 Now, we talk about these various parts of digestive tract one by one. So, let us have a look on the human digestive system, its diagram then we will go ahead. In this diagram, you can see that the digestive system, the human alimentary canal starts from mouth, mouth is to ingest the food, ingest the food inside. Then comes the oral cavity, there are different parts of oral cavity. You can see that there are different types of glands associated with the oral cavity. There is a submandibular gland, there is a sublingual gland and there are salivary glands which are associated with this oral cavity. Then there is the tongue is important part of the oral cavity, the teeth are the part of oral cavity. Then comes the pharynx which is actually present on the back side of oral cavity, just below the nasal cavity and above the trachea and esophagus. Then we can see a long canal or long tube called esophagus. After esophagus we can see the stomach, esophagus actually enters inside the stomach. Then stomach enters inside the intestine, a highly convoluted small intestine which enters into a large intestine and that large intestine as we can see ends into a rectum and an innus. We also can observe in the diagram on the left side a large organ called liver and just below the liver the gallbladder, below the gallbladder is the pancreas. So, this is a generalized diagram of the digestive system of human beings. You can see two other parts attached to the large intestine. One of these is called cecum and the other is called appendix. Appendix is a vestigial organ in human beings. Appendix is of no use apparently sometimes it get infected and by a surgery we have to remove it. But in few other organisms appendix is very important for example in rabbits who have to eat up the vegetables. This cellulose, this cecum and appendix these are very long structures and they have certain enzymes called cellulases and certain bacteria which break down the cellulose present inside the plant material. So, they can acquire a lot much more energy from these plant materials in comparison to the human being. Now, we come next sensing the food as we know that acquiring food is itself very important property. When we start acquiring food first of all we sense it, we look at the food how it looks like, is it looks good in its appearance, we smell it, is it smells good or it smells bad, if it smells bad we reject it. If it smells bad then we assume that there is a problem in the food and we reject it. This is also a property of human beings, a quality, in the same way organisms, animals also in the same way if there is a smell in the food and it does not smell good then we reject it. So, we look at the sensory qualities of food as this is the first step of food selection. If food looks improper it is rejected, it is not ingested. Then comes the oral cavity, through mouth food enters inside the oral cavity. Oral cavity also have its own selection, our tongue have taste buds, when food enters inside the mouth we taste it, our taste buds feel it, that what is its taste. If it tastes bad it is again rejected, so this is another part of food selection that even it smells quite ok, it is looking ok, but its taste is bad then this is again rejected. So, food selection is a very important property of particularly the human digestive system. Now the second function of the oral cavity is to grind the food, we have teeth to grind the food. Because the next parts of the digestive system the alimentary canal the asafigas or stomach or intestines need smaller parts and food is there food that we take in or ingest by mouth is in larger pieces. Teeth help in grinding of the food, teeth grind the food and convert them into smaller pieces. Then the next property of oral cavity, your next function of oral cavity is to lubricate the food, that is add some water and salts to it. As we know that salivary glands, the submandibular glands and sublingual glands all of these produces some sort of mucous and they also have some enzymes for example, from salivary gland MILase enzyme is released which break down the starch and the mucous released by different enzymes and different glands lubricate the food that is make it wet. So that when it goes into the next parts of the alimentary canal it do not gives any resistance to those parts. Then another part of the oral cavity, tongue, tongue also have its own function, tongue have to roll up the food into the form of a bolus, tongue roll up the food and convert them into a rough circular mass which is called a bolus. This bolus can then go down the asafigas, the next part of the alimentary canal between the asafigas and the oral cavity, there is another part called pharynx which is present behind the oral cavity above the trachea and asafigas and below the nasal cavity. The pharynx have one important part called because we know that asafigas and the trachea the wind pipe which is going towards lung are going side by side and when food is going down from the oral cavity inside the asafigas body have to or organism have to make sure that this food should not enter inside the wind pipe or the trachea otherwise it will be blocked. There is a small part on the back of the oral cavity which is called epiglottis which is cartilaginous, when tongue make the food convert the food in form of a bolus and throw it back then the epiglottis goes down and close the wind pipe and the upper part of the oral cavity, the roof of the oral cavity goes slightly up and closes the nose so that food when it is going down towards the asafigas through pharynx cannot enter into the asafigas cannot enter into the wind pipe the trachea due to epiglottis because it is covering the trachea and it also cannot go into the nasal cavity because the upper part of oral cavity is