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The Review on a Structure of the Digestive System

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 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 , and anal canal, the last terminating musculomembranous tube, about 9 metres long, on the surface of the body at the anus. extending from the 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 and (mastication), and for its admixture with a fluid , are two large glands in the , secreted by the ; beyond this are the secretions of which, in addition to that of the organs of deglutition, the and the numerous small glands in the walls of the , which convey the food into the alimentary canal, assist in the process of , 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 (),

Fig. 3. Structure of a tooth. Fig. 4. Permanent teeth. 5 by the apposition of the stomodeal ectoderm with the 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 , teeth, and gums are formed from the walls of the , but the 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 , the underlying . The central cells of 6. The region of the . 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 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 . 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 , the pericardial area and substances pass into the oral cavity. the come to lie on The lips are mobile muscular folds, which the ventral surface of the embryo. With the formed by covered further expansion of the brain, and the forward externally by 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 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 and 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 . 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 . 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 - 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 ) 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 , 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 - 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 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 . There are composed of dentin, that is covered by also groups of these glands around the terminal enamel over the crown - and over part of the , 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 . 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. The filiform papillae - numerous, rough, and support of metabolic processes in the teeth. threadlike. They are arranged in rows parallel to the sulcus terminalis and contain afferent 2. THE GINGIVAE. endings that are sensitive to touch. They are composed of fibrous tissue that is 2. The fungiform papillae- are small and covered with mucous membrane. They are firmly mushroom-shaped. They usually appear as attached to the margins of the alveolar pink or red spots. Contain taste receptors processes (tooth sockets) of the jaws and to the located in the taste buds. of the teeth. The alveolar mucosa ('loose 3. The vallate (circumvallatae) papillae - are gingiva') is normally shiny red and the largest papillae (1 to 2 mm in diameter). nonkeratinizing. The gingiva proper ('attached They lie just anterior to the sulcus terminalis gingiva') is normally pink, stippled, and and appear similar to short, flat-topped keratinizing. cylinders sunken into the mucosa. A deep, 3. THE TONGUE. circular trench (trough), the walls of which are The tongue is a highly mobile muscular studded with taste buds, surrounds the vallate that can vary greatly in shape. It is concerned papillae. with mastication, taste, deglutition (swallowing), 4. The foliate papillae - are small lateral folds articulation (speech), and oral cleansing, but its of the lingual mucosa: they are poorly main functions are squeezing food into the developed in humans. It contains taste pharynx when swallowing and forming words receptors located in the taste buds. during speaking. The tongue is situated partly 5. The conic papillae (conicae). in the mouth and partly in the oropharynx. It consists of three parts: 2. THE PHARYNGEAL PART OF THE TONGUE a) a tip, is locates posterior to the terminal sulcus and b) a body, the palatoglossal arches. Its mucous membrane c) a root. has no papillae, but contents lymphoid nodules The root, its posterior part, is attached-mainly to (lingual follicles) are collectively known as the the floor of the mouth. At rest the tongue fills lingual tonsil. most of the mouth proper. It is mainly composed MUSCLES OF THE TONGUE of muscles and is covered by a mucous The tongue is divided into halves by a median membrane on its dorsum, tip, and sides. The fibrous that lies deep to the dorsum of the tongue is divided by a V-shaped median groove. In each half of the tongue, there sulcus terminals into: are four extrinsic and four intrinsic muscles. 1. ANTERIOR ORAL (presulcal) - forms EXTRINSIC MUSCLES OF THE TONGUE about 2/3; contents of four muscles: 2. POSTERIOR PHARYNGEAL (postsulcal) parts - forms about 1/3 of the dorsum of 1. The Genioglossus Muscle, the tongue. At the apex of the terminal 2. The Hyoglossus Muscle, sulcus is a small median depression, the 3. The Styloglossus Muscle, foramen cecum (Morgagni's foramen). 