MASTICATION AND DEGLUTITION

The mastication process includes the biting and tearing of food into manageable pieces. This usually involves using the incisors and cuspid teeth. The grinding of food is usually performed by the molars and premolars. During the mastication process, food is moistened and mixed with saliva.

Deglutition is the swallowing of food and involves a complex and coordinated process. It is divided into three phases; the first phase of swallowing is voluntarily; phases two and three are involuntary.

Phase one: the collection and swallowing of masticated food.

Phase two: passage of food through the pharynx into the beginning of the esophagus.

Phase three: the passage of food into the stomach.

Table 3-5.\List of the Three Major Salivary Glands

CHAPTER 4 ORAL ANATOMY

This chapter covers the oral anatomy and physiology of the teeth, the histology of their tissues and supporting structures, and concentrates on the external features of the teeth. To understand the material in this section, you must become familiar with the terms used to describe the external features of the teeth. In addition, you must know the numbering system by which the teeth are identified on the standard dental chart used by the armed services. As a basic dental assistant, you must be aware that teeth differ in size, shape, and other characteristics from one person to another. Such knowledge will be useful to you when you fill in dental charts, expose radiographs, clean teeth, and assist in all phases of dentistry throughout your career.

FORMATION PERIOD

As all living things are forming, they go through a developmental process to reach maturity or a final outcome. When teeth are in the odontogenesis phase (tooth formation), every tooth goes through three developmental processes. They are categorized into the growth, calcification, and eruption periods (illustrated in figure 4-1). The term emergence is used to describe the tooth as it is breaking through the gingival tissue.

GROWTH PERIOD

Dental development usually begins in the fifth or sixth week of prenatal life. By the seventh week, skin cells of the mouth called epithelium thicken along the ridge of the developing jaws creating a horse-shoe shaped band called the dental lamina. The growth period of development is divided into the bud, cap, and bell stages.

Bud Stage

From the dental lamina, patches of epithelial cells grow into the underlying tissues. These patches of cells are called tooth buds. As soon as the dental lamina is formed, the tooth buds for the primary teeth develop. Usually 10 tooth buds are present in each dental arch and they give rise to future primary teeth. Tooth buds for the permanent teeth form between the 17th week of fetal life through the age of 5 years. When the primary teeth are lost, permanent teeth will replace them.

Cap Stage

This stage is also known as proliferation (reproduction or multiplication) in which the cells of the tooth grow and the tooth bud takes a hollowed caplike shape. The epithelium of the cap will give rise to the enamel. The zone under the cap is called the dental papilla. The dental papilla gives rise to the dentin, cementum, and the pulp.

Bell Stage

The last period of growth is also known as histodifferentiation (the acquisit on of tissue characteristics by cell groups) or bell stage. It is here the ameloblast cells form the enamel, odontoblast cells form the dentin, and the cementoblast cells form the cementum.

MORPHODIFFERENTIATION

As the tooth is in the bell stage, it begins to take shape and form through a process called morphodifferentiation. Enamel forming cells (ameloblast) and dentin forming cells (odontoblast) line up on a boundary line called dentinoenamel junction.

APPOSITION

Apposition refers to the depositing of the matrix for the hard dental structures. This matrix is deposited by cells along the boundary line at the end of morphodifferentiation.

CALCIFICATION

Calcification (fig. 4-1) is the process by which organic tissue (the matrix formed during apposition) becomes hardened by a deposit of calcium or any mineral salts. Next, the tooth crown receives layers of enamel that start at the top of the crown and go downward over the sides to the cementonenamel junction.

Figure 4-1.\Stages in development of a tooth.

ERUPTION

After the crown of the tooth has formed, the root begins to develop. Now the tooth begins to erupt (fig. 4-l), which is a movement of the tooth into its proper position in the mouth. For permanent teeth, it takes about 3 years from crown completion to the time the tooth emerges into the mouth. Figures 4-2 and 4-3 list the average emergence periods of primary and permanent teeth.

EXFOLIATION

When primary teeth get ready to fall out and make way for the eruption of permanent teeth, they go through a process called exfoliation (shedding). The root of the primary tooth resorbs (looses structure) as the permanent tooth erupts from beneath. The primary teeth act as guides for the developing permanent teeth. The premature loss of primary teeth can have a serious impact on the eruption of permanent teeth and how they will be in position in the dental arch.