1. This paper describes the macroscopic and microscopic changes that are seen in posterior intervertebral joints after anterior vertebral fusion.

2. We now have a reasonably clear view of the types of change seen under these circumstances. The type varies from case to case and in different parts of the same specimen. So far we have no clear idea of the sequence or the pattern that leads from the normal to complete fibrosis or osseous ankylosis.

3. Further experimental work is needed in order to build up a clear concept of the sequence of events and of their relative importance. To do this it will be necessary to immobilise joints for longer than before.

1. Direct injury to skeletal muscle results in fragmentation and necrosis of muscle fibres, though this is patchy in distribution.

2. The sarcolemmal basement membranes form the interface along which fibre regeneration takes place.

3. Phagocytosis of disorganised sarcoplasm is an essential prelude to the reconstitution of severely damaged fibres.

4. Regeneration of injured muscle begins with proliferation of basophilic cells probably originating from muscle satellite cells. After a few days typical myoblast nuclear chains are present. By a week following injury the chains of myoblasts have formed myotubes, which possess myofibrils and sarcomeres.

5. By twelve days in the monkey and by eighteen days in man the muscle fibre regenerative process shows many new fibres which have not reached a mature diameter.

6. Much collagen may be formed in the tissue space at the site of injury. It appears that as the muscle fibres increase in diameter the collagen decreases in extent.

7. In the monkey by three weeks the muscle at the fracture site appears normal. This is also true in the specimens examined at four, six and twelve weeks.

8. In the monkeys the injured limb was immediately used to run and jump. A parallel intense and early activity of muscle and joints was a cardinal point in the management of this series of fracture patients. The clinical results were satisfactory.

9. It is concluded that in both the monkey and in man, given active limb movements, permanent and functionally useful muscle regeneration occurs following soft-tissue injury associated with a bone fracture.