none the less the result of abiding sensory impressions graven in the substance of the brain, from which, through the action of memory, they may at any favorable moment reënter consciousness. Sleep does not wholly arrest this process. A certain amount of projection into the field of consciousness continues, even during profound repose; and the ideas thus aroused form the material of our dreams. It has already been remarked that the invasion of sleep is not an instantaneous process. One by one the senses fall asleep, and long before the final cessation of their activity, sleepiness hinders their function. Hence a progressive narrowing of the range of external perception; hence a reduction of the vividness of impressions derived from the outside world; hence, also, a simplification of the actions and reactions which constitute the “polygon of forces” active within the brain. But the suppression of certain lines in this polygon does not suppress life, nor does it necessarily destroy consciousness. It only occasions a redistribution of force, and a proportionate narrowing of the stream of related ideas. Since this process of suppression, just mentioned, is not an absolute quantity, but a variable factor, the polygon of physical forces within the brain and the corresponding succession of ideas in consciousness must necessarily be in a state of continual change. Consequently, our dreams must be as variable as the clouds that drift upon the currents of the air. As, on a hot day in summer, when the equatorial draught has ceased to guide the wind, we may observe all manner of local tides among the masses of vapor which arise from the earth, so, in sleep, when the guiding influence of the senses is withdrawn, the ideas that still arise are chiefly dependent for their origin and association upon the automatic and endogenous activities of the brain. Undisturbed by impulses from the external world, the brain seems then to become more sensitive to impressions that have their origin within the body. An overloaded stomach, an enfeebled heart, a turgid sexual apparatus, or an irritable nervous ganglion, may become the source of irregular and uncompensated impulses which, without disturbing the organs of special sense, may invade the cerebral cortex, and may there set in motion a whole battery of mechanisms whose influence upon consciousness would remain quite unnoticed were the external senses in full operation.