 Okay, so before I start describing the circumventricular organs, let me give you a quick overview of what is the structure of these circumventricular organs, the microscopic structure. These circumventricular organs in general have got a certain few characteristics which are very specific. First of all, they are highly vascularized, secondly, they have got fenestrations of their endothelial cells of the capillaries, they are surrounded by a permeable basement membrane and the surrounding photosites of the astrocytes, the foot processes or the perivascular foot processes do not form a complete circle. So these characteristics enable blood and sampling of the CSF by the circumventricular organs. The only notable exception as we shall see a little later is the subcommissural organ which does not have fenestrated capillaries. With this few background, let's start with the circumventricular organs and we will follow a clockwise direction around the third ventricle. So what you are seeing in front of you is the third ventricle. Just to give you a quick overview, this is the lateral wall of the third ventricle which is formed by the hypothalamus, the lower one third and the upper two thirds is formed by the thalamus, this is the interthalamic adhesion, this is the midbrain here, this is the pons, this is the medulla, this is the aqueduct of sylvius which connects the third ventricle to the fourth ventricle. So first of our description will be restricted to the third ventricle and only a little bit of it we shall go into the fourth ventricle. And we shall follow the circumventricular organs in a clockwise direction. So let's start with the first one. The first circumventricular organ is