Puncture of the cisterna magna apparently is a relatively easy clinical procedure, and the dangers associated with it are readily overlooked. Ayer, from the time of his earliest reports to the present, has not ceased to emphasize the precautions to be taken. A characteristic statement in his papers is as follows: "It must be remembered that those who have performed the puncture reported here thoroughly realize the possible dangers of the procedure. Without a knowledge of the subarachnoid spaces and an appreciation of the physiology of the cerebrospinal fluid, but especially without preliminary experience on the cadaver, cisternal puncture should not be undertaken."1
Sagittal section through the brain stem. The cerebellum is displaced caudally in fixation; in addition, there is a pronounced pressure cone. Note the two puncture wounds transfixing the medulla at the level of the olives and the hemorrhage into the fourth ventricle. The scattered gray areas