Final answer:
Bone growth in diameter, known as appositional growth, occurs through the actions of osteoblasts adding new tissue beneath the periosteum and osteoclasts resorbing old bone in the medullary cavity. This process continues even after longitudinal growth has stopped, allowing bones to thicken in response to stimuli like increased muscle activity.
Step-by-step explanation:
Appositional Growth and Bone Diameter
The increase in bone diameter is known as appositional growth. This process involves the activity of osteoblasts and osteoclasts. Osteoblasts lay down new bone tissue beneath the periosteum, leading to an increase in the diameter of the diaphysis, while osteoclasts resorb old bone from the medullary cavity, also contributing to the bone's thickening. The modeling of bones, therefore, not only increases the diameter but also maintains the balance by enlarging the medullary cavity to prevent the bones from becoming too heavy.
Bone growth in diameter occurs even after the completion of longitudinal bone growth, which finalizes when the epiphyseal plates turn into epiphyseal lines. Notably, the capacity of bones to thicken can still persist in adulthood, especially in response to certain stimuli such as increased muscle activity or changes in body weight.