The dictum has been formulated that poliomyelitis virus has an almost obligate affinity for gray nerve fibers.1 When the virus is injected into medullated peripheral nerves of a normal Macaca mulatta monkey its spread is resisted, unless there has been previous manipulation whereby gray-fibered axis-cylinders have become exposed.2 Even under these conditions, the disease is difficult to produce. It may be concluded, therefore, that poliomyelitis virus has but a slight affinity for the exposed axons of peripheral medullated nerves, and, by inference, it might be deduced that its spread to the central nervous system is stopped by the myelin contained in the sheath which covers the gray fibers. This would explain why there is not in every case of poliomyelitis involvement of the vagus nerve, which is actually a bundle of well medullated connector fibers, the postganglionic unmedullated gray fibers of which are found in the walls of