Fig. 43-11 The course of the cerebrospinal fluid (C.S.F.). Arrows lead from the choroid plexuses of the lateral and third ventricles toward the aqueduct. The fluid thereby formed is joined by that produced in the fourth ventricle and passes through the median aperture to the cerebellomedullary cistern of the subarachnoid space. The fluid then extends (1) upward around the brain and (2) downward around the spinal cord. The inset (a coronal section at the sagittal suture) shows the drainage of the C.S.F. into the venous system (superior sagittal sinus,S, and lateral lacunae, L, by way of arachnoid granulations, G). Various adjacent vessels are also included. A, cerebral artery; C, cerebral vein; 0, diploic vein; E, emissary vein; M, meningeal vein; 3 and 4, third and fourth ventricles. The lowermost part of the figure shows the caudal end of the spinal cord. lumbar puncture (L.P.) is performed in the part of the subarachnoid space that lies below the termination of the spinal cord. Ct. fig. 41-1.