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Mouth and buccal cavity are first organs (Fig 14a) which receives the food material. Mouth contains a set of teeth arranged in upper and lower jaw. Teeth are meant for cutting and mastication of food in finer particles, so that it can pass easily through narrow digestive tract. Tongue is mobile muscular organ and it helps in movement of food and taste perception through numerous taste buds. Third structure associated with month is set of salivary glands that secrete saliva. One pair of parotid glands located below the external ear contributes more than half of total saliva secreted into mouth. Other saliva secreting glands are submandibular (situated below the jaw bones) and sublingual (situated below tongue) glands. Pharynx is a common chamber located behind the mouth connecting oral cavity and oesophagus. Oesophagus is muscular tube like structure connecting pharynx and stomach and it passes through neck and thorax. Simple stomach is pouch like organ which is connected anteriorly to oesophagus through cardiac sphincter and posteriorly to duodenum through pyloric sphincter. Numerous glands called gastric glands are situated in the wall of stomach and produce gastric juice. Small intestine comprises of a long coiled tube extending between stomach and large intestine. It is subdivided into three segments namely duodenum (U-shaped tube in which pancreas is lodged), jejunum and ileum. Small intestine has a narrow lumen and produce intestinal juice through crypts of Liberkuhn which helps in final digestion of food ingredients. The secretion of exocrine pancreas (called pancreatic juice) and liver (called bile) are mixed with food in the duodenum through pancreatic and bile duct, respectively. Large intestine is much wider in diameter and further divided into three segments namely caecum, colon and rectum. Caecum is poorly developed or rudimentary in carnivores and man and serves no function. It is well developed in all herbivores and ruminants and responsible for microbial fermentation of cellulose and other plant materials. Colon is important in herbivores as it absorbs most nutrients arising from caecal fermentation. The formation of feces also occurs in colon. Rectum is a last organ of digestive tract and stores the feces till its expulsion. Feces are composed of indigestible food ingredients, dead cells of digestive tract, microorganisms and water.
The human digestive system is a complex series of organs and glands that convert our meals into body fuel when it passes along the digestive tract starting from mouth and ending with the anus. Along the way, the food is broken down, stored, and reprocessed before being circulated around the body to nourish and replace cells and supply energy to our muscles and different organs. It has also a role to excrete out the undigested waste material.
Most animals have one of three main types of diets: they are either carnivoresherbivores, oromnivores. Most humans fall into the last category. In humans, herbivores are usually called vegetarians. Strict vegetarians who consume no animal products, whatsoever, are called vegans. Some people are lacto-ovo-vegetarians, meaning that they also eat dairy products and eggs.
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