In this example each individual detail could have been easily traced to its source in memory. Pictures, and actually existent scenery furnished the detached items which were combined in a brain that for the time being was released from the control of the reason and the will. Irritated by painful sensations the brain was inordinately excitable, and sleep was less profound. Hence the remarkable intensity of the pictures which were presented to the eye of the mind. The indescribable richness and variety of the vision was probably due to the fact of extensive bodily disturbance, opening a wide range of territory from which impressions were communicated to the morbidly sensitive brain. The unusual permanence of the whole dream in memory may be explained by the observation of Maury, that the ease with which dreams are recollected varies inversely with the depth of the sleep in which they occur. Dreams which are produced in sound sleep are seldom recalled after waking, because they are but slightly connected with impressions received by the brain during wakefulness.[62] But dreams which occupy the mind when sleep is light and partial are excited by cerebral movements which are closely associated with external impressions that originate either at the moment of awaking or immediately after that event; consequently, the bond of union between the ideas of the dream and our waking ideas is nearly if not quite as perfect as the bonds which serve to connect the thoughts that occupy any portion of our conscious life. Hence such dreams are more easily reproduced from memory by any disposition that arouses a retrograde association of ideas.