Fig. 4. In dogs anesthetized with halothane-oxygen, we placed a needle into the cisterna magna, removed 5-10 ml of cerebrospinal fluid, and injected an equal volume of air.12At time 0, we administered 75% nitrous oxide and monitored the associated increase in cisternal fluid pressure. Pressure increased to a relatively steady level over the next 10 min. This figure was reprinted from fig. 1of the original report (Anesthesiology 1965; 26:67-72).12