The first stage, the cephalic phase of digestion, begins with secretions from gastric glands in response to the sight and smell of food.

Saliva contains the digestive enzymes amylase, and lingual lipase, secreted by the salivary and serous glands on the tongue.

Digestion is helped by the chewing of food carried out by the muscles of mastication, the tongue, and the teeth, and also by the contractions of peristalsis, and segmentation.

Peristalsis is the rhythmic contraction of muscles that begins in the esophagus and continues along the wall of the stomach and the rest of the gastrointestinal tract.

This initially results in the production of chyme which when fully broken down in the small intestine is absorbed as chyle into the lymphatic system.

Mucous membranes vary in structure in the different regions of the body but they all produce a lubricating mucus, which is either secreted by surface cells or more usually by underlying glands.

The palate is hard at the front of the mouth since the overlying mucosa is covering a plate of bone; it is softer and more pliable at the back being made of muscle and connective tissue, and it can move to swallow food and liquids.

[7] The surface of the hard palate allows for the pressure needed in eating food, to leave the nasal passage clear.

Within the oral mucosa, and also on the tongue, palates, and floor of the mouth, are the minor salivary glands; their secretions are mainly mucous and they are innervated by the facial nerve (CN7).

[11][13] Sensory information can stimulate the secretion of saliva providing the necessary fluid for the tongue to work with and also to ease swallowing of the food.

Amylase starts to work on the starch in carbohydrates, breaking it down into the simple sugars of maltose and dextrose that can be further broken down in the small intestine.

If the taste is agreeable, the tongue will go into action, manipulating the food in the mouth which stimulates the secretion of saliva from the salivary glands.

The teeth are named after their particular roles in the process of mastication—incisors are used for cutting or biting off pieces of food; canines, are used for tearing, premolars and molars are used for chewing and grinding.

Mastication of the food with the help of saliva and mucus results in the formation of a soft bolus which can then be swallowed to make its way down the upper gastrointestinal tract to the stomach.

At the same time that protein is being digested, mechanical churning occurs through the action of peristalsis, waves of muscular contractions that move along the stomach wall.

The pylorus, the lowest section of the stomach which attaches to the duodenum via the pyloric canal, contains countless glands which secrete digestive enzymes including gastrin.

The parietal cells in the fundus of the stomach, produce a glycoprotein called intrinsic factor which is essential for the absorption of vitamin B12.

[5] Medicine treats the spleen solely as belonging to the lymphatic system, though it is acknowledged that the full range of its important functions is not yet understood.

Bile is released into the small intestine in order to help in the digestion of fats by breaking down larger molecules into smaller ones.

A milky fluid called chyle, consisting mainly of the emulsified fats of the chylomicrons, results from the absorbed mix with the lymph in the lacteals.

When the chyme is exhausted of its nutrients the remaining waste material changes into the semi-solids called feces, which pass to the large intestine, where bacteria in the gut flora further break down residual proteins and starches.

[33] In the large intestine,[2] the passage of the digesting food in the colon is a lot slower, taking from 30 to 40 hours until it is removed by defecation.

The celiac artery supplies the liver, stomach, spleen and the upper 1/3 of the duodenum (to the sphincter of Oddi) and the pancreas with oxygenated blood.

[35] The enteric nervous system consists of some one hundred million neurons[36] that are embedded in the peritoneum, the lining of the gastrointestinal tract extending from the esophagus to the anus.

By the end of the fourth week, the developing duodenum begins to spout a small outpouching on its right side, the hepatic diverticulum, which will go on to become the biliary tree.

A number of problems including malnutrition and anemia can arise from malabsorption, the abnormal absorption of nutrients in the GI tract.

In the middle of the 17th century, a Flemish physician Jan Baptist van Helmont offered the first chemical account of digestion which was later described as being very close to the later conceptualised enzyme.

Physiological research into secretion and the gastrointestinal tract was pursued with experiments undertaken by Claude Bernard, Rudolph Heidenhain and Ivan Pavlov.

The first to be discovered was secretin by Ernest Starling in 1902, with ensuing results from John Edkins in 1905 who first suggested gastrin with its structure being determined in 1964.

[56] Art historians have often noted that banqueters on iconographic records of ancient Mediterranean societies almost always appear to be lying down on their left sides.