2 hrS . AN L ES Ll - - - - 2 & hrS . - - - - - - -

Two problems confront the spinal anaesthetist when he makes his injection :i. Which are the spinal nerve roots carrying the pain impulses from the operation area. This problem will not be further considered. It involves basic anatomical knowledge with which the anaesthetist must make himself familiar. 2. How is the local anaesthetic, deposited in the lumbar region, to be brought into contact with these nerve roots. In the case of an abdominal operation the nerve roots to be anaesthetized may be some 9 in. or io in. away from the spinal puncture. An understanding of how the injected analgesic is brought into contact with the desired spinal roots is the very basis of spinal analgesia, and no apologies are needed for the manner in which it will be dealt. Consider a few simple, almost childish experiments which each of us can perform for himself in his own kitchen. For the first experiment a beaker of water is needed, a mark being made on the glass a little way up from the bottom. A long needle is inserted to the bottom of the glass and some coloured sugar solution injected into the water until the solution reaches half-way to the mark (Fig. iA). To bring the coloured sugar solution into contact with the mark, the obvious thing to do is to tilt the glass. The sugar solution responds to the pull of gravity and moves inside the water until it reaches the mark. This experiment demonstrates the ability of gravity to move one fluid inside another. If the contents of the glass are not stirred, the sugar solution will remain separate for many months.' In this instance, diffusion in the scientific sense of the word, that is, intermingling by molecular movement, is so slow that it can be ignored, for it must be reckoned in months, instead of minutes as in the case of gases. The term ' diffusion' therefore should not be used in speaking of the spread of spinal solutions. Now for another simple experiment. The same beaker is completely filled with water (Fig. 2A). Some coloured sugar solution is again injected. As the injection is made, some of the water spills out of the beaker (Fig. 2B). If sufficient solution is injected, its level will ultimately reach the mark on the beaker. This experiment teaches that any injected solutidn occupies space, and the extent of the glass wall with which it comes in contact depends on the volume of the solution.