In the stillness and darkness of the night we are excluded from most of the external stimuli, yet a massive stream of systemic sensations keeps the sensitive mechanism active, and in sleep directs the dreams. The cramps and epileptiform attacks which occur during sleep are most probably due to the over-excitability produced by surplus carbonic acid. To temporary anæmia may be assigned the strange exaggeration of our sensations during the moments which precede awakening; and the greater vividness of dream-images. It is only needful to mention in passing the varied stimuli by which cerebral changes act upon the organism. The mention of a name will cause a blush, a brightening of the eye, a quickening of the pulse. The thought of her absent infant will cause a flow of milk in the mother’s breast. 175. We may formulate the foregoing considerations in another law: LAW II. The neural excitation, which is itself a change, directly causes a change in the organ innervated, and indirectly in the whole organism. The significance of this law is, that although for the convenience of research and exposition we isolate one organ from the rest of the organism, and one process from all the co-operant processes, we have to remember that this is an artifice, and that in reality there is no such separation. STIMULATION. 176. Passing now from these general considerations to their special application, we may formulate the law of stimulation: LAW III. A faint or moderate stimulation increases the activity of the organ; but beyond a certain limit, increase of stimulation diminishes, and finally arrests, the activity. Duration of stimulation is equivalent to increase. A muscle stimulated contracts; if the stimulation be repeated, the muscle becomes tetanized, and in this state has reached its limit; a fresh stimulation then _relaxes_ the muscle. A very faint stimulation of the vagus quickens the pulsation of the heart, but a slight increase, or duration of the stimulation, slackens and arrests the heart.[193] Every one knows how a moderate feeling of surprise, pleasure, or pain quickens the heart and the respiration; and how a shock of surprise, joy, grief, or great physical pain depresses, and even arrests them. Excess of light is blinding; excess of sound deafening.