In attempting to explain these phenomena Dercum and Parker refer to the induction of Spencer as to the universality of the rhythm of motion. Through the whole nervous system of every healthy animal a constant rhythmical interchange of motion takes place. What might be called nervous equilibration results. In man the will modifies and controls the action of the nervous system; it assists in maintaining nervous equilibrium when it is threatened. The will being withdrawn from the nervo-muscular apparatus, and this being subjected to strain, a disturbance takes place. This same explanation may be applied to some of the convulsive and other phenomena of hysteria. Comparing and analyzing the various views, it may be concluded with reference to the pathology of hysteria as follows: (1) The anatomical changes in hysteria are temporary. (2) These changes may be at any level of the cerebro-spinal axis, but most commonly and most extensively cerebral. (3) They are both dynamic and vascular: the dynamic are of some undemonstrable molecular character; the vascular are either spastic or paretic, most frequently the former. (4) The psychical element enters in that, either, on the one hand, violent mental stimuli which originate in the cerebral hemispheres are {214} transmitted to vaso-motor conductors,[30] or, on the other hand, psychical passivity or torpor permits the undue activity of the lower nervous levels. [Footnote 30: Rosenthal.] ETIOLOGY.—Heredity has much to do with the development of hysteria. It is not that it is so frequently transmitted directly after its own kind, but this disorder in one generation generally indicates the existence of some ancestral nervous, mental, or diathetic affection. Briquet[31] has shown that of hysterical women who have daughters, more than half transmit the disease to one or several of these, and, again, that rather more than half of the daughters of the latter also become hysterical. Amann, according to Jolly,[32] has stated that in 208 cases of hysteria he proved with certainty an hereditary tendency 165 times—that is, in 76 per cent. This is too big to be true. [Footnote 31: _Op. cit._] [Footnote 32: _Ziemssen's Cyclopædia of the Practice of Medicine_, vol. xiv., American translation.] Briquet has also made some careful investigations into the subject of the health of infants born of hysterical mothers. The investigations were based upon a study of 240 hysterical women, with whom he compared 240 other patients affected with such diseases as fever, phthisis, cancer, diseases of the heart, liver, and kidneys, but without any hysterical symptoms. In brief, the result of his investigations was that children born of hysterical mothers die more frequently and at a younger age than those who are born of mothers not hysterical.