The hardest part of the head can be defined in two ways: the hardest biological material or the most structurally resistant region of the skull. The hardest material in the body is found in the head, but it does not form the brain’s main protective structure. The skull relies on bone thickness and specialized engineering for protection. Therefore, the answer requires looking at both chemical composition and physical architecture.
The Hardest Biological Substance
The hardest biological substance found in the head is dental enamel, the highly mineralized tissue that forms the outer layer of the teeth. Its remarkable hardness is due to its composition, which is up to 96% mineral by weight. This mineral content is far greater than that of bone, which is only about 60% mineralized. The primary mineral component is a crystalline form of calcium phosphate known as hydroxyapatite.
The dense, tightly packed arrangement of these crystals gives enamel stiffness and resistance to wear, allowing it to withstand the forces of chewing and grinding. Unlike bone, enamel is an acellular tissue, meaning it contains no living cells and cannot regenerate or repair itself. This lack of organic components contributes significantly to its superior hardness compared to the more flexible tissue of bone.
Identifying the Densest Cranial Region
When considering the structural defense of the brain, the thickest region of the cranium is the occipital bone, which forms the back and lower part of the skull. This area is subjected to high forces during falls, as the head often strikes the ground backward, requiring robust protection. Studies consistently show that the occipital bone is the most substantial, with average thicknesses ranging from 7.7 millimeters to over 10 millimeters in certain areas.
The thickness of the cranial bone is not uniform across the head; it varies significantly depending on the location and the mechanical stresses it endures. For instance, the temporal bones, located near the temples, are the thinnest parts of the skull, often measuring as little as 3.4 to 6 millimeters. The frontal bone, which makes up the forehead, is commonly the second thickest. This variation demonstrates a design that balances structural strength with the need to minimize overall head mass.
Structural Features That Enhance Resilience
Beyond the thickness of any single bone, the overall resilience of the skull is enhanced by its specialized, layered architectural design. The cranial bones are not solid blocks but are constructed as a “sandwich structure,” providing a highly effective mechanism for shock absorption. This structure consists of two layers of dense, compact bone, known as the inner and outer tables.
These tables surround a middle layer of spongy, porous bone tissue called the diploë. When an impact occurs, the outer table may fracture, but the porous diploë layer absorbs and dissipates the energy before it reaches the inner table and the brain. The dome shape of the skull further contributes to resilience by spreading impact forces over a larger surface area. Furthermore, the interlocking, fibrous joints between the skull bones, called sutures, also dissipate mechanical stress, preventing localized force from being transmitted directly to the brain.