Human tooth.

 Human tooth.

Human tooth,  any of the hard structures.

Teeth are used for catching and masticating food, for defense, and for other specialized purposes. The human teeth function to mechanically break down items of food by cutting and crushing them in preparation for swallowing and digesting. Humans have four types of teeth: incisors, canines, premolars, and molars, which each have a specific function. The incisors cut the food, the canines tear the food and the molars and premolars crush the food. The roots of teeth are embedded in the maxilla (upper jaw) or the mandible (lower jaw) and are covered by gums. Teeth are made of multiple tissues of varying density and hardness.

We have two sets of teeth. The first is called primary and grows in by the age of 2, the second set are called permanent teeth. These are the teeth that grow in between the ages of 6 and12. We start with 20 primary teeth and end with 32 permanent teeth, including the four wisdom teeth.

The incisors are typically the first to arrive when babies turn 6 months old. The permanent molars are usually the first to come in when kids are 6. They emerge behind the last primary teeth. Most adult teeth come in by age 12. Wisdom teeth grow in between ages 17 and 25.

The incisors:

The incisors are chisel shaped teeth located anteriorly within the oral cavity. The four front teeth in both the upper and lower jaws are called incisors.  Their primary function is to cut food.  The two incisors on either side of the midline are known as central incisors.  The two adjacent teeth to the central incisors are known as the lateral incisors.  Incisors have a single root and a sharp incisal edge, both children and adults have eight incisors — four central incisors at the front of the mouth, two on each row, with one lateral incisor positioned on either side of them.
Adult humans normally have eight incisors, two of each type. The types of incisor are:

-maxillary central incisor (upper jaw, closest to the center of the lips)
-maxillary lateral incisor (upper jaw, beside the maxillary central incisor)
-mandibular central incisor (lower jaw, closest to the center of the lips)
-mandibular lateral incisor (lower jaw, beside the mandibular central incisor)
Children with a full set of deciduous teeth (primary teeth) also have eight incisors, named the same way as in permanent teeth. Young children may have from zero to eight incisors depending on the stage of their tooth eruption and tooth development. Typically, the mandibular central incisors erupt first, followed by the maxillary central incisors, the mandibular lateral incisors and finally the maxillary laterals. The rest of the primary dentition erupts after the incisors. Apart from the first molars, the incisors are also the first permanent teeth to erupt, following the same order as the primary teeth, among themselves.

Canine tooth:

The Canine Teeth (dentes canini) are four in number, two in the upper, and two in the lower arch. Your four canine teeth sit next to the incisors. You have two canines on the top of your mouth and two on the bottom. Canines have a sharp, pointy surface for tearing food.  

The first baby canines come in between the ages of 16 months and 20 months. The upper canines grow in first, followed by the lower canines. 

Lower adult canines emerge in the opposite way.  First, the lower canines poke through the gums around age 9, then the upper canines come in at age 11 or 12.  

The crown is large and conical, very convexe on its labial surface, a little hollowed and uneven on its lingual surface, and tapering to a blunted point or cusp, which projects beyond the level of the other teeth. The root is single, but longer and thicker than that of the incisors, conical in form, compressed laterally, and marked by a slight groove on each side.

The upper canine teeth (popularly called eye teeth) are larger and longer than the lower, and usually present a distinct basal ridge.

The lower canine teeth (popularly called stomach teeth) are placed nearer to the middle line than the upper, so that their summits correspond to the intervals between the upper canines and the lateral incisors.
Canine teeth play important roles in eating, speaking, maintaining the shape of the lips and guiding the other teeth into position. When you eat, canines help cut and tear food into bite-sized pieces, and when you speak, they work alongside the incisors to help form words.   In addition, canine teeth function as guideposts to the rest of the teeth. They help maintain the position of the upper and lower teeth when the lower jaw moves side to side. What's more, canine teeth guide the upper and lower teeth into place.

Premolar:

The premolars, also called premolar teeth, or bicuspids. The permanent premolar teeth are placed between the anterior teeth and molars. In humans, there are two premolars per quadrant in the permanent set of teeth, making eight premolars total in the mouth. They have at least two cusps. 

Premolars can be considered transitional teeth during chewing, or mastication. They have properties of both the canines, that lay anterior and molars that lay posterior, and so food can be transferred from the canines to the premolars and finally to the molars for grinding, instead of directly from the canines to the molars.  Premolars are bigger than canines and incisors. They have a flat surface with ridges for crushing and grinding food into smaller pieces to make it easier to swallow.  Baby molar teeth are replaced by adult premolars.

Infants and young children don’t have premolars because these teeth don’t start to come in until around age 10.  

There is always one large buccal cusp, especially so in the mandibular first premolar. The lower second premolar almost always presents with two lingual cusps.  The lower premolars and the upper second premolar usually have one root. The upper first usually has two roots, but can have just one root, notably in Sinodonts, and can sometimes have three roots.

Molar:

The molars or molar teeth are large, flat teeth at the back of the mouth. The Molar Teeth (dentes molares) are the largest of the permanent set, and their broad crowns are adapted for grinding and pounding the food. They are twelve in number; six in each arch, three being placed posterior to each of the second premolars. The crown of each is nearly cubical in form, convex on its buccal and lingual surfaces, flattened on its surfaces of contact; it is surmounted by four or five tubercles, or cusps, separated from each other by an important depression; hence the molars are sometimes termed multicuspids. The neck is distinct, large, and rounded.

Upper Molars: As a rule the first is the largest, and the third the smallest of the upper molars. The crown of the first has usually four tubercles; that of the second, three or four; that of the third, three. Each upper molar has three roots, and of these two are buccal and nearly parallel to one another; the third is lingual and diverges from the others as it runs inwards. The roots of the third molar (dens serotinus or wisdom-tooth) are more or less fused together.

Lower Molars: The lower molars are larger than the upper. On the crown of the first there are usually five tubercles; on those of the second and third, four or five. Each lower molar has two roots, an anterior, nearly vertical, and a posterior, directed obliquely backward; both roots are grooved longitudinally, indicating a tendency to division. The two roots of the third molar (dens serotinus or wisdom tooth) are more or less united.

Read more: Teeth anatomy, maxilla.