The mouth

The mouth is a cavity bounded externally by the lips and cheeks and leading into the pharynx. The roof is formed by the hard and soft palates, and the anterior two-thirds of the tongue fills the floor of the mouth (Fig. 18.2). The walls are formed by the muscles of the cheeks. The mucous membrane, which lines the mouth, is continuous with the skin of the lips and with the mucous lining of the pharynx. The lips enclose the orbicularis oris muscle, which keeps the mouth closed. The hard palate is formed by parts of the palatine bones and the maxillae; its upper surface forms the floor of the nasal cavity. The soft palate is suspended from the posterior border of the hard palate and extends downwards between the oral and nasal parts of the pharynx. Its lower border hangs like a curtain between the mouth and the pharynx and a small conical process, called the uvula, hangs down from it. Two curved folds of mucous membrane extend sideways and downwards from each side of the base of the uvula, called the palatoglossal and palatopharyngeal arches, between which lie the masses of lymphoid tissue known as the palatine tonsils.

Fig. 18.2 Looking into the mouth with the tongue depressed.

Tongue

The tongue is a muscular organ that is attached to the hyoid bone and the mandible. It is covered in certain areas with modifications of the mucous membrane, which appear as projections called papillae, to increase the surface area. In addition, specialized areas called taste buds (Fig. 18.3) are widespread over almost the entire area of the tongue. The undersurface of the anterior part of the tongue is connected to the floor of the mouth by a fold of mucous membrane called the frenulum. The functions of the tongue are:

• It is the organ of taste.

• It assists in the mastication of food.

• It assists in swallowing.

• It assists with speech.

Fig. 18.3 A taste bud in the tongue.

Teeth

Humans have two sets of teeth, which make their appearances at different periods of life. The first set are deciduous or primary teeth and erupt through the gums during a child’s first and second years. The second set begins to replace the first at about the sixth year and the process is usually complete by the 25th year. Since they cannot be replaced, and may be retained until old age, they are known as the permanent teeth.

Each tooth consists of three parts (Fig. 18.4):

1. crown – projecting beyond the gum

2. root – embedded in the alveolus of the maxilla or mandible (a tooth may have one, two or three roots)

3. neck – the constricted part between the crown and the root.

Fig. 18.4 Section of a tooth, showing its structure.

In the centre of all these parts is the pulp and immediately outside the pulp is a yellowish-white layer, dentine, which forms the main part of the tooth. The outer layer of the tooth is in two parts: that covering the crown is called enamel and is a hard, white layer; that covering the root is called cement and is a thin layer resembling bone in structure. The pulp is richly supplied with blood vessels and nerves, which enter the tooth through foramina at the apex of each root.

There are four types of teeth:

1. Incisor teeth have chisel-shaped crowns giving a sharp cutting edge for biting food.

2. Canine teeth have large conical crowns.

3. Premolar (or bicuspid) teeth have almost circular crowns with two cusps for grinding food.

4. Molar teeth are the largest, and they have broad crowns with four or five cusps.

There are 20 deciduous teeth and 32 permanent teeth (Table 18.1).

Table 18.1 The deciduous teeth and permanent teeth

The pharynx

When food is well chewed and moistened the tongue rolls it into a bolus and carries it towards the oropharynx. The soft palate rises up to occlude the nasopharynx and the epiglottis moves upwards and forwards so that the bolus passes over the closed inlet of the larynx and on into the laryngopharynx and then to the oesophagus. This is an excellent example of muscular co-ordination and if it is not achieved correctly, choking will result (see Fig. 17.2).

The oesophagus

The oesophagus is a muscular canal approximately 25 cm long, extending from the pharynx to the stomach. It begins at the level of the sixth cervical vertebra and descends through the mediastinum in front of the vertebral column and behind the trachea. It passes through the diaphragm at the level of the tenth thoracic vertebra and ends at the cardiac orifice of the stomach at the level of the eleventh thoracic vertebra. On each side of the upper part of the oesophagus are the corresponding common carotid artery and part of the thyroid gland.

The oesophagus has four layers and is similar in structure to the remainder of the alimentary canal (Fig. 18.5):

Fig. 18.5 Cross-section of the alimentary tract.

The muscular coat of the upper two-thirds of the oesophagus is of striped voluntary muscle; the lower third contains unstriped involuntary muscle. The oesophagus is innervated by the vagus nerve. Movement of food through the oesophagus is by peristaltic action (peristalsis is a wave of dilatation followed by a wave of contraction as succeeding muscle fibres relax and contract). It takes approximately 9 seconds for a wave of peristalsis to pass the bolus of food from the pharynx to the stomach.

The stomach

The stomach is the most dilated part of the digestive tube and is situated between the end of the oesophagus and the beginning of the small intestine. Its shape and position are altered by changes within the abdominal cavity and by the stomach contents. It lies below the diaphragm, slightly to the left of the midline (Fig. 18.6).

Fig. 18.6 The stomach: (a) empty and (b) containing food.

The stomach is approximately J-shaped and has two curvatures. The lesser curvature forms the right (or posterior) border of the stomach. The greater curvature is directed mainly forwards and first forms an arch upwards and to the left to form the fundus of the stomach; it then passes downwards and finally turns right to the point where it joins the duodenum. The capacity of the stomach is about 1500 mL in the adult.

The upper opening from the oesophagus is called the cardiac orifice and the circular muscle fibres of the oesophagus are slightly thicker at this point and constitute a weak sphincter muscle.

The lower opening, into the duodenum, is called the pyloric orifice and is guarded by the strong pyloric sphincter, which prevents regurgitation of food from the duodenum into the stomach.

The wall of the stomach consists of four layers (Fig. 18.7):