur, and that external factors play a less important role.

1. In five series of experiments in eighty-two rabbits we succeeded in causing rarefaction of the calcaneum of all the animals soon after it was relieved from muscular compressing forces; new bone was generated when the calcaneum was subjected again to the stresses and strains of muscle contraction.

2. We found evidence that during muscle action pressure forces are transmitted through the bone, and that the presence or absence of these pressure forces conditions the balance between bone formation and bone removal.

3. In the calcaneum of the rabbit lack of muscular action seems to be the most important factor inducing osteoporosis. It is possible that the origin of post-traumatic osteoporosis has the same basis.

4. In our experiments bone rarefaction was characterised by a great increase in the vascularity of the bone; this increase ceased when the bone reached its final precarious bone density. Thus, vascular over-activity accompanied the removal of bone; but bone reconstruction was also seen to be accompanied by a more localised increase in vascularity.

5. From our experiments we cannot suggest that the inhibition of muscle contraction accompanying Sudeck's syndrome is responsible for this disorder, because we were unable in our animals to cause any of the other signs characteristic of Sudeck's bone atrophy. But the constancy with which we caused bone atrophy by the removal of muscle action may possibly help to explain the mechanism of bone absorption accompanying Sudeck's disease.