4. The Palatoglossus Muscle. This pit is the remnant of the opening (point They originate outside the tongue and attaches of origin) of the embryonic thyroglossal to it. These muscles mainly move the tongue, duct. but they can alter its shape as well. 1. THE GENIOGLOSSUS MUSCLE is bulky, 1. ANTERIOR ORAL (PRESULCAL) PART is fan-shaped muscle. freely movable, but it is loosely attached to Origin: arises by a short from the the floor of the mouth by the lingual . superior mental spine of the mandible. Its On each side of the frenulum is a deep lingual most inferior fibers insert into the body of the . The inferior surface and sides of the hyoid . tongue are covered with smooth, thin mucous Insertion: enters the tongue inferiorly and its membrane. On the dorsum of the oral part of fibers attach to the entire dorsum of the 8 tongue. Actions: curls the tip of the tongue inferiorly, Actions: depresses the tongue and its posterior making the dorsum of the tongue convex. part protrudes it. 3. THE TRANSVERSE MUSCLE muscle lies 2. THE HYOGLOSSUS (ZAGLAS'S) MUSCLE deep to the superior longitudinal muscle. is thin, quadrilateral muscle. Origin: the fibrous lingual septum. Origin: arises from the body and greater horn Insertion: submucous fibrous tissue. of the . Actions: narrows and increases the height of Insertion: the side and inferior aspect of the the tongue. tongue. 4. THE VERTICAL MUSCLE is small muscle. Actions: depresses the tongue, pulling its sides Origin: dorsum of the tongue. inferiorly and also aids in retrusion of the Insertion: site of the tongue. tongue. Actions: flattens and broadens the tongue; 3. THE STYLOGLOSSUS MUSCLE is small, acting with the transverse muscle, it increases short muscle. the length of the tongue. Origin: the anterior border of the styloid process near its tip and from the stylohyoid ligament. 4. THE PALATE forms the arched roof of the Insertion: the side and inferior aspect of the oral and the floor of the nasal cavities and tongue. separates the oral cavity from the nasal cavities Actions: retrudes the tongue and curls its sides and the nasal part of the pharynx or nasopharynx to create a trough during swallowing. (Fig. 5). The palate consists of two regions: 4. THE PALATOGLOSSUS MUSCLE is small 1. - the anterior two-thirds or muscle. bony part. Origin: the palatine aponeurosis of the soft 2. THE - the mobile posterior palate. one-third or fibromuscular part. Insertion: the side and the lateral part of the 1. THE HARD PALATE. The palatine processes tongue. of the maxillae and the horizontal plates of Actions: elevates the posterior part of the the palatine form the anterior bony part tongue. of the palate. Anteriorly and laterally, the alveolar processes and the gingivae bound THE INTRINSIC MUSCLES OF THE TONGUE the hard palate. Posteriorly the hard palate are four pairs. The intrinsic muscles are mainly is continuous with the soft palate. concerned with altering the shape of the 2. THE SOFT PALATE contains a membranous tongue, making it broad or narrow. Their fibers aponeurosis and is a movable, fibro-muscular run in three directions. fold that is attached to the posterior edge of 1. The Superior Longitudinal Muscle, the hard palate. The soft palate or velum 2. The Inferior Longitudinal Muscle, palatinum extends postero-inferiorly to a 3. The Transverse Muscle, curved free margin from which hangs a 4. The Vertical Muscle. conical process - the uvula. It separates the nasopharynx superiorly from the oropharynx 1. THE SUPERIOR LONGITUDINAL MUSCLE inferiorly. Laterally, the soft palate is forms a thin layer deep to the mucous membrane continuous with the wall of the pharynx and on the dorsum of the tongue, running from its is joined to the tongue and pharynx by the tip to its root. palatoglossal and palatopharyngeal arches, Origin: the submucous fibrous layer and the between which locate the palatine tonsil. lingual septum. Deep to the palatal mucosa are mucous Insertion: mainly into the mucous membrane. glands. The soft palate is strengthened by Actions: curls the tip and sides of the tongue the palatine aponeurosis, which the mastoid superiorly, making the dorsum of the tongue process, and the anterior border of the concave. sternocleidomastoideus muscle. 2. THE INFERIOR LONGITUDINAL MUSCLE consists of a narrow band close to the interior 5. THE SALIVARY GLANDS. There are three surface of the tongue. salivary glands (Fig. 6), which are: Origin: the tip to the root of the tongue. 1. The parotis gland, Insertion: the hyoid bone. 2. The submandibular glands, 9 3. The . the transverse facial are above it. The deep surface is in relation with the 1. THE PAROTIS GLAND (GLANDULA mylohyoideus hyoglossus, styloglossus, PAROTIDEA). stylohyoideus, and posterior belly of the The superficial surface, slightly lobulated, digastricus muscles; in contact with it are the is covered by the integument, the superficial mylohyoid nerve and the mylohyoid and fascia containing the facial branches of the submental vessels. great auricular nerve and some small lymph 2. THE SUBMANDIBULAR GLANDS lie along glands, and the fascia which forms the the body of the mandible. The capsule of the gland. The external carotid arises from the portion artery lies at first on the deep surface, and of the gland that lies between the mylohyoid then in the substance of the gland. The artery and hyoglossus muscles. The submaxillary gives off its posterior auricular branch gland is irregular in form and about the size which emerges from the gland behind. of a walnut. A considerable part of it is Superficial to the are the superficial situated in the submaxillary triangle, reaching temporal and internal maxillary , uniting forward to the anterior belly of the digastricus to form the posterior facial vein; in the lower muscle and posterior to the stylomandibular part of the gland this vein splits into anterior ligament, which intervenes between it and and posterior divisions. The anterior division the . Above, it extends under emerges from the gland and unites with the cover of the body of the mandible. Posterior, anterior facial to form the common facial vein; it usually overlaps the intermediate tendon the posterior unites in the gland with the of the digastricus and the insertion of the posterior auricular to form the external jugular stylohyoideus muscles, while from its deep vein. On a still more superficial plane is the surface a tongue-like deep process extends , the branches of which emerge forward above the mylohyoideus muscle. from the borders of