Law of ossification:
This law states that," The ossification center that appears first is the last to unite with the diaphysis".
As we all know that most of the long bones in our body are ossified in cartilagenous tissue (some are also ossified in intramembranous tissue i.e.clavicle). There are primary and secondary centers of ossification . Primary centers appears first mostly in prenatal life and form the diaphysis in long bones. Diaphysis forms the shaft of most long bones in our body.Then there are secondary centers which appear later on, mostly in the postnatal life and form the epiphysis of long bones and these later on fuse with the diaphysis.
Now according to the law of ossification the secondary ossifying center in the epiphysis of long bones that start ossifying first is the last to unite with the diaphysis. In other words we can say that the secondary ossifying centers which appear first, they become last to unite with the diaphysis. In the same way, the secondary ossifying centers which appear later on, they unite first to the diaphysis.
Here I would like to add some more points about ossification.
- If the epiphysis of a bone is larger, its ossification center will appear earlier.
- If the epiphysis of a bone consists of more than one ossification centers, all these centers will coalesce (join ) with one another before fusing with the diaphysis.
- The growth of the long bones in our body ceases after the fusion of epiphysis with diaphysis occurs.
All the long bones in our body ( except fibula) follow the law of ossification. For instance, femur, humerus, tibia, radius and ulna are the long bones that follow the law of ossification.
Exception / Violation of law of ossification:
Fibula is the only long bone which violates the law of ossification. The reason behind this is that the distal end of fibula which appears first doesn't fuses last as the law of ossification describes. It unites with the diaphysis before the fusion of proximal end which appears later on.