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Published on : Mar 01, 2020 | Views : 2751 | Downloads : 5

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DIGESTIVE SYSTEM PART 1
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The gastrointestinal system The esophagus conducts food from the oral cavity to the stomach where fragmentation is completed and digestion initiated. Digestion is the process by which food is enzymaticahly broken down into molecules, which are small enough to be absorbed into the circulation. Ingested proteins are first reduced to polypeptides. They are further degraded to small peptides and amino acids, which can then be absorbed. In the large intestine water is absorbed from the liquid residue, which becomes progressively more solid as it passes towards the anus. The gastrointestinal system is primarily involved in breaking down food for absorption into the body. This process has five main phases: ingestion, fragmentation, digestion, absorption, and elimination of waste products. Ingestion and initial fragmentation of food occur in the oral cavity, resulting in the formation of а bolus of food, this is then conveyed to the esophagus by the action of the tongue and pharyngeal muscles during swallowing. Fragmentation and swallowing are facilitated by the secretion of saliva from three pairs of major salivary glands and numerous small accessory glands.
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The histology of the alimentary canal Mucosa The lumen of the alimentary canal is lined by an epithelium, deep to which is a loose connective tissue known as the lamina propria. This richly vascularized connective tissue houses glands as well as lymph vessels and lymphoid nodules. Surrounding this connective tissue coat is the muscularis mucosae, composed of an inner circular and an outer longitudinal layer of smooth muscle. The epithelium, lamina propria and muscularis mucosae are collectively called the mucosa. Sub mucosa The mucosa is surrounded by a dense, irregular fibroelastic connective tissue layer, the submucosa, this layer houses glands in the esophagus and duodenum. The submucosa also contain blood and lymph vessels as well as a parasympathetic neural plexus, Meissner’s submucosal plexus, which controls the motility of the mucosa and secretory activity of the glands.
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Muscularis externa The submucosa is invested by a thick muscular layer, the muscularis externa, responsible for peristaltic activity, which moves the contents of the lumen along the alimentary tract. The muscularis externa is composed of skeletal muscle in oral cavity, of smooth muscle in second part of alimentary canal and of skeletal muscle in rectum and anal canal and is usually organized in an inner circular and an outer longitudinal layer. A parasympathetic nerve plexus, Auerbach’s myenteric plexus, is situated between the two layers and regulates the activity of the muscularis externa. Serosa or adventitia The muscularis externa is enveloped by a thin connective tissue layernthat may or may not be surrounded by a simple squamous epithelium of the visceral peritoneum. If the region of the alimentary canal is intraperitoneal, it is invested by peritoneum, and the covering is known as the serosa; however, if the organ is retroperitoneal, it adheres to the body wall by its adventitia.
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The wall of the oral cavity is made up partly of bone and partly of muscle and connective tissue (lips, cheeks, the soft palate, and the floor of the mouth). These structures are lined by mucous membrane. The mucous membrane is lined by stratified nonkeratinized squamous epithelium, which rests on connective tissue. Over the alveolar processes (where the mucosa forms the gums) and over the hard palate, the mucous membrane is closely adherent to the underlying periosteum. Elsewhere it is connected to the underlying structures by loose connective tissue. In the cheeks, this connective tissue contains many elastic fibers and much fat (specially in children). Each lip has an "external" surface lined by skin and an "internal" surface lined by mucous membrane. The "external" surface of the lip is lined by true skin in which hair follicles and sebaceous glands are located. The internal substance of each lip (upper or lower) is predominantly muscular (skeletal muscle). The oral cavity
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The tongue has a dorsal surface, a ventral surface, and two lateral surfaces. Observation of its dorsal surface clearly delineates the tongue into two unequal regions, the larger anterior two thirds and the smaller posterior one third. The two regions are separated from one another by a shallow, V-shaped groove, the sulcus terminalis, whose apex points posteriorly and contains a deep concavity, the foramen caecum. The dorsal surface of the posterior one third of the tongue is uneven because of the presence of the lingual tonsils. Lingual papillae, most of which project above the surface, cover the anterior two thirds of the tongue’s dorsal surface. Tongue
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Lingual papillae Filiform papillae are numerous slender structures that impart a velvety appearance to the dorsal surface. These papillae are covered by stratified squamous keratinized epithelium and function in scraping food off a surface. The high degree of keratinization is especially apparent in the sandpaper-like quality of the cat tongue. Filiform papillae do not have taste buds. Each fungiform papilla resembles a mushroom, whose slender stalk connects a broad cap to the tongue surface. The epithelail covering of these papillae are stratified squamous nonkeratinized; thus the blood coursing through the subepithelail capillary loops is evident as red dots distributed randomly among the filiform papillae on the dorsum of the tongue. Fungiform papillae have taste buds on the dorsal aspect of their cap.
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There are 8 to 12 large circumvallate papillae in a V-shaped arrangement just anterior to the sulcus terminalis. These papillae are submerged into the surface of the tonge so that they surrounded by an epithelially lined groove, whose base is pierced by slender ducts of glands of von Ebner. The epithelial lining of the groove and the side of these papillae have taste buds. Foliate papillae are located along the posterolateral aspect of the tongue. They appear as vertical furrows, reminiscent of pages of a book. These papillae have functional taste buds in the neonate, but these taste buds degenerate by the second or third year of life.
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The bilateral palatine tonsils are located at the boundary of the oral cavity and the oral pharynx, between the palatoglossal and the palatopharyngeal folds. The deep aspect of each palatine tonsil is isolated from the surrounding connective tissue by a dense fibrous capsule. The superficial aspect of the tonsils is covered by stratified squamous nonkeratinized epithelium that dips into the 10 to 12 deep crypts that invaginate the tonsilar parenchyma. The crypts frequently contain desquamated epithelail cells, dead leucocytes, bacteria and other antigenic substances, and food debris. The parenchyma of the tonsils is composed of numerous lymphoid nodules, many of which display germinal centers, indicative of B cell formation. Palatine tonsils