Closely akin to these states are expressions of degeneracy manifesting themselves with some approach to regularity in periods, as in epilepsy and the periodical insanities. The periodical insanities may be simply emotional states of exaltation (as in mania), of extreme depression (as in melancholia), of stupor, or of mental confusion. They may show themselves in periodical acts, as in dipsomania. This condition differs from the condition called inebriety in the fact that it is a periodical expression of degeneracy whose form has been accidently determined, but which would exist even were its form changed. The differences are excellently outlined by Dana,[261] who divides intemperate drinkers into four classes: Periodical inebriates or dipsomaniacs, pseudo-inebriates, common drunkards and victims of delirium inebriosum. The disease in the first class is a periodical insanity. In the pseudo-inebriates the desire for drink is only one of many manifestations of a weakened constitution or inherently unstable nervous system. The third type are those which approximate the occasional criminals. Another periodical insanity is kleptomania, in which insane stealing occurs at intervals of greater or lesser regularity. Nymphomania or satyrasis is a periodical insanity in which there is an insane impulse for sexual intercourse. Pyromania is a periodical tendency to commit arson. All of these periodical conditions may occur alone or in combination with other degeneracies. Closely related to the periodical insanities are the epileptic states which play so large a part in many of the phenomena presented by the degenerates. In the epileptic the mental rather than the gross nervous expression merits attention. From what has already been said about epilepsy, it and the periodical insanities are in no small degree the effect of mal-development of the fore brain as compared with the centres of organic life. The great convulsive centre is, according to Spitzka,[262] the reticular grey matter of the brain isthmus, particularly of the pons and medulla. All characteristic features of the full epileptic onset can be produced in animals deprived of the related cerebral cortex. It needs but a slight puncture with a thin needle to produce typical convulsions in the rabbit, and some of the convulsive movements reported by Nothnagel have not only shown the true epileptic character but also that peculiar automatism noted in aberrant attacks. It is in this segment of the nervous system that all the great nerve strands conveying motor impulses, both of a voluntary and automatic and some of a reflex character, are found united in a relatively small area, and just here a relatively slight irritation might produce functional disturbances involving the entire bodily periphery.