Mature age is a very strong accessory cause. The disease never occurs with the first parturition, and rarely with the second. It appears with the third, fourth, fifth, or sixth--after the growth of the cow has ceased and when all her powers are devoted to the production of milk. Calving is an essential condition, as the disturbance of the circulation consequent on the contraction of the womb and the expulsion into the general circulation of the enormous mass of blood hitherto circulating in the walls of the womb fills to repletion the vessels of the rest of the body and very greatly intensifies the already existing plethora. If this is not speedily counterbalanced by a free secretion from the udder, kidneys, bowels, and other excretory organs, the most dire results may ensue. Calving may thus be held to be an exciting cause, and yet the labor and fatigue of the act are not active factors. It is after the easy calving, when there has been little expenditure of muscular or nervous energy and no loss of blood, that the malady is seen. Difficult parturitions may be followed by metritis, but they are rarely connected with parturition fever. All these factors coincide in intensifying the one condition of plethora and point to that as a most essential cause of the affection. It is needless to enter here into the much-debated question as to the mode in which the plethora brings about the characteristic symptoms and results. As the results show disorder or suspension of the nervous functions mainly, it may suffice to say that this condition of the blood and blood vessels is incompatible with the normal functional activity of the nerve centers. How much is due to congestion of the brain and how much to bloodlessness may well be debated, yet in a closed box like the cranium, in which the absolute contents can not be appreciably increased or diminished, it is evident that, apart from dropsical effusion or inflammatory exudation, there can be only a given amount of blood; therefore, if one portion of the brain is congested, another must be proportionately bloodless; and as congestion of the eyes and head generally and great heat of the head are most prominent features of the disease, congestion of the brain must be accepted. This, of course, implies a lack of blood in certain other parts or blood vessels.