Fig. 3

Exposure of the fourth ventricle under microscope; the two cerebellar tonsils are retracted laterally to expose the floor of the fissure, i.e., inferior medullary velum and telachoroidea. The telachoroidea which forms the caudal part of the lower half of the roof of the fourth ventricle is incised from the foramen of the magendie and then followed laterally to the foramen of the luschka on both sides