THE DIGESTIVE SYSTEM 1
The digestive system consists of the mouth, the pharynx, the oesophagus, the stomach, the small intestine and the large intestine. It also includes the associated glands as the salivan glands, the liver, and the pancreas.
– The mouth is the first part of the digestive tube and it is also used in breathing. The mouth is composed of a smaller outer part called the vestibule and a larger inner part called the mouth cavity proper.
– The vestibule is bounded externally by the cheeks and lips and internally by the teeth and gums.
– The operture between the lips is called the oral fissure.
– The mouth cavity proper is bounded anteriorly and on each side by the teeth and gums. Posteriorly it communicates with the oropharynx. The roof is formed b> the hard and soft palates which separate it from the nasal cavity The floor of the mouth is principally formed b> the tongue and the sublingual salivary glands, which raise a ridge of mucous membrane called the sublingual fold.
The lips are the two muscular folds surrounding the oral fissure. /They are covered externally by the skin and internally by mucous membrane. Between the outer skin and inner mucosa is the vermilion which is the red zone to which lipstick is sometimes applied. It appears red because of the presence of capillary loops close to the surface.
The gums are composed of fibrous tissue, which is covered by mucous membrane. They are attached to the margins of the alveolar processes of the upper and lower jaws and to the neck of the teeth.
The tongue is a highly mobile muscular organ concerned with chewing, taste, swallowing, articulation (speech), and oral cleaning. It has an .upper surface, a lower surface, and two lateral borders.
The teeth are set in sockets in the alveolar processes of the jaws. There are two sets of teeth: deciduous (or milk) teeth, and permanent teeth.
Milk teeth are ten in each jaw i.e. 5 in each half of a jaw: 1 central incisor, 1 lateral incisor, 1 canine, and 2 molar teeth. They erupt during the first 2 years of life starting usually at the sixth month. They are usually shed from the sixth to the twelfth year as they are replaced by the permanent teeth.
Permanent teeth (Fig. 24) replace the milk teeth between the 6th and 18th year. Each adult jaw contains 16 permanent teeth i.e. each half of a jaw contains 8 teeth: 1 central incisor, 1 lateral incisor, 1 canine, 2 premolars and 3 molars.
N.B. incisors are the cutters. Canines are piercers. Premolars and molars are the grinders.
Each tooth (Fig. 25) has a crown above the gingivae a neck embedded in the gingivae, and a root fixed in the socker of the alveolar process of the jaw by a fibrous periodontal membrane. Most of the tooth is composed of dentine which is covered b\ enamel over the crown and cementum over the root. The pulp cavity contains connective tissue, blood vessels and nerves and is continuous with the periodontal tissue through the root canal.
See the respiratory system.
The oesophagus is a muscular tube about 25 cm in length. It extends from the lower end of the pharynx to the cardiac orifice of the stomach. It passes in the neck (cervical part), in the thorax (thoracic part), and in the abdomen (abdominal part). It transmits the food from the pharynx to the stomach.