What is Digestion
The process by which complex food is converted into simplest food with the help of digestive enzymes(Hydrolytic enzymes) is called digestion.Hence process of digestion is a hydrolytic process. Type of digestion (1) Intracellular digestion: When the process of digestion occurs within the cell in the food vacuole.Examples: Protozoa,porifera,Coelenterata and free living platyhelminthes. (2)Extracellular digestion: When the process of digestion occurs outside the cell.Examples:Coelenterata and phylum platyhelminthes to phylum chordata. Digestive system of human Digestion in vertebrates occurs in the digestive tract or alimentary canal.The various parts involved in digestion can be broadly grouped in the two groups- (i) Digestive tract or alimentary canal (ii)Digestive glands. Digestive tract or alimentary canal On the basis of the embryonic origin,the alimentary canal of vertebrates can be divided into three parts- (1)Fore gut/Stomodaeum: Ectodermal.It includes buccal cavity/oral cavity, pharynx,oesophagus,stomach and small part of duodenum. (2) Mid gut/Mesodaeum: Endodermal.It includes small intestine and large intestine. (3)Hind gut/Proctodaeum: Ectodermal.It includes anal canal and anus. Part of alimentary canal and its histology Mouth: The mouth is a transverse slit bounded by two movable lips or labia,upper lip and lower lip. Upper lip has small ridges on the sides,a tubercle in the middle and a vertical groove(philtrum) above. Vestibule: It is a narrow space between lips and gums in front and gums and cheeks on the sides. Its lining contains mucous glands.In the vestibule,a small median fold of mucous membrane,the superior labial frenulum,connects the middle of the upper lip to the gum and usually a similar but smaller inferior labial frenulum connects the middle of the lower lip to the gum.