The description of long bones is correct.

A, osteoblasts in periosteum of bone surface have regenerative function. It can also continuously produce new osteoblasts, making the bone after fracture longer and thicker. A is correct;

B, long bone, long tube, there is a cavity in the spine to accommodate bone marrow. When I was young, the bone marrow cavity was red bone marrow, which had hematopoietic function. B is right.

Long bones are a kind of human bones. According to the different shapes of bones, bones are often divided into long bones, short bones, flat bones and irregular bones. Long bones are mainly distributed in limbs, and upper limbs include humerus, ulna, radius, metacarpal bone and phalanx. The lower limbs are femur, tibia, fibula, metatarsal and phalange. Long bones have two complete ends, called bone ends, with cortical bone on the surface and cancellous bone inside. Between the two ends is a hollow vertebra, and the vertebra is a hard cortical bone. The cancellous bone is rich in blood supply and has strong fracture healing ability after fracture, while the cortical bone has relatively poor blood supply, poor fracture healing ability and long healing time.

Long bones are generally tubular and divided into two ends (epiphysis), that is, the middle part is thin (to provide space for the muscle abdomen) and the two ends are thick (to form a larger articular surface). In addition, because it is tubular, it is usually hollow and has a bone marrow cavity.

The bones of limbs, except the wrist, tarsus and bone (clavicle is generally considered as a special long bone because it has no bone marrow cavity), are all long bones.

Trunk bone and skull have no long bones.