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The Review on a Structure of the Digestive System

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The Review on a Structure of the Digestive System THE REVIEW ON A STRUCTURE OF THE DIGESTIVE SYSTEM Fig 2.. Oral region. purposes of description into three parts, the duodenum, the jejunum, and ileum. In the small intestine the process of digestion is Fig. 1. Parts of the digestive system. completed and the resulting products are The apparatus for the digestion of the food absorbed into the blood and lacteal vessels. consists of THE DIGESTIVE TUBE and of Finally the small intestine ends in the large certain ACCESSORY ORGANS. THE intestine, which is made up of cecum, colon, DIGESTIVE TUBE (ALIMENTARY CANAL) is a rectum, and anal canal, the last terminating musculomembranous tube, about 9 metres long, on the surface of the body at the anus. extending from the mouth to the anus, and lined The accessory organs are: the teeth; the three throughout its entire extent by mucous pairs of salivary glands, the parotid, membrane (Fig. 1). It has received different submaxillary, and sublingual the secretion names in the various parts of its course: at its from which mixes with the food in the mouth commencement is the mouth, where provision and converts it into a bolus and acts chemically is made for the mechanical division of the food on one of its constituents. The liver and (mastication), and for its admixture with a fluid pancreas, are two large glands in the abdomen, secreted by the salivary gland; beyond this are the secretions of which, in addition to that of the organs of deglutition, the pharynx and the numerous small glands in the walls of the esophagus, which convey the food into the alimentary canal, assist in the process of stomach, in which it is stored for a time and in digestion. which also the first stages of the digestive THE ORAL REGION. process take place; the stomach is followed by The oral region (Fig.2) includes: the small intestine, which is divided for 1. The oral cavity, 2. The teeth, 3. The gingivae (gums), Fig. 3. Structure of a tooth. Fig. 4. Permanent teeth. 5 by the apposition of the stomodeal ectoderm with MAXILLA the foregut entoderm. At the end of the third week it disappears, and thus a communication is established between the mouth and the future pharynx. No trace of the membrane is found in the adult; and the communication just mentioned must not be confused with the permanent isthmus faucium. The lips, teeth, and gums are formed from the walls of the stomodeum, but the tongue is developed in the floor of the pharynx. The visceral arches extend in a ventral MANDIBULA direction, between the stomodeum and the pericardium, and with the completion of the MAXILLA mandibular arch and the formation of the maxillary processes, the mouth assumes the appearance of a pentagonal orifice. The orifice is bounded in front by the fronto-nasal process, posteriorly by the mandibular arch, and laterally by the maxillary processes With the inward growth and fusion of the palatine processes , the stomodeum is divided into an upper nasal, and a lower buccal part. Along the free margins of the processes bounding the mouth cavity a MANDIBULA shallow groove appears. The last is termed as Fig. 4-А. Permanent and decidui teeth. the primary labial groove, and from the bottom 4. The tongue, of it a downgrowth of ectoderm takes place into 5. The palate, the underlying mesoderm. The central cells of 6. The region of the palatine tonsil. the ectodermal downgrowth degenerate and The mouth or oral cavity is where the ingestion secondary labial groove is formed. By the of food occurs and where it is prepared for deepening of this, the lips and cheeks are digestion in the stomach and small intestine. The separated from the alveolar processes of the teeth chew food and saliva from the salivary maxillae and mandible. glands is added to the food to facilitate the formation of a manageable bolus. Deglutition 1. THE ORAL CAVITY. or swallowing is voluntarily initiated in the oral The oral cavity (mouth) consists of two parts: cavity. In this situation saliva make primary A) THE VESTIBULE digestion of carbohydrates by means of B) THE MOUTH PROPER. amylaze. After this process food pushes as the A) THE VESTIBULE is the slitlike space between bolus into the pharynx. the lips, cheeks, the teeth and the gingivae. This The mouth is developed partly from the is the entrance to the digestive tract. It is also stomodeum, and partly from the floor of the used in breathing. The vestibule communicates anterior portion of the foregut. By the growth of with the exterior through the orifice of the mouth the head end of the embryo, and the formation - the opening through which food and other of the