 Coming to the last but not the least important circumventricular organ. This is not located anywhere around the third ventricle, this is located in the floor of the fourth ventricle. So to give you a quick overview of the floor of the fourth ventricle, this is the superior medullary vealum which is the roof of the fourth ventricle. This is the inferior medullary vealum which is the inferior part of the roof of the fourth ventricle. And this is the rhomboid fossa composed of the pons and the medulla. So this is the floor of the fourth ventricle. Now just at the inferior angle of the floor of the fourth ventricle through which the fourth ventricle communicates with the foramina magindai, just adjacent to that there is a circumventricular organ which is known as the aria postrema. This aria postrema has got modified astrocyte like neurons. It is covered with fenestrated capillaries and this has got 5 HD3, 5 hydroxytryptamine, 3 excitatory ionotropic receptors which means they respond to serotonin. Once there is excessive levels of serotonin in the blood or whether there is any other toxic substances like morphine or oxytocin, they all stimulate the aria postrema and therefore they induce vomiting. So therefore they play a role in determining toxins in the blood. This aria postrema also receives afference from the chemoreceptor trigger zone and also receives afference from the GIT. So therefore if we eat any toxic substances or any noxious substances they send impulses to the aria postrema and cause us to vomit. This is also, it is postulated that the aria postrema also plays a role in food intake and cardiovascular system control. So this is the last but not the least important circumventricular organ.