Teeth

Humans and other mammals have heterodont dentition. This
means that they have various types of teeth (fig. 18.8) that are
adapted to handle food in particular ways. The four pairs (upper
and lower jaws) of anteriormost teeth are the incisors (in-si'sorz).
The chisel-shaped incisors are adapted for cutting and shearing
food. The two pairs of cone-shaped canines (cuspids) are located
at the anterior corners of the mouth; they are adapted for holding
and tearing. Incisors and canines are further characterized by
a single root on each tooth. Located behind the canines are the
premolars (bicuspids), and molars. These teeth have two or three
roots and somewhat rounded, irregular surfaces called dental
cusps for crushing and grinding food. The buccal surface of the
premolars and molars is adjacent to the cheek. The labial surface
of the incisors and canines is adjacent to the lip. The lingual
surface of all teeth is adjacent to the tongue.

Humans are diphyodont (di-fi'o˘-dont); that is, normally two
sets of teeth develop in a person’s lifetime. Twenty deciduous
(milk) teeth begin to erupt at about 6 months of age (fig. 18.9
and tables 18.2 and 18.3), beginning with the incisors. All of the
deciduous teeth normally erupt by the age of 2 1/2. Thirty-two
permanent teeth replace the deciduous teeth in a predictable sequence.
This process begins at about age 6 and continues until
about age 17. The third molars (“wisdom teeth”) are the last to
erupt. There may not be room in the jaw to accommodate the
wisdom teeth, however, in which case they may grow sideways
and become impacted, or emerge only partially. If they do erupt
at all, it is usually between the ages of 17 and 25. Presumably, a
person has acquired some wisdom by then—hence, the popular
name for the third molars.

A dental formula is a graphic representation of the types,
number, and position of teeth in the oral cavity. Such a formula
can be written for each species of mammal with heterodontia.
Following are the deciduous and permanent dental
formulae for humans:

Formula for deciduous dentition:
I 2/2, C 1/1, DM 2/2 = 10 × 2 = 20 teeth
Formula for permanent dentition:
I 2/2, C 1/1, P 2/2, M 3/3 = 16 × 2 = 32 teeth
where I = incisor; C = canine; P = premolar; DM = deciduous
molar; M = molar.


The dental cusps of the upper and lower premolars and molars
occlude for chewing food, whereas the upper incisors normally
form an overbite with the incisors of the lower jaw. An
overbite of the upper incisors creates a shearing action as these
teeth slide past one another. Masticated food is mixed with saliva,
which initiates chemical digestion and aids swallowing. The soft,
flexible mass of food that is swallowed is called a bolus (bo'lus).
A tooth consists of an exposed crown, which is supported
by a neck that is anchored firmly into the jaw by one or more
roots (fig. 18.10). The roots of teeth fit into sockets, called dental
alveoli, in the alveolar processes of the mandible and maxillae.
Each socket is lined with a connective tissue periosteum,
specifically called the periodontal membrane. The root of a
tooth is covered with a bonelike material called the cementum;
fibers in the periodontal membrane insert into the cementum
and fasten the tooth in its dental alveolus. The gingiva (jı¯n-ji'va˘)
(gum) is the mucous membrane surrounding the alveolar
processes in the oral cavity.
The bulk of a tooth consists of dentin, a substance similar to
bone but harder. Covering the dentin on the outside and forming
the crown is a tough, durable layer of enamel. Enamel is composed
primarily of calcium phosphate and is the hardest substance in the
body. The central region of the tooth contains the pulp cavity.
The pulp cavity contains the pulp, which is composed of connective
tissue with blood vessels, lymph vessels, and nerves. A root
canal, continuous with the pulp cavity, opens to the connective
tissue surrounding the root through an apical foramen. The tooth
receives nourishment through vessels traversing the apical foramen.
Proper nourishment is particularly important during embryonic
development. The diet of the mother should contain an
abundance of calcium and vitamin D during pregnancy to ensure
the proper development of the baby’s teeth.
Even though enamel is the hardest substance in the body,
bacterial activity may result in dental caries (kar'e¯z), or tooth
decay. Refluxed stomach acids also destroy tooth enamel and constant
vomiting, as in the eating disorder bulimia nervosa, contributes
to the development of dental caries. Cavities in the teeth must be artificially
filled because new enamel is not produced after a tooth
erupts. The rate of tooth decay decreases after age 35, but then
problems with the gums may develop. Periodontal diseases result
from plaque or tartar buildup at the gum line. This buildup pulls the
gum away from the teeth, allowing bacterial infections to develop.

Mouth

Ingested food is changed into a bolus by the mechanical action of
teeth and by the chemical activity of saliva. The bolus is swallowed
in the process of deglutition.

The functions of the mouth and associated structures are to form a
receptacle for food, to initiate digestion through mastication, to
swallow food, and to form words in speech. The mouth can also
assist the respiratory system in breathing air. The pharynx, which
is posterior to the mouth, serves as a common passageway for both
the respiratory and digestive systems. Both the mouth and pharynx
are lined with nonkeratinized stratified squamous epithelium,
which is constantly moistened by the secretion of saliva.
The mouth, also known as the oral cavity (fig. 18.5), is
formed by the cheeks, lips, hard palate and soft palate. The
vestibule of the oral cavity is the depression between the cheeks
and lips externally and the gums and teeth internally (fig. 18.6).
The opening of the oral cavity is referred to as the oral orifice,
and the opening between the oral cavity and the pharynx is
called the fauces (faw/se¯z).

Cheeks, Lips, and Palate

The cheeks form the lateral walls of the oral cavity. They consist
of outer layers of skin, subcutaneous fat, facial muscles that assist
in manipulating food in the oral cavity, and inner linings of
moistened stratified squamous epithelium. The anterior portion
of the cheeks terminates in the superior and inferior lips that surround
the oral orifice.

The lips are fleshy, highly mobile organs whose principal
function in humans is associated with speech. Lips also serve for
suckling, manipulating food, and keeping food between the upper
and lower teeth. Each lip is attached from its inner surface to the
gum by a midline fold of mucous membrane called the labial
frenulum (fren'yu˘-lum) (fig. 18.5). The lips are formed from the
orbicularis oris muscle and associated connective tissue, and are
covered with soft, pliable skin. Between the outer skin and the
mucous membrane of the oral cavity is a transition zone called
the vermilion. Lips are red to reddish brown because of blood
vessels close to the surface. The numerous sensory receptors in
the lips aid in determining the temperature and texture of food.



The palate, which forms the roof of the oral cavity, consists
of the bony hard palate anteriorly and the soft palate posteriorly
(figs. 18.5). The hard palate, formed by the
palatine processes of the maxillae and the horizontal plates of
the palatine bones, is covered with a mucous membrane.
Transverse palatine folds, or palatal rugae (roo'je), are located
along the mucous membrane of the hard palate. These structures
serve as friction ridges against which the tongue is
placed during swallowing. The soft palate is a muscular arch
covered with mucous membrane and is continuous with the
hard palate anteriorly. Suspended from the middle lower border
of the soft palate is a cone-shaped projection called the
palatine uvula (yoo'vyu˘-la˘). During swallowing, the soft palate
and palatine uvula are drawn upward, closing the nasopharynx
and preventing food and fluid from entering the nasal
cavity.

Two muscular folds extend downward from both sides of
the base of the palatine uvula (figs. 18.5 and 18.6). The anterior
fold is called the glossopalatine arch, and the posterior fold is
the pharyngopalatine arch. Between these two arches is the
palatine tonsil.