The Human Body


Teeth, hard, bony structures in the mouths of humans and animals used primarily to chew food, but also for gnawing, digging, fighting, and catching and killing prey. Teeth are the body’s hardest, most durable organ—long after bones and flesh have dissolved, archaeologists find well-preserved teeth from humans and other animals that lived thousands of years ago.

Structure of a Tooth
The sensitive nerves and blood vessels at the center of each tooth are protected by several layers of tissue, the outermost (the enamel) being the hardest substance in the body. Under the enamel, surrounding the pulp from crown to root, lies a layer of bonelike dentin. A hard tissue called cementum separates the root from the periodontal ligament, which in turn holds the root in place and cushions the tooth against the gum and jaw during the grinding, jarring activity of chewing.

Human teeth are made of four distinct types of tissue: enamel, dentin, pulp, and cementum. Enamel, the clear outer layer of the tooth above the gum line, is the hardest substance in the human body. In human teeth, the enamel layer is about 0.16 cm (about 0.06 in) thick and protects the inner layers of the teeth from harmful bacteria and changes in temperature from hot or cold food. Directly beneath the enamel is dentin, a hard, mineral material that is similar to human bone, only stronger. Dentin surrounds and protects the pulp, or core of the tooth. Pulp contains blood vessels, which carry oxygen and nutrients to the tooth, and nerves, which transmit pain and temperature sensations to the brain. The outer layer of the tooth that lies below the gum line is cementum, a bonelike substance that anchors the tooth to the jawbone.

The visible portion of the tooth is called the crown. Projections on the top of each crown, used primarily for chewing and grinding, are called cusps. The portion of the tooth that lies beneath the gum line is the root. The periodontal ligament anchors the tooth in place with small elastic fibers that connect the cementum in the root to a special socket in the jawbone called the alveolus.