cephalic flexure, the pericardial area and substances pass into the oral cavity. the buccopharyngeal membrane come to lie on The lips are mobile muscular folds, which the ventral surface of the embryo. With the formed by orbicularis oris muscle covered further expansion of the brain, and the forward externally by skin and internally by mucous bulging of the pericardium, the buccopharyngeal membrane and surround the mouth are the membrane is depressed between these two entrance to the oral cavity. Around the oral prominences. This depression constitutes vestibule and between the mucous membrane the stomodeum. It is lined by ectoderm, and is and the orbicularis oris muscle are located separated from the anterior end of the foregut labial salivary glands. The ducts of these small by the buccopharyngeal membrane. This glands open into the vestibule. The upper and membrane is devoid of mesoderm, being formed lower lips are attached to the gingivae in the 6 Fig. 5. Oral region. Fig. 6. Salivary glands. median plane by raised folds of mucous B) THE MOUTH PROPER is bounded: membrane, called the labial frenula (Bridle's 1. Superiorly - by the palate; frenula). The junction of the upper lip and cheek 2. Posteriorly -it communicates with the is clearly demarcated by the nasolabial sulcus. oropharinx, Running laterally from the margin of the nose to 3. Inferiorly - by the epiglottis, the angle of the mouth. The mentolabial sulcus 4. Laterally - by the palatoglossal arches. is the junction of the lower lip and chin. The upper 1. THE TEETH. Twenty deciduous teeth lip has a median, shallow, vertical groove called (primary or 'milk' teeth) begin to develop in the the philtrum. jaws before birth (Fig. 3; Fig. 4; Fig. 4-А). The Each lip has the following four parts: first tooth - incisor- usually erupts (or "cutting 1. A cutaneous zone. teeth") at 6 to 8 months after birth and the last by 2. A vermilion border - where the skin of the 20 to 24 months of age. The deciduous teeth are lip approaches the mouth, it changes color usually shed between 6 and 12 years of age and abruptly, to red. are replaced by permanent teeth. Eruption of 3. A transitional zone (red area) - between the the permanent teeth (normally 14 to 16 in each skin and the mucous membrane. The jaw) is usually complete by 18 years of age, transitional zone appears red because of the except for the third molars ('wisdom teeth'). presence of capillary loops close to the Parts and Types of Teeth. surface, which is composed of extremely thin Each tooth consists of three parts: and hairless skin. 1. The crown, 4. A mucosal zone. 2. The neck, The cheeks formed the lateral walls of the 3. The root. vestibule of the oral cavity. 1. The crown is the part of a tooth, that They have the same structure as the lips with projects from the gingiva and occludes (meets) which they are continuous. The muscular with one or more teeth in the opposite jaw. The component of the cheeks is the buccinator crown has 5 surfaces: muscle. Superficial to the fascia covering this 1. The lingual, muscle is the buccal fat pad - Bisha's body. It 2. The labial or buccal, gives the cheeks their rounded contour, 3. The occlusal, particularly in infants and female. This relatively 4. The lateral contact, large fat deposit is thought to reinforce the 5. The mesial contact. cheeks in infants and keep them from collapsing 2. The neck is the part of the tooth between during sucking. The lips and cheeks function as the crown and the root. a unit (e. g. - during blowing, eating, sucking, 3. The root is fixed in the alveolus (tooth socket) and kissing). They act as an oral sphincter in by a fibrous periodontal ligament. The number pushing food from the vestibule to the oral cavity of roots varies - the incisors and canines have a proper. single root each, the maxillary molars have three The buccal glands - are small mucous glands roots: the mandibular molars two. that are situated between the mucous Structure of a tooth. Most of the tooth is membrane and the buccinator muscle. There are composed of dentin, that is covered by also groups of these glands around the terminal enamel over the crown - and cementum over part of the parotid duct, which opens on a small the root. The pulp cavity contains of: papilla on the oral surface of the cheek, opposite connective tissue, the crown of the second maxillary molar tooth. blood vessels, 7 nerves. the tongue is a median groove. The mucous It is continuous with the periodontal tissue membrane on the oral part of the tongue is through the root canal and the apical foramen rough, owing to the presence of numerous (root foramen). The root canals transmit the papillae. They are: nerves and vessels and form the pulp cavity for 1.
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