Until the adult condition is reached and growth ceases, the pad of cartilage between the epiphysis and the shaft continues to grow, its outer (epiphysial) half growing by the formation of fresh cartilage as fast as its inner half is encroached on by the growth of bone from the shaft. The terminal or articular surfaces of the bone remain throughout life covered by layers of articular cartilage.
Even after the adult condition is reached the bone is subject to continual change, processes of absorption and fresh formation going on for a time and tending to render the bone more compact.
Methods in which bones are united to one another.
The various bones composing the endoskeleton are united to one another either by sutures or by movable joints.
When two bones are suturally united, their edges fit closely together and often interlock, being also bound together by the periosteum.
In many cases this sutural union passes into fusion or ankylosis, ossification extending completely from one bone to the other with the obliteration of the intervening suture. This feature is especially well marked in the cranium of most birds.
The various kinds of joints or articulations[6] may be subdivided into imperfect joints and perfect joints.
In imperfect joints, such as the intervertebral joints of mammals, the two contiguous surfaces are united by a mass of fibrous tissue which allows only a limited amount of motion.
In perfect joints the contiguous articular surfaces are covered with cartilage, and between them lies a synovial membrane which secretes a viscid lubricating fluid.
The amount of motion possible varies according to the nature of the articular surfaces; these include—