190. The Law of Arrest is only another aspect of the Law of Discharge, and may be regarded as the conflict of excitations. If a stranger enters the room where a woman lies in labor, there will often be caused a sudden cessation of the uterine contractions.[198] Again, every one knows how the breathing and the beating of the heart are arrested by the idea of danger. The arrest is in each of the three cases only temporary, because when the shock of the new stimulus has caused its discharge (arrest), the peripheral irritation which caused the former discharges resumes its influence, and uterus, heart, and diaphragm begin to move again, even more energetically. Note, moreover, that not only will the cerebral excitation arrest the spinal discharge--an idea check the contractions of the uterus or the heart--but the reverse also takes place. The brain of the woman may be intently occupied with some scheme for the education or welfare of her expected child, but no sooner do the labor pains set in, than all these cerebral combinations are arrested. 191. One sensation arrests another; one idea displaces another. If the foreleg of a headless frog be irritated, the hind-leg will also be moved by the stimulation; or _vice versa_. Here there has been a propagation of the excitation in either direction. But if while the legs are thus irritated, and the