If these forces are too great the shaft may fracture. Fractures usually heal spontaneously, albeit rather slowly in some cases, but the broken surfaces need to be manipulated into the right place and may need to be held with casts, pins or wires. Towards the ends of the shaft the marrow cavity tends to be wider and filled with trabecular bone, arranged along lines of force which has a skeletal function in its own right and supports the marrow. The ends of the bone are specialised to allow growth with as little loss of strength as possible. The epiphysis permits this, but looks complicated.
Lets go back to the fish long bone and try to understand the logic underlying the structure.
The fish accomplishes this by ending the bone with a simple plug of cartilage. This works until the bone has to bear weight, when the large, floppy cartilage becomes an embarrassment. The new radially arranged cartilage is still strong but less bulky. These problems can be overcome by producing a secondary centre of ossification, already mentioned, - forming another lump of strong bone in the epiphysis. This is popular, occurring in at least seven groups of land vertebrates.