the gland. Branches of Superficial surface consists of an upper and the great auricular nerve pierce the gland to a lower part. The upper part is directed join the facial, while the auriculotemporal outward, and lies partly against the nerve issues from the upper part of the gland. submaxillary depression on the inner surface The parotid duct (ductus parotideus; of the body of the mandible, and partly on Stensen's duct) is about 7 cm long (Fig. 6). the pterygoideus medialis muscle. The lower The parotid duct is dense, its wall being of part is directed downward and outward, and considerable thickness; its canal is about the is covered by the skin, superficial fascia, size of a crow-quill, but at its orifice on the platysma muscle, and deep cervical fascia. oral surface of the cheek its lumen is greatly It is crossed by the anterior facial vein and reduced in size. It consists of a thick external by filaments of the facial nerve. In contact fibrous coat which contains contractile fibers, with it, near the mandible, are the and of an internal or mucous coat lined with submaxillary lymph glands. short columnar . It begins by The deep process of the gland extends numerous branches from the anterior part of forward between the mylohyoideus muscle the gland, crosses the masseter, and at the below and externally, and the hyoglossus and anterior border of this muscle turns inward styloglossus muscle internally; above it is the nearly at a right angle, passes through the lingual nerve and submaxillary ganglion. corpus adiposum of the cheek and pierces The submaxillary duct (ductus the buccinator muscle. It then runs for a short submaxillaris; Wharton's duct) is about 5 distance obliquely anterior between the cm. long, and its wall is much thinner than buccinator muscle and mucous membrane that of the parotid duct. It begins by numerous of the mouth, and opens upon the oral branches from the deep surface of the gland, surface of the cheek by a small orifice, and runs forward between the Mylohyoideus opposite the second upper molar tooth. While and the hyoglossus and genioglossus crossing the masseter muscle, it receives the muscles, then between the sublingual gland duct of the accessory portion; in this position and the genioglossus muscle, and opens by it lies between the branches of the facial a narrow orifice on the summit of a small nerve; the accessory part of the gland and papilla, at the side of the frenulum lingua.

10 Fig. 7. Pharynx. Fig. 8. Esophagus. On the hyoglossus muscle it lies between the the oral cavity (Fig. 7). The pharynx is located lingual and hypoglossal nerves, but at the posterior to the nasal and oral cavities and the anterior border of the muscle it is crossed and is the common route for air and food laterally by the lingual nerve; the terminal (conducts food to the esophagus and air to the branches of the lingual nerve ascend on its larynx and ). In its superior part, the pharynx medial side. receives the posterior openings of the nasal 3. THE SUBLINGUAL (RIVINUS'S OR cavities, called choanae. The pharynx SIZZANNE'S) GLANDS (GLANDULA communicates with three cavities: the nose, SUBLINGUALIS) are the most deep and the mouth, and larynx. The pharynx is divided into smallest of the big salivary glands, which lies three parts: in the floor of the mouth between the 1. THE NASOPHARYNX or EPIPHARYNX - mandible and the genioglossus muscle. The lies posterior to the nose and superior to the paired glands unite to form a horseshoe- soft palate and is a posterior extension of the shaped glandular mass around the lingual nasal cavities. It has a respiratory function. frenulum. Numerous small ducts (8 to 20) The nose opens into the nasopharynx by two open into the floor of the mouth. Sometimes large posterior apertures called choanae or one of the duct opens into the submandibular internal nares. They are separated by the duct. It is narrow, flattened, shaped bony . The roof and posterior somewhat like an almond, and weighs nearly wall of the nasopharynx form a continuous 2 gm. It is in relation, above, with the mucous surface that lies inferior to the body of the membrane; inferior, with the mylohyoideus sphenoid bone and the basilar part of the muscle; posterior, with the deep part of the . In the mucous membrane of submaxillary gland; laterally, with the the roof and posterior wall of the nasopharynx mandible; and medially, with the is a collection of lymphoid tissue - the pha­ genioglossus muscle, from which it is ryngeal tonsil. The pharyngeal orifice of the separated by the lingual nerve and the auditory tube is on the lateral wall of the submaxillary duct. Its excretory ducts are from nasopharynx. It has a hoodlike tubal elevation eight to twenty in number. Of the smaller over it - the torus of the auditory tube or sublingual ducts (ducts of Rivinus), some the torus tubarius inferiorly to which vertical join the submaxillary duct; others open fold of mucous membrane the separately into the mouth, on the elevated salpingopharyngeal fold. The collection of crest of mucous membrane (plica lymphoid tissue in the submucosa of the sublingualis), caused by the projection of the pharynx is the tubal tonsil. Posterior to the gland, on either side of the frenulum lingua. torus and the salpingopharyngeal fold, there One or more join to form the larger is a slitlike lateral projection of the pharynx sublingual duct ( Bartholin's duct), which called pharyngeal recess. opens into the submaxillary duct. 2. THE OROPHARYNX or MESOPHARYNX) - it extends from the soft palate to the superior THE PHARYNX border of the epiglottis. It has a digestive It is a funnel-shaped fibromuscular tube that is function and is connect with the oral cavity the continuation of the digestive system from through the oropharyngeal isthmus. The 11

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