Human Digestive System consists of Alimentary Canal and the Accessory organs. Alimentary canal comprises organs of the digestive tract starting from Mouth, Pharynx and Esophagus, Stomach, Small Intestine (Duodenum, Jejunum, Ileum), Large Intestine (Cecum, Colon, Rectum) to Anus.

• Liver, Gall bladder and Pancreas are the accessory organs of the digestion.
• Salivary glands are associated glands for digestion.
• Teeth and Tongue are the accessory digestive organs.

Organs of the Digestive System

Mouth

• Mouth, also referred as Oral or Buccal cavity, is formed by the cheeks, hard and soft palates, and tongue and teeth.
• Lips or Labia are fleshy folds surrounding the opening of the mouth. The inner surface of each lip is attached to its corresponding gum by a mid-line fold of mucous membrane called Labial Frenulum.
• Oral vestibule (entrance to a canal) of the oral cavity is the space bounded externally by the cheeks and lips and internally by the gums and teeth.
• Palate is a wall or septum that separates the oral cavity from the nasal cavity, and forms the roof of the mouth. It makes possible to chew and breathe at the same time.
• Palate is divided into Hard palate and Soft palate.
• Hard palate is the anterior portion of the roof of the mouth (forms a bony partition between the oral and nasal cavities). Soft palate  forms the posterior portion of the roof of the mouth (arch-shaped muscular partition between the oropharynx and nasopharynx).
• Hanging from the free border of the soft palate is a finger-like muscular structure called the Uvula.
• During swallowing, the soft palate and uvula are drawn superiorly, closing off the nasopharynx and preventing swallowed foods and liquids from entering the nasal cavity.

Structure of the Mouth

Tongue

• Tongue is a freely movable muscular organ. It forms the floor of the oral cavity. It is an accessory digestive organ.
• Lingual frenulum is a fold of mucous membrane in the mid-line of under-surface of the tongue, which is attached to the floor of the mouth and helps in limiting the movement of the tongue
• Upper and lateral surfaces of the tongue are covered with Papillae. These are the elevations found on the tongue in which taste buds are present. These also increase the surface area of tongue and provide a rough texture to the upper surface of the tongue.
• Taste buds are the receptors for Gustation (taste). Some papillae lack taste buds, but they contain receptors for touch and increase friction between the tongue and food, making it easier for the tongue to move food in the oral cavity.
• Taste or gustation is a chemical sense. Three types of papillae contain taste buds:

1. Vallate papillae or Circumvallate papillae form an inverted V-shaped row at the back of the tongue. Each of these papillae houses 100–300 taste buds.
2. Fungiform papillae are mushroom-shaped elevations scattered over the entire surface of the tongue that contain about five taste buds each.
3. Foliate papillae are located in small trenches on the lateral margins of the tongue, but most of their taste buds degenerate in early childhood.

• Filiform papillae present on entire surface of the tongue. These pointed, threadlike structures contain tactile receptors but no taste buds. They increase friction between the tongue and food, making it easier for the tongue to move food in the oral cavity.
• Five primary tastes that can be distinguished: sour, sweet, bitter, salty, and umami. Umami taste is described as “meaty” or “savory.” Umami taste is stimulated by L-glutamate and nucleotides, substances present in many foods. Monosodium glutamate (MSG), added to foods as flavor enhancer, confers the umami taste to foods.

Tongue Showing Papilla and Taste