Development

The bones of living people, like their other tissues, are very much alive. The living cells in bones, and the food and oxygen brought to bones in the blood, enable children’s bones to grow and adults’ bones to maintain themselves. Occasionally something goes seriously wrong, but usually the life processes work well.

Skull
The skull is formed from many separate bones. Most of these bones remain separate throughout life, but the two bones in a baby’s forehead almost always unite to form a single bone in adults. Where the bones meet, they are connected by interlocking sutures, like exceedingly complicated pieces from a jigsaw puzzle. Collagen fibers hold the sutures together, attaching each bone very firmly to its neighbours. Collagen is the protein that also forms the deeper layers of the kin, the membranes that wrap many of the organs of the body and the tendons that connect muscles to bones.

The human brain is remarkably large, in comparison with the brains of animals. Its mass averages 1.4kg in men, and 1.3kg in women — the difference is simply due to men having bigger bodies than women.
The brain occupies most of the space inside the skull, but it is not a tight fit. The bones of the skull roof have grooves on their inner surfaces. These are the paths of some of the blood vessels that serve the brain and its membranes. A good blood supply is needed to bring oxygen and foodstuffs to the brain because it uses a remarkable amount of energy. The brain makes up only 2% of the mass of the body, but it accounts for 20% of the energy consumption of the resting body.

Teeth
Unlike their bones, young children’s teeth do not grow to adult size. Just as we discard our clothes and shoes as we grow, replacing them with larger sizes, we discard and replace our first teeth. Unlike clothes, we replace teeth only once. Adult teeth are larger than the milk teeth of young children, but not sufficiently larger to fill the mouth of a full-grown adult if there were no additional teeth. As well as the 20 teeth that are direct replacements for milk teeth, adults have 12 additional teeth at the back of the mouth: three molars on each side of each jaw.

The milk teeth erupt more or less in order from the front backwards, in the first 2.5 years of life. The canines commonly appear out of order, after the teeth immediately behind them. The permanent teeth start appearing at age 6 or 7 years, starting with the first incisors and the first molars. The last teeth to erupt are the third molars or wisdom teeth. They do not appear until the young adult has stopped growing, usually between 17 and 21 years. We do not have a full set of 32 teeth in place until we have a full-sized jaw to hold them.

Apart from the difference in size, the skeletons of new-born babies are very different from those of adults. They have cartilage (gristle) in many places where adults have bone. There is only cartilage in the wrist where adults have many small carpal bones. They are tiny pieces of bone that will become epiphyses at the lower end of the femur and the upper end of the tibia, but all the other long bones end in lumps of cartilage. Many more bony epiphyses appear during the first year of life, but the set is not complete until an age of about five years.

Baby Bones
Babies’ skulls grow very rapidly, both before and after birth. At birth, the head is one quarter of the total length of the body, whereas in adults it is only one eighth of stature. It has to be disproportionately big to hold the brain, which makes up 10% of the weight of the baby compared to only 2% in the adult. With an average mass of 350g, the baby’s brian is already one quarter of adult size. Though it starts big, the brain grows fast and reaches 90% of adult size as in adults, but the rest of the face and the jaws are relatively much smaller.

Newborn babies have thin skull bones. The bones do not meet at the top of the head, but are joined by a strong leathery membrane. This structure allows fast growth, but has its dangers. The skull roof is flexible, and easily distorted by anything that presses against it. Even lying always in the same position may alter the shape of the skull. Pressure on the sides of the skull may make it abnormally long and narrow, the condition known as scaphocephaly. Pressure on one side of the back of the skull may make it asymmetrical, the condition of plagiocephaly. Happily, the flexibility that allows the skull to distort also makes correction possible. A suitably shaped head band or helmet will usually mould the growing skull to a normal shape within a few months. Scaphocephaly and plagiocephaly can, however, result from some of the bones meeting and suturing together too soon, making it difficult for the growing skull to expand in some direction. In these cases, surgery may be needed. In normal growth, the gaps between the bones are not fully closed until the child is about a year old.

The growth of babies’ skulls seems to be driven by the growth of the brain inside. In the rare disorder known as microcephaly, the brain fails to grow as it should. Though the face grows to normal size, the rest of the skull does not. Common results include mental retardation and early death. In contrast, in hydrocephaly far too much fluid accumulates in the cavities of the brain. The brain swells, and the skull grows with it. This condition can be treated by surgery designed to let the excess fluid drain away. Both microcephaly and hydrocephaly are often associated with abnormalities of the chromosomes.

Jaw Bones
Babies have two lower jaw bones, one on each side, joined by collagen fibres at the chin. The two bones fuse together within three years, so older children and adults have just one lower jawbone. Rounded knobs (called condyles) at the back of the lower jaw fit into hollows in the underside of the skull to form the jaw joints. Looking at the bones you might think that the joints worked as simple hinges, but the ligaments that hold them together are loose enough to allow more varied movement. As well as opening and closing our mouths (the hinge action) we can slide our jaws forward and back a little. Moving the jaw forward dislocates the joint, sliding the condyles forward out of the hollow that they normally rest in. We can also move our jaws from side to side. All these movements play their part in eating.