Finally, we must remark that, if every external or internal sensation, whatever its nature, is a transmission of movements coming from without, a new acquisition for the nervous system and the brain, _every_ sensation ought at first to produce at least a momentary increase of energy. Féré, who has foreseen the possibility of this objection, admits in all cases a primary excitement. “If there appear to be cases in which phenomena of depression arise suddenly and subsist by themselves, it is only because they have been insufficiently observed.”[63] There would then be a very short phase of increase, immediately masked, according to him, by the phase of diminution. The physiologists, as we have seen, are always inclined to explain pain by intensity of sensation; but, if we take into account its nature, its quality, and the susceptibility of the nervous system to certain modes of motion received, nothing prevents the loss being immediate. Footnote 63: _Pathologie des émotions_, p. 226. Meynert, in his _Psychiatrie_, is the only writer who has attempted to advance any nearer to an explanation, and to determine the _mechanism_ which produces pleasure and pain. His hypothesis, in its principal points, is as follows:— As far as pain is concerned, his theory may be summed up as an arrestive action of two categories of reflex movements, motor and vascular. The painful state is the translation into consciousness of this physiological mechanism. 1. _Motor reflexes._—Let us suppose the head of a sleeping child to be slightly tickled. As the child’s sleep is sound, and there is no pain, nothing takes place but a slight withdrawal of the hand. If we suppose a slight prick, there will be few movements, and these limited to a small part of the body. But if we suppose some severe pain, such as the extraction of a tooth, or a burn extending over a large portion of the skin, the result will be prolonged and terrible reflex movements in _all_ parts of the body, which (in our opinion) may be considered as defensive. So much for external facts; what is taking place internally? We know that vibrations are conducted slowly in the grey matter (twelve times as slowly, according to Helmholtz, as in the white). When an excitation increases, as we have seen, the number of muscular groups set in motion, resistance to transmission increases in the same proportion. “The sensation of pain presupposes a reflex movement and an arrest of nervous conduction in the grey substance of the spinal marrow.” It is this process of inhibition, in varying degrees, which is felt by the consciousness as pain.