If we study the phenomena of sleep, we can easily imagine that there are links between the centres of the will and the muscles which may, in certain circumstances, be severed. We all know what nightmare is; we all remember the oppression we have suffered when, in a dream, we have felt ourselves suffocating under an immovable weight on the chest, or from a noose round the throat which we cannot unloose. These dreams, in which we feel ourselves paralysed, are a positive torture; the ground gives way under us and we are precipitated into an abyss; we fall while being pursued and cannot get up again; we find ourselves stretched out in the middle of the street and hear the approaching roll of wheels that will crush us, or see a horse gallop up to trample us with its hoofs. We cannot even scream, hands and feet try in vain to move, the oppression and despair increase until the nightmare passes and we awake with beating heart and laboured breath. Women and children overcome by violent fear turn their back, cover their eyes with their hands, or creep into a corner without looking behind them. In terror even the most intrepid men do not think of flight; it seems as though the nerves of defence were severed and they were left to their fate. Even in slight emotions we notice a partial failure of the power of the will over the muscles of the hand. Anyone weeping bitterly or laughing heartily cannot steady the pen between the fingers, and the writing is altered. Whytt noticed that after the head is cut off an animal, its excitability increases greatly after the lapse of a few minutes. The electric stimuli applied to the skin of the trunk immediately after decapitation do not produce any movement of reaction, but a few minutes later the same electric current causes vigorous movements of the legs. This unexpected phenomena gave rise to the belief that there were mechanisms in the spinal cord which, when irritated by the violent blow of the axe, were capable of arresting the reflex movements. But there are many other experiments which lead us rather to suspect the existence in the nerve-centres of some mechanism which, in certain conditions, nullifies the power of the will over the muscles. Anyone who has tritons in an aquarium can try the experiment of seizing hold of one by the leg with a pair of pincers; he will see that it remains motionless, almost rigid, for a few minutes. Frogs, when suffering a strong irritation of the sensory nerves, are no longer capable of making a single movement. There are also many other experiments which show us how, under the influence of violent and supernormal excitation, the molecular work of the cells of the spinal cord, requisite for the production of muscular movement through voluntary stimuli, is impeded. II