Each bone is developed in cartilage and ossified from three centres, one for the shaft and one for either end : the upper epiphysis is the " growing " one in both bones.

The centres in the tibia precede those in the fibula. Tibia :-Shaft, seventh week ;

Upper end, just after birth ; Lower end, second year. Fibula :-Shaft, eighth week ;

Upper end, third year ; Lower end, second year.

The lower ends fuse with the shaft a few years after puberty in the tibia and about twenty-one in the.fibula. The proximal end is fused before twenty-four: in the tibia occasionally as early as nineteen or twenty.

The upper epiphysis in the tibia includes (Fig. 131) the upper half of the tubercle : in this there is occasionally an additional centre, appearing about twelve or thirteen and joining the epiphysis as a rule.