Anatomical Structure of the Tooth

The teeth are specialized, hard, calcified organs attached to the maxilla and mandible. The function of the teeth is to incise, masticate food and assist in phonation.

Anatomically, a tooth consists of two fundamental parts: the crown and the root, or roots. The crown of the tooth is the part that appears above the gum line; it is only about 1/3 of the whole tooth. Underneath the gum line lie the roots. The root is that part of the tooth which is in the alveolar bone and is covered by mucous membrane. Incisors have only one root; other teeth, two or three. The crown and the root of the tooth are connected together by the neck.

The crown of each tooth is covered with dental enamel, the hardest substance in the body. The roots are encased in cementum, not quite as hard. Inside the enamel and cementum is the softer bony substance called dentin, which is the bulk of the tooth. The dentin surrounds a pulp chamber (dental pulp), where nerves and blood vessels of the tooth are located.

The nerves and blood vessels run up and down through narrow channels, called root canals, through which they connect with the rest of the circulatory and nervous systems of the body.

The peridental membrane or alveolar-dental periosteum is a fibrous membrane. Covering the roots of the tooth it holds the tooth in its socket in the jaw and takes up some of the shock of chewing. This is why the teeth are slightly movable. The periodontal membrane serves to attach the tooth to the bone.

A thin layer of bone called the lamina dura serves to line the alveolus or tooth socket.

III. Работа над грамматикой

Времена группы Continuous

Cказуемое в форме Continuous выражает действие, которое протекает (длится) в оп­ределенный отрезок или момент времени в на­стоящем, прошедшем и будущем.

Форма Continuous образуется при помощи вспомогательного глагола be, за которым следует Participle I смыслового глагола.

Вспомогательный глагол be изменяется, указывая на время, число и лицо, и не пере­водитcя; Participle I смыслового глагола не из­меняется, но переводится в том времени, на которое указывает глагол be, например:

am writing — Present Continuous Active — (сейчас) пишу

were making — Past Continuous Active — де­лали (в определенный момент в прошлом)

shall be making — Future Continuous Active — буду (будем) делать (в определенный момент в будущем).

Выполнение упр Учебника

IV. Реферирование текста. Направлено на формирование компетенции ок-5

Make a summary considering the stages of teeth development.

Tooth development

Before a human embryo is 3 weeks old, the stomodeum is established. The primitive mouth is lined with ectoderm, beneath which is mesenchyme. The ectoderm gives rise to the oral epithelium, and the mesenchyme becomes the underlying connective tissue.

Odontogenesis is the name given to the origin and tissue formation of the teeth. Not all teeth start development at the same time. A tooth is formed from ectoderm and ectomesenchyme. Ectomesenchyme is derived from neural crest cells. Development begins with the formation of the primary dental lamina, extending along the jaws on a line where the teeth will later appear.

Concurrently with the development of primary dental lamina, at 10 places in the mandibular and at 10 places in the maxillary arch some cells of the dental lamina multiply and 10 little knobs of epithelial cells are forming on the dental lamina in each jaw. Each of these knob-shaped structures is an early enamel organ. It is the beginning of the tooth germ of a primary tooth. A tooth germ is derived from two embryonic tissues: the part that develops from the dental lamina originate from ectoderm and the remaining parts originate from mesenchyme underlining ectoderm.

As the dental lamina enlarges, the enamel organ acquires the shape of a cap. By the 8^th week in utero this cap formation is seen in the enamel organs of the deciduous incisor tooth germs. The connective tissue inside the cap undergoes a number of changes and becomes the dental papilla. The connective tissue beneath the dental papilla becomes fibrous and encircles the papilla forming the dental sac.

The crown and the root of the tooth grow as the result of the deposition of new layers of enamel and dentin on previously formed layers. When the final size of the enamel crown has been attained, the enamel-forming cells disappear and further formation of enamel is impossible.

The fetus derives its mineral from mother through placenta. That’s why the dentin formed in utero is always of a more homogenous texture and more mineralized than that laid down postnatally. In the young child, the pulp cavity is quite large relative to amount of dentin already formed. For this reason, capping of teeth may be delayed for several years until the formation of the dentin has been completed. So the infant is fully dependent on the mineral obtained through ingestion.