While I have dwelt on Ramon y Cajal's theory, because for years it has been familiar, of course I must re-echo his own warning that it is, after all, only a theory. It presupposes an active interposition of the glia cells between the axon of one neuron and the dendrons of another. This cannot be demonstrated. A third theory of mental operations, then, has been suggested, and the English school, so ably led by Sherrington ("Integrative Action of the Nervous System," London, 1903) and McDougal ("Synapse Theory of Fatigue," _Brain_, 1910) has deservedly attracted wide attention. They contend that all the phenomena can be more simply explained without postulating the movement required for the Duval Theory or the glial activity of Ramon y Cajal's hypothesis. They consider that each nerve cell has, as it were, a certain potential energy which it sends forth in nerve impulses. These are transferred from neuron to neuron through the synapse. If what we might call, to borrow a figure from electricity, the voltage of the cell impulse be sufficient to overcome the resistance at the synapse, the impulse passes from neuron to neuron. In fatigue the potential energy of the cell is gradually dissipated. The impulses become feebler till they cease to pass. This occurs in the state we usually experience as tiredness and in analogous states such as sleep, unconsciousness, narcosis and the like. Obviously this {124} theory can be elaborated and applied parallel with the neuroglia theory except that here we are substituting synapse resistance for the hypothetical, undemonstrated action of the glial cells. But, as the latter seems a simpler process upon which to explain the various phenomena, especially to those not familiar with very recent developments in nervous histology and studies in nervous mechanism, and as it merely involves a question of the nature of the resistance and not of its site, I have used it for explanatory purposes without advocating either theory in the present state of our knowledge. CHAPTER V BRAIN CELLS AND MENTAL OPERATIONS