“ The Lat. intervallum, from which the English word has come through the French, originally meant a space between the palisades on a rampart (vallum) , or between the rampart and the tents of the legionaries. In medical language “interval” is used of the intervening periods between attacks or paroxysms of a disease, particularly of the periods of a rational or normal condition of mind sometimes experienced by an insane person, a “lucid interval”; this phrase frequently occurs in legal documents from the 13th to the 15th centuries, non compos mentis sed gaudet lucidis intervallis. ”