Food enters the digestive tract through the mouth. The labia (lips) protect its anterior opening, the cheeks form its lateral walls, the hard palate forms its anterior roof, and the soft palate forms its posterior roof. The space between the lips and cheeks externally and the teeth and gums internally is the vestibule. The area contained by the oral cavity is the oral cavity proper. The tongue occupies the floor of the mouth and has many bony attachments. As food enters the mouth, it is mixed with saliva and masticated (chewed). This is where the breaking down of food begins.
From the mouth, food passes posteriorly into the pharynx, which is the common path for food, fluids, and air. The pharynx is divided into three sections: the nasopharynx (air from the nose passes here), the oropharynx (food and air from the mouth passes here), and the laryngopharynx (air going to the lungs passes here).The walls of the pharynx contain two skeletal muscle layers. The cells of the inner layer run longitudinally; those of the outer layer (the constrictor muscles) run around the wall in a circular fashion. Alternating contractions of these muscle layers propells food through the pharynx into the esophagus below. This propelling mechanism is called peristalsis.
The esophagus runs from the pharynx through the diaphram to the stomach. The esophagus conducts food to the stomach by peristalsis. Begining with the esophagus, the walls of the GI tract have a basic pattern that reflects their common functions. Because the tissue arrangement in the alimentary canal walls is modified along its length to serve special functions, here are the basic wall functions for reference.
The walls of the alimentary canal organs from the esophagus to the large intestine have four characteristic layers:
1- The mucosa is the innermost layer. Its a moist membrane that lines the cavity or lumen of the organ. It consists primarily of a surface epithelium, plus small amounts of connective tissue, and a scanty smooth muscle layer. 2- The submucosa is just beneath the mucosa. It is a soft connective tissue layer containing blood vessels, nerve endings, and lymphatic vessels. 3- The mascularis externa is muscle layer made up of a circular inner layer and a longitudinal outer layer of smooth muscle cells. 4- The serosa is the outermost layer of the wall. It consists of a single layer of flat serous fluid-producing cells, the visceral peitoneum. All layers of the alimentary canal wall except the mucosa contain a nerve plexus, and intrinsic network of nerve fibers that is actually part of the autonomic nervous system.
These plexuses help to regulate the mobility of the GI tract organs Stomach
The stomach is a muscular, hollow, dilated part of the digestion system which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects (mid-gut), and molluscs. It is involved in the second phase of digestion, following mastication (chewing). The stomach is located between the esophagus and the small intestine. It secretes protein-digesting enzymes and strong acids to aid in food digestion, (sent to it via oesophageal peristalsis) through smooth muscular contortions (called segmentation) before sending partially digested food (chyme) to the small intestines.
The small intestine (or small bowel) is the part of the gastrointestinal tract following the stomach and followed by the large intestine, and is where much of the digestion and absorption of food takes place. In invertebrates such as worms, the terms “gastrointestinal tract” and “large intestine” are often used to describe the entireintestine. This article is primarily about the human gut, though the information about its processes is directly applicable to most placental mammals. The primary function of the small intestine is the absorption of nutrients and minerals found in food.  (A major exception to this is cows; for information about digestion in cows and other similar mammals, see ruminants.)
large intestine The large intestine (or large bowel) is the last part of the digestive system invertebrate animals. Its function is to absorb water from the remaining indigestible food matter, and then to pass useless waste material from the body. This article is primarily about the human gut, though the information about its processes are directly applicable to most mammals. The large intestine consists of the cecum, colon, rectum and anal canal. It starts in the right iliac region of the pelvis, just at or below the right waist, where it is joined to the bottom end of the small intestine. From here it continues up the abdomen, then across the width of the abdominal cavity, and then it turns down, continuing to its endpoint at the anus. The large intestine is about 4.9 feet (1.5 m) long, which is about one-fifth of the whole length of the intestinal canal.
Rectum and Anus The rectum provides temporary storage for feces before they are expelled. As the rectal walls expand due to collecting feces, stretch receptors in the rectal walls stimulate the desire to defecate. Peristaltic waves then push the feces out of the rectum. The anus controls the expulsion of the feces. The flow of feces through the anus is controlled by the anal sphincter muscle. The internal and external sphincter muscles relax, allowing the feces to be passed by muscles and pulling the anus up over the exiting feces.
The accessory organs of digestion include the salivary glands, pancreas, liver, and gallbladder. As stated earlier, during the digestive process, the accessory organs produce secretions that assist the organs of the alimentary canal.
The salivary glands are located in the mouth (fig. 1-53). Within the salivary glands are two types of secretory cells, serous cells and mucous cells. The serous cells produce a watery fluid that contains a digestive juice called amylase. Amylase splits starch and glycerol into complex sugars. The mucous cells secrete a thick, sticky liquid called mucus. Mucus binds food particles together and acts to lubricate during swallowing. The fluids produced by the serous and mucous cells combine to form saliva. Approximately 1 liter of saliva is secreted daily.
Pancreas The pancreas is a large, elongated gland lying posteriorly to the stomach (fig. 1-53). As discussed earlier in “The Endocrine System,” the pancreas has two functions: It serves both the endocrine system and the digestive system. The digestive portion of the pancreas produces digestive juices (amylase, proteinase, and lipase) that are secreted through the pancreatic duct to the duodenum. These digestive juices break down carbohydrates (amylase), proteins (proteinase), and fats (lipase) into simpler compounds.
Liver The liver is the largest gland in the body. It is located in the upper abdomen on the right side, just under the diaphragm and superior to the duodenum and pylorus (fig. 1-53).
Of the liver’s many functions, the following are important to remember: · It metabolizes carbohydrates, fats, and proteins preparatory to their use or excretion. · It forms and excretes bile salts and pigment from bilirubin, a waste product of red blood cell destruction. · It stores blood; glycogen; vitamins A, D, and B12; and iron. · It detoxifies the end products of protein digestion and drugs. · It produces antibodies and essential elements of the blood-clotting mechanism.
The gallbladder is a pear-shaped sac, usually stained dark green by the bile it contains. It is located in the hollow underside of the liver (fig. 1-53). Its duct, the cystic duct, joins the hepatic duct from the liver to form the common bile duct, which enters the duodenum. The gallbladder receives bile from the liver and then concentrates and stores it. It secretes bile when the small intestine is stimulated by the entrance of fats.