The incisors are sharp, chisel-like teeth used for grasping, cutting, and gnawing food during chewing. In humans, there are eight incisor teeth located at the front of the mouth. Many other animals have incisor teeth as well.
The two sets of incisors on the top of the mouth are called the maxillary central incisors and maxillary lateral incisors. The central incisors are those closest to the midline of the body, while the lateral incisors are distal, or further from the midline. The maxillary incisors attach to the maxillary, or upper jaw. Attached to the bottom jaw, or mandible, are the mandibular central and lateral incisors.
Primary or deciduous incisors grow in the mouth during infancy. These are replaced by secondary or permanent incisors in childhood. Each incisor is attached to the jaw with one, long cone-shaped root per tooth. The front, or outward facing side, of the tooth is smooth and polished with a convex shape. The back, or inward facing side of the tooth, has a concave shape, and the upper incisors have a v-shaped ridge at the top of the tooth near the maxillary arch.
There are several different systems for labeling the teeth in the human jaw. The universal notation system, which is commonly used in the United States, labels deciduous teeth by assigning each one a capital letter. Permanent teeth are labeled according to number. The upper deciduous teeth, from the patient's right to left, are labeled D, E, F, and G. The upper permanent teeth are accordingly labeled 7, 8, 9, and 10. The lower deciduous teeth from patient's left to right are N, O, P, and Q, while the permanent teeth are 23, 24, 25, and 26.
The FDI World Dental Federation developed a two-digit notation used by dentists internationally. This system labels both deciduous and permanent teeth by number. Deciduous incisors, from patient's right to left are 52, 51, 61, and 62 while the permanent teeth are 12, 11, 21, and 22. Lower deciduous incisors are, from right to left, 82, 81, 71, and 72. Permanent teeth are 42, 41, 31, and 32.
In addition to humans, this type of tooth is found in herbivores, those who eat only plants, like rabbits. It is also found in other omnivores, or those who eat both plants and meats. Some omnivores that have incisor teeth are horses, cats, and rodents.
Horses and cats have 12 incisors, while rodents only have four. These teeth are different in rodents, because they continue to grow throughout the animal's life and are worn down by the gnawing action when it eats. In elephants, the upper incisors have evolved into curved tusks.