Digestion by definition is the process by which food substances are chemically changed into forms that can be absorbed through cell membranes. The digestive tract, also known as the alimentary canal, extends from the mouth to the anus and includes several accessory organs that release secretions into the canal. The primary organs include the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. Accessory organs include the salivary glands, liver, gall bladder, and the pancreas.

The walls of the digestive tract contain four layers. The inner most layer is the mucous (mucosa) membrane. It serves to protect the other tissues by adding a layer of mucous and it also carries on absorption and secretion. The next layer is the submucosa which contains loose connective tissue along with blood vessels, lymphatic vessel, and nerves. The vessels serve to nourish surrounding tissues and to carry away absorbed materials. The next layer is the muscular layer that consists of two coats of smooth muscle tissue. One coat has circular fibers for decreasing the diameter of the digestive tube and the other has longitudinal fibers that shorten the tube. Finally, the outer layer called the serous membrane, is composed of the visceral peritoneum. It secrete serous fluid that keeps the tube's outer surface moist.

Food moves through the alimentary canal in two ways. The first, called mixing, occurs in the mouth and stomach. Mixing movements are rhythmic contractions that mix together the foods with various enzymes. The second movement is called peristalsis. These wavelike contractions move food through the esophagus and the intestines.

The next several paragraphs will discuss the primary organs of digestion beginning with the mouth. The function of the mouth is to receive food, prepare it for digestion, and to begin the digestion of starch. The oral cavity is the chamber between the palate and the tongue. The vestibule is the space between the teeth, cheeks and lips. The cheeks are composed of an outer layer of skin, pads of subcutaneous fat, muscles, and an inner lining of stratified squamous epithelium. The lips, on the other hand, are skeletal muscles and sensory receptors. Their reddish color is due to an abundance of blood vessels near the surface.

The mouth also holds the tongue which is attached at one end by the hyoid bone and at the other end by the frenulum, a tiny fold of skin attached to the floor of the mouth. On the surface of the tongue are rough projections called papillae which provide friction for handling of food. (How rough are these compared to a cat or cow? Would we have the same ability to lap milk from a bowl??) These also contain the taste buds. Check page 256 in your book to locate the four types of taste buds, sweet, sour, bitter and salty. Found at the root of the tongue are the lingual tonsils which are lymph tissue. (These are not the tonsils that you would have removed!)

The palate forms the roof of the mouth. The hard palate is anterior to the soft palate. The uvula hangs down from the soft palate and together they close the nasal cavity when swallowing. (This is why food can come out the nose when you laugh and eat at the same time!) The palatine tonsils are lymph tissue found on either side of the palate. (These are the ones that get inflamed and may be removed.) The pharyngeal tonsils are also known as the adenoids. They are found on the posterior wall of the pharynx, above the border of the soft palate.

The teeth are also found in the mouth (dah!!) and function in breaking food into smaller pieces. This allows digestive enzymes to react more effectively with the food molecules. Teeth fit into sockets called alveoli. The gums, known as gingiva, are composed of stratified squamous epithelium and dense fibrous connective tissue. The periodontal membrane lines the alveoli (sockets). We have 20 baby or deciduous teeth. These include 8 incisors (4 in each jaw), 2 canines or cupids, and 4 molars. We have 32 secondary or permanent teeth. The 8 incisors are adapted for cutting. The 4 cuspids (canines) are used for tearing food. The 8 bicupids (premolars) are for grinding food. The 12 molars are for crushing and grinding food.

The structure of a tooth consists of the crown, neck and root. The crown is the part of the tooth above the surface of the gum. It is covered in enamel which is the hardest material in the body. Dentin composes most of the insides of the crown. It is a hard, calcified substance. The neck is the middle section of the tooth right at the gum line. The root is below the gum. It is covered by cementum which anchors the tooth to the periodontal membrane. The pulp cavity is the central region of the root where the blood vessels, nerves and pulp are located. The root canals are tubes that extend into the roots. A hole is located at the end of each root canal through which the blood and nerves enter the tooth. This is called the apical foramen. (Do you remember what a foramen is?) If you go to the dentist and need a "root canal" done, guess what the dentist is about to do to you!!!! Check out the diagram on page 416 in your text.

The salivary glands are also located in the mouth region. They function in secreting saliva, moistening food and bringing it together. They also begin the digestion of carbohydrates and the dissolving of various food chemicals which are necessary for us to taste. They produce two types of secretions from cells which are named for the secretion. The serous cells secrete serous fluid, a watery fluid that contains the enzyme, amylase. This enzyme splits starch and glycogen into disaccharides. The second secretion is mucous from mucous cells. This is a thick, stringy liquid that binds food particles together and acts as a lubricant during swallowing.

The three salivary glands are the parotid, the submaxillary (submandibular) and the sublingual. The largest of the three is are the parotids, located in front of and somewhat below each ear, between the skin of the cheek and the masseter muscle. The parotids secrete serous fluid. The submaxillary glands are located in the floor of the mouth of the inside surface of the jaw. They secrete both serous and mucous. The sublingual glands are the smallest of the salivary glands. They secret mostly mucous. Check out the location of the glands by looking at the diagram on page 417 in your textbook.

The pharynx is the throat, the cavity behind the mouth connecting to the esophagus. It is divided into three parts. The nasopharynx is the upper portion that is contiguous with the nasal cavity and provides a passageway for air during breathing. The oropharynx is the middle portion that functions as a passageway for food moving from the mouth and for air moving to and from the nasal cavity. Finally, the layngopharynx is the lower portion which opens into the larynx and esophagus.

