The changes in the nervous system of the pregnant woman correspond to those in the vascular system. There is the same increase of activity, a heightening of tension. Bruno Wolff, from experiments on bitches, concluded that the central nervous system in women is probably more easily excited in the pregnant than in the non-pregnant state, though he was not prepared to call this cerebral excitability "specific."[180] Direct observations on pregnant women have shown, without doubt, a heightened nervous irritability. Reflex action generally is increased. Neumann investigated the knee-jerk in 500 women during pregnancy, labor, and the puerperium, and in a large number found that there was a progressive exaggeration with the advance of pregnancy, little or no change being observed in the early months; sometimes when no change was observed during pregnancy the knee-jerk still increased during labor, reaching its maximum at the moment of the expulsion of the foetus; the return to the normal condition took place gradually during the puerperium. Tridandani found in pregnant women that though the superficial reflexes, with the exception of the abdominal, were diminished, the deep and tendon reflexes were markedly increased, especially that of the knee, these changes being more marked in primiparæ than in multiparæ, and more pronounced as pregnancy advanced, the normal condition returning with ten days after labor. Electrical excitability was sensibly diminished.[181] One of the first signs of high nervous tension is vomiting. As is well known, this phenomenon commonly appears early in pregnancy, and it is by many considered entirely physiological. Barnes regards it as a kind of safety valve, a regulating function, letting off excessive tension and maintaining equilibrium.[182] Vomiting is, however, a convulsion, and is thus the simplest form of a kind of manifestation--to which the heightened nervous tension of pregnancy easily lends itself--that finds its extreme pathological form in eclampsia. In this connection it is of interest to point out that the pregnant woman here manifests in the highest degree a tendency which is marked in women generally, for the female sex, apart altogether from pregnancy, is specially liable to convulsive phenomena.[183]