If these forces are too great the shaft mayfracture. Fractures usuallyheal spontaneously, albeit rather slowly in some cases, but the brokensurfaces need to be manipulated into the right place and may need to beheld with casts, pins or wires. Towards the ends of the shaft the marrow cavity tends to be widerand filled with trabecular bone, arranged along lines of force whichhas a skeletal function in its own right and supports the marrow. The ends of the bone are specialised to allow growth with as littleloss of strength as possible. The epiphysis permits this, but lookscomplicated.
Lets go back to the fish long bone and try to understandthe 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, floppycartilage becomes an embarrassment. The new radially arrangedcartilage is still strong but less bulky. These problems can beovercome by producing a secondary centre of ossification, already mentioned, - forming another lump of strong bone in the epiphysis. Thisis popular, occurring in at least seven groups of land vertebrates.