Biology Reference
In-Depth Information
requirements through the remainder of life. Humans are also heterodonts, which means that
we have different types of teeth. Moving from the front to the back of the oral cavity, the four
different types of teeth that we have include incisors, canines, premolars (or bicuspids), and
molars (
Figure 10.1
).
Within the oral cavity, four quadrants are drawn from a line that runs anterioposteriorly
along the sagittal plane between the central incisors. The tooth row runs posteriorly in
a curved manner. Tooth surfaces closest to the midline, or facing more towards the front of
the mouth within the curve of the dental arch are mesial while the more posterior surfaces
facing away from the midline are distal. The surface facing the tongue is the lingual surface
and the cheek side is the buccal surface (or labial when referring to the incisors). The chewing
surface of the tooth crown is referred to as the occlusal surface (or incisal when referring to the
incisors). The cervical region is where the crown meets the root, near what is called the cemen-
toenamel junction (or CEJ), and the apical region is that nearest the bottom or apex of the root tip.
The four main components of a tooth as shown in
Figure 10.2
are enamel, dentin, cementum,
and pulp. Enamel is the outermost surface of the tooth crown. Dentin is the next layer, and in
addition to forming the inner portion of the tooth crown, it makes up a large portion of the root
of each tooth. The innermost layer of each tooth is the pulp chamber. Once the tooth is nearing
full eruption, a layer of cementum begins to develop on the outside of the root. The portion of
the jaws that the teeth sit in is called the alveolus. Teeth are held in place by periodontal liga-
ments that adhere to the cementum and the alveolus. The joint formed by this connection
between the tooth and the alveolus is called a gomphosis and is relatively immovable.
In humans, the premolars and molars have separate cusps on their occlusal surfaces. The
cusps on these teeth are separated by fissures or grooves that run between them. Premolars
have a buccal cusp and a lingual cusp in keeping with the directional terminology presented
above. Cusps in the molars are identified by their position on the tooth (i.e., mesiolingual,
mesiobuccal, etc.), by numbers, and also by a naming system that derives from the ancestral
tooth form (
Figure 10.3
).
Maxillary molars normally have four cusps, the three largest of which (mesiobuccal,
mesiolingual, and distobuccal) form a somewhat raised triangle while the distolingual
cusp (the hypocone) is often considerably smaller. In addition, the groove running between
the distobuccal and mesiolingual cusps of the maxillary molars is known as the crista obliqua.
The first mandibular molars normally present with five cusps. In modern humans, the size
of permanent molars tends to decrease fromM1 (1st permanent molar) to M3 (3rd permanent
FIGURE 10.1
Tooth classes. From mesial to distal: incisors, canine, premolars, molars. Drawing by Lon Hunt.
