Digestive System

• Tubular structures that collect and transport lymph from body tissues to the heart are known as lymphatic vessels.
• Valves are a feature of most of the larger lymphatic vessels, and are closer together than in veins.
• Valve cusps in the peripheral vessels point outward the node: flow is one-way from the periphery towards the hilus.
• The wall of the lymphatic vessels is similar to that in veins.
• The digestive system consists of organs involved in the prehension, mastication, ingestion, and physical and enzymatic digestion of food into small units absorbed through blood circulation and used to nourish the animals for growth and/or reproduction.
• Submucosa in the caudal part of the large intestine contains circumanal glands and glands of the anal sac.
• Adventitia may present anal or circumanal glands in some species.
• Muscularis is the inner circular smooth muscle layer forming the internal anal sphincter.
• Renewal, replacement and repair of the large intestinal epithelium is similar to that in the small intestine.
• Digestive organs are derived from the Stomodeal Ectoderm and Primitive Gut.
• Morphological differences exist between the digestive systems of mammals and birds.
• The mammalian digestive system consists of a long tubular structure called the alimentary canal or digestive tube and extramural glands derived from it.
• The oral cavity is the first part of the alimentary canal guarded by the lips, laterally by the cheeks, dorsally and caudally by the palate.
• The wall of the oral cavity consists of four tunics: Tunica Mucosa, Tunica Submucosa, Tunica Muscularis, Tunica Adventitia/Serosa.
• The oral cavity contains the tongue and teeth and receives the excretory ducts of major salivary gland (Parotid Gland, submandibular Gland, Sublingual Gland).
• The functions of the oral cavity include prehension of food, vocalization, manipulation, mastication, digestion, chemoreception, and swallowing.
• The lip is a mucocutaneous junction between the skin and mucous membrane of the oral cavity, presenting external and internal surfaces that are continuous with each other at the transition zone (free margin).
• The transition zone (free margin) of the lip is a narrow tip of the lip, covered by skin with no hairs and a thin transparent epidermis that thickens toward the mucous membrane of the internal surface.
• The internal (labial or buccal) surface of the lip is lined by the mucosa (mucous membrane), which is thicker than the mucosa of the external surface.
• The lamina propria (LP) of the lip is a typical loose connective tissue.
• The muscularis mucosa (MM) of the lip forms the core of the lip, containing the skeletal muscle orbicularis oris with fibers cut tangentially or transversely, a loose connective tissue and labial salivary glands.
• The adventitia of the lip is replaced by the hypodermis and dermis of the skin.
• Cementoblasts produce cementum.
• Enamel is the thickest in the crown where it is covered by enamel, and tapers down towards the root where it is covered by a thin layer of cementum.
• Enamel covers the entire dentin in the crown and root of hypsodont tooth.
• Hypsodont teeth do not possess definite crowns, roots and necks.
• Cementum is deposited on both the dentin and the enamel.
• The esophagus contains mucosa that presents longitudinal folds/ridges and furrows.
• Cementum is a thin covering of the root of the brachydont tooth, but doesn't project above the gum line.
• Cementum is histologically similar to bone but without canaliculi, Haversian system and blood vessels.
• Enamel also forms downward infoldings called enamel plicae, lateral to the infundibulum.
• Ameloblasts do not degenerate; enamel is continuously formed and the hypsodont tooth erupts continuously.
• Cementum is formed only after the tooth has completely erupted.
• The cementum and enamel in the occlusal surface of the tooth invaginate into the dentin to form an infundibulum.
• The esophagus is a muscular tube modified for voluntary and involuntary movement of food from the oropharynx to the stomach and vice versa (in ruminants).
• Hypsodont teeth are longer than brachydont teeth and include the cheek teeth of ruminants, all the teeth of horse, incisor teeth of rodents and canine teeth of pig.
• The muscularis of the esophagus is composed of inner circular and outer longitudinal muscle layers that usually begin as skeletal at the cervical region (voluntary control of swallowing) and change to smooth near the stomach.
• The oropharynx contains mucosa that presents longitudinal folds/ridges and furrows.
• In humans, the inner circular smooth muscle layer and outer longitudinal skeletal muscle layer are present in the cervical region; in the thoracic region the outer layer is mixed smooth and skeletal; in the caudal region the outer layer is completely smooth.
• Enamel has a radially striated appearance due to the presence of numerous minute canals called dentinal tubules.