closing the nasal cavity so food goes down into the asafigas. So, asafigas is the next part we call this process swallowing we are swallowing the bolus part of food. If we have a look on the diagram above we can see that there is tongue on the back of the tongue there is an extension which is called epiglottis and epiglottis is just present just above the trachea and on the back of this trachea we have asafigas. So, when tongue rolls up then the roof of the oral cavity goes up and block the nose so food cannot enter into the nasal cavity and this epiglottis you can see it is in slanting position towards upside it goes down and closes the trachea so that food directly goes down into the asafigas. Here in the diagram you can see the process of a bolus going down bolus is shown here with the help of a yellow color you can see in the first diagram A from left side a bolus is going down in the diagram B it is present it is passing from the pharynx and in this position you can see that epiglottis is closed and the nasal cavity side is also closed in the third part you can see the food is entering into the asafigas. Now the asafigas after pharynx asafigas starts asafigas is a muscular tube which on one side it is attached to the pharynx on the other side it is connected to the stomach. In the asafigas food is present in the form of the bolus which is coming from the oral cavity the muscles of the asafigas contract and relax at specific intervals so that the bolus move downwards that is from oral cavity towards the stomach and this happen due to the alternate contractions and relaxations of the muscles of the asafigas. Asafigas is specifically designed to move the food down we call this type of movement parastalysis. Sometimes we experience a situation that is opposite to the parastalysis we call it antiparastalysis when we are ill we are not well we experience a vomiting that is actually due to the antiparastalitic movements that is the muscles of this asafigas rather they are contracting and relaxing towards the stomach they contract and relax in an opposite direction that is from stomach towards the oral cavity. The result is this that food goes out of the stomach and asafigas from the oral cavity and out so we experience vomiting. This is the function of asafigas that is by parastalitic movement transferring food the bolus from the oral cavity to the stomach. Here in the diagram you can see a bolus is moving down the asafagas towards the stomach. One part of muscles of the asafagas you can see are contracting and the other parts they are relaxing in this way this parastalitic movements they continue. Now we talk about the stomach. Stomach is more precisely a storage organ stomach is for the temporary storage of the food. As you can see in the diagram in human beings stomach is continuous from asafagas and on the other side it enters inside the small intestine. Stomach have three parts as you can see in the diagram. Main part called cardiac the main body of the stomach called fundus and then come the third part of the stomach called pylorus. Cardiac is the part in which the asafagas enters. Then the pylorus is the main the fundus is the main body of the stomach and then the pylorus is the lower part of stomach which is enters entering inside the intestine. Now what are the functions of the stomach? Stomach also have highly contractile walls. Inside the stomach when food enters in the form of bolus the stomach releases three major things. One is bucus that is to lubricate the food further then its walls releases HCl the hydrochloric acid to acidify the food with the help of which microorganisms in present inside the food they are involved and an enzyme called pepsinogen. This enzyme when it is interacted with the HCl the hydrochloric acid released by the walls of the stomach convert the inactive enzyme pepsinogen into pepsin. So HCl performs dual function number one it gives the microorganisms in the food and it activates the inactive enzyme pepsinogen into pepsin. Pepsin is the enzyme that break down the proteins it means that digestion of the proteins starts inside the stomach. Now we know that digestion of start is already started inside the mouth because of the mylase enzyme produced by the salivary glands. Digestion actually continues in the stomach by the action of pepsin proteins are broken down into peptides. Secondly stomach by its contractions its strong contractions causes mixing of the contents of the food. Food which is present inside the stomach is mixed up with the mucus and HCl and the pepsinogen and the pepsin enzyme due to the contraction and relaxation of this walls of the stomach. We call it churning the churning process that is food is ground further and mixed up with the enzymes HCl and the mucus. Now one important thing stomach releases the HCl the hydrochloric acid which is a very strong acid is the microbes help the pepsinogen and convert it into pepsin but how the walls of stomach stomach itself are protected from the HCl. Actually the walls of stomach have a layer a thick layer of mucus present upon them which is secreted by the gastric cells itself. When we use them gastric is always something related to stomach. So the cells of the stomach the walls of the stomach releases mucus thick mucus which covers the layers of the or walls of the stomach and protect them from the action of hydrochloric acid. But sometimes what happened that if there is a abnormal condition stomach is not able to produce enough mucus or HCl is released in higher quantities HCl can start digesting the walls of the stomach and this may lead to a very serious condition called ulceration ulcer of the stomach. But normally walls of the stomach are covered by very thick layers of mucus and they are not affected by the hydrochloric acid.