To understand this we must recall what happened when we first learned the letters in their order. Someone repeated A, B, C, D to us over and over again, and we imitated the sounds. Sensory cells corresponding to each letter were awakened in succession in such wise that each one of them (by virtue of our second law) must have 'drained' the cell just previously excited and left a path by which that cell tended even afterwards to discharge into the cell that drained it. Let Sa , Sb, Sc in figure 89 stand for three of these cells. Each later one of them, as it discharges [p. 588] motorwards, draws a current from the previous one, Sb from Sa, and Sc from Sb. Cell Sb having thus drained Sa, if Sa ever gets excited again, it tends to discharge into Sb; whilst Sc having drained Sb, Sb later discharges into Sc, etc., etc. - all through the dotted lines. Let now the idea of the letter A arise in the mind, or, in other words, let Sa be aroused: what happens? A current runs from Sa not only into the motor cell Ma for pronouncing that letter, but also into the cell Sb. When, a moment later, the effect of Ma's discharge comes back by the afferent nerve and re-excites Sa, this latter cell is inhibited from discharging again into Ma, and
reproducing the 'primordial motor circle' (which in this case would be
the continued utterance of the letter A), by the fact that the process
in Sb, already under headway and tending to discharge into its own motor associate Mb, is, under the existing conditions, the stronger drainage-channel for Sa's excitement. The result is that Mb discharges and the letter B is pronounced; whilst at the same time Sc receives some of Sb's overflow; and, a moment later [p. 589] when the sound of B enters the ear, discharges into the motor cell for pronouncing C, by a repetition of the same mechanism as before; and so on ad libtum. Figure 90 represents the entire set of processes involved.