For psychological purposes there are two major changes in recent ideas of nervous function. One concerns the single cell, the other an "arousal" system in the brain stem. The first I shall pass over briefly; it is very significant, but does not bear quite as directly upon our present problem. Its essence is that there are two kinds of activity in the nerve cell: the spike potential, or actual firing, and the dendritic potential, which has very different properties. There is now clear evidence (12) that the dendrite has a "slow-burning" activity which is not all-or-none, tends not to be transmitted, and lasts 15 to 30 milliseconds instead of the spike's one millisecond. It facilitates spike activity (23), but often occurs independently and may make up the greater part of the EEG record. It is still true that the brain is always active, but the activity is not always the transmitted kind that conduces to behavior. Finally, there is decisive evidence of primary inhibition in nerve function (25, 14) and of a true fatigue that may last for a matter of minutes instead of milliseconds (6, 9). These facts will have a great effect on the hypotheses of physiological psychology, and sooner or later on psychology in general. Our more direct concern is with a development to which attention has already been drawn by Lindsley (24): the nonspecific or diffuse projection system of the brain stem, which was shown by Moruzzi and Magoun (34) to be an arousal system whose activity in effect makes organized cortical activity possible. Lindsley showed the relevance to the problem of emotion and motivation; what I shall attempt is to extend his treatment, giving more weight to cortical components in arousal. The point of view has also an evident relationship to Duffy's (13). The arousal system can be thought of as representing a second major pathway by which all sensory excitations reach the cortex, as shown in the upper part of Fig. 1; but there is also feedback from the cortex and I shall urge that the psychological evidence further emphasizes the importance of this "downstream" effect. In the classical conception of sensory function, input to the cortex was via [p. 249] the great projection systems only: from sensory nerve to sensory tract, thence to the corresponding sensory nucleus of the thalamus, and thence directly to one of the sensory projection areas of the cortex. These are still the direct sensory routes, the quick efficient transmitters of information. The second pathway is slow and inefficient; the excitation, as it were, trickles through a tangled thicket of fibers and synapses, there is a mixing up of messages, and the scrambled messages are delivered indiscriminately to wide cortical areas. In short, they are messages no longer. They serve, instead, to tone up the cortex, with a background supporting action that is completely necessary if the messages proper are to have their effect. Without the arousal system, the sensory impulses by the direct route reach the sensory cortex, but go no farther; the rest of the cortex is unaffected, and thus learned stimulus-response relations are lost. The waking center, which has long been known, is one part of this larger system; any extensive damage to it leaves a permanently inert, comatose animal.