But of all these important reactions in nervous tissues none bears so closely on the problem of the formation of reflex-arcs as that of Facilitation. This is equivalent to the Law of Neural Habit of the physiological psychologist, and is bound up with the highly important Law of Forward Direction, which Professor Starling says might as well be spoken of as the Irreciprocal conduction of nerve-arcs. The Law of Forward Direction of sensori-motor arcs is too well known to need here any description. But when this law is taken into account the phenomenon of Facilitation is seen to throw a strong light upon the earliest and rudimentary formation of specialized nerve-fibres, reflex-arcs and Final Common Paths leading to the effector glands or muscles. Facilitation is described shortly by Professor Starling as follows. If the passage of a nervous impulse across a synapse or series of synapses in the central nervous system be too often repeated, fatigue is produced, and there is an increase of the block at each synapse. If, however the stimulus be not excessive and the impulse not too frequently evoked, the effect of a passage of an impulse once is to diminish the resistance, so that a second application of the stimulus provokes the reaction more easily, and he adds that the result of summation of stimuli is in fact in the direction of removal of block. When an impulse has passed once through a certain set of neurones to the exclusion of others it will tend, other things being equal, to take the same course on a future occasion, and each time it traverses this path the resistance in the path will be smaller. Education then is the laying down of nerve-channels in the central nervous system, while still plastic, by this process of Facilitation along fit paths, combined with inhibition (by pain) in the other unfit paths. He makes the important statement that Facilitation is of great interest in connection with the development of “long paths” in the central nervous system and, _more especially with the acquirement of new reactions by the higher animals_. (Italics not in the original). Raw Materials of the Central Nervous System.