Final answer:
Bone thickness increases through a process known as appositional growth, which involves osteoblasts adding new bone tissue and osteoclasts removing old bone.
Step-by-step explanation:
Bone can grow in thickness only by a process called appositional growth. This biological process involves the addition of bony tissue at the surface of bones. Osteoblasts on the outer surface secrete new bone matrix, while osteoclasts on the inner surface break down old bone. As osteoblasts secrete bone matrix, they differentiate into osteocytes, and the balance between their activity and that of the osteoclasts allows for an increase in bone thickness without the bone becoming excessively heavy. This modeling ensures that the diameter of the medullary cavity is also increased, thus maintaining the structural integrity of the bone. Appositional growth can continue even after longitudinal growth has ceased, often occurring in response to increased muscle activity or body weight changes.