THE STEPS IN SWALLOWING ARE LISTED ON YOUR OUTLINE.

The esophagus is the foodtube. It begins at the base of the pharynx and descends behind the trachea, ending at the stomach. There is an opening in the diaphragm through which the esophagus passes called the esophageal hiatus. Sometimes this gets inflamed and will close as the food descends. People who have this problem have a hiatal hernia.

The stomach receives food from the esophagus, mixes it with gastric juices, initiates the digestion of proteins, carries on some absorption and moves food into the small intestine. It consists of four major regions. The first is the cardiac region which is a small area near the esophageal opening. The opening itself is called the cardiac orifice and a small muscle known as the cardiac sphincter allows food to enter while trying to prevent it from going back up. The fundus region bulges above the entrance to the esophagus. It acts as a temporary storage area and sometimes becomes filled with swallowed air. This is how 'burps' form!! The body is the main region of the stomach it is located between the fundus and the pyloric regions. The pyloric region joins the small intestine. The canal is a narrow portion of the pyloric that connects with the small intestine. The sphincter allows food to go one way. . .to the small intestine.

Rugae are folds in the stomach that allow it to expand as food is placed into it. There are gastric pits in the lining of the stomach in which gastric glands are located. These glands contain three types of cells--the mucous cells (secrete mucous giving protection to the stomach from the other acids), chief cells (secrete digestive enzymes, including pepsinogen which begins the digestive of proteins) and parietal cells (secrete hydrochloric acid). Together the products of these three cells produce gastric juice. Chyme is the substance produced from the partial digestion of the food and the gastric juice. You would see this if you vomit. The scientific term for the vomiting reflex is emesis. The vomiting action is triggered by the medulla of your brain. You take a deep breath, raise the soft palate, relax the cardiac sphincter, contract the diaphragm and then contract the abdominal muscles. You know the rest!

The small intestine is the longest digestive organ at well over 20 feet. It receives secretions from the pancreas and liver, completes the digestion of the nutrients in chyme, abosorbs the various products of digestion, and transports the remaining residue to the large intestine. The small intestine is divided into three sections. The duodenum is the first 10 inches. It receives the secretions from the pancreas and the liver, as well,as from its own walls. This the where most digestion occurs. The second section is the jejunum and is 8 feet long. The ileum, 12 feet long, is the last part of small intestine. The jejunum and the ileum absorb digested food.

The mesentery is a double-layered fold of peritoneum that contains the blood vessels, nerves and lymphatic vessels that supply the intestinal wall and connect the jejunum and ileum. In addition, the intestines are lined with villi, finger-like projections that increase the intestines' absorbing area.

The large intestine is so named because it is larger in diameter. It is only 5 to 6 feet in length. It begins in the lower right side of the abdominal cavity, where the ileum joins the cecum, then travels upward on the right side, across to the left, and then descends into the pelvis. It functions in secreting mucous to protect the intestinal wall, it holds particles together, and it absorbs the proper amount of water. It is divided into several parts. The first 2 to 3 inches is called the cecum. It contains the appendix that projects downward from the cecum. This structure is a narrow closed ended tube that sometimes gets infected and may even rupture.

The ileocecal valve allows the food to enter from the small intestine. The colon begins after the cecum. It is divided into the ascending where it travels upward against the posterior abdominal wall just below the liver; the transverse where it turns left and goes across; the descending where it turns downward near the spleen; and the sigmoid where it makes an S-shaped curve before becoming the rectum.

The rectum lies next to the sacrum and attaches to it by the peritoneum. It ends about 2 inches below the tip of coccyx where it becomes the anal canal. This canal is formed by the last 2 or 3 inches of the large intestine. There are two sphincters here. The internal anal sphincter is made of smooth muscle and is therefore, involuntary. The external anal sphincter is make of skeletal muscle. You can control this muscle (except under dire emergencies!) The end result is the production of waste products called feces. Feces is composed of 75% water, indigestible wastes, electrolytes, mucus and bacteria. It is the bacteria that gives the feces its distinctive odor.

One of the accessory organs of the digestive system is the pancreas. Remember that it is also an endocrine gland?!? The pancrease is located beneath the stomach, in the C-shaped curve of the small intestine. It function in secreting the pancreatic juices that aids in the digestion of carbohydrates, fats, proteins, and nucleic acids.

A second accessory organ is the liver. It is located in the upper right portion of the abdominal cavity, just below the diaphragm. It is the largest gland in the body, weighing 3 to 4 pounds. It is reddish brown in color and is well supplied with blood vessels. It is divided into two lobes, the right and the left with the right being much larger. The liver detoxifies the blood by removing poisonous substances like drugs and alcohol by phagocytosis. It is also involved in the metabolism of excessive glucose, amino acids, and fats.

Every day the liver secretes a pint of bile. This is a yellowish green liquid that breaks down fats. It is composed of water, bile salts, bile pigments, cholesterol, and electrolytes. It is stored in the gall bladder which is attached to the ventral surface of the liver by the cystic duct. The common bile duct is formed by the union of the hepatic and cystic ducts. This leads to the duodenum where a sphincter keeps the bile from backing up.

The entire abdomen is lined with a membrane called the peritoneum. It has two layers, the parietal that lines the abdominal cavity and the visceral that covers the organs. Other membranes include the mesentery that attaches the small intestine, the omenta that attaches the stomach to the intestine and ligaments that attach the liver and spleen to the stomach, intestines, and kidneys.