The cerebrospinal fluid is contained within, and surrounds, the brain and spinal cord (Fig. 1). Thus, the internal fluid is contained within the ventricles —a set of intercommunicating cavities—while the external fluid occupies the subarachnoid spaces, i.e., the spaces on the surface of the brain and cord contained by the pia, internally, and the arachnoid membrane, externally. The internal fluid connects with the external fluid by way of one or more foramina. Thus, in man the principal connection is by way of theforamen of Magendie, which is essentially a gap in the roof of the IVth ventricle that permits flow out of this into the large adjacent subarachnoid space, the cisterna magna. In lower animals this foramen is not present, so that connection is made through the two foramina of Luschka, which are holes in the lateral recesses of the IVth ventricle opening into the subarachnoid spaces at the base of the brain. Each foramen is situated in the angle between the pons and medulla and opens into the cisterna pontis.