This is divided into an anterior and a posterior part. From the roof of the anterior division arises the cerebellum, and from its floor the pons Varolii.

The posterior division gives rise to the medulla oblongata.

The cavity of this vesicle is called the fourth ventricle. It is continuous with the central canal of the spinal cord. Its upper wall is thinned and forms the valve of Vieussens. It communicates with the subarachnoid space through the foramen of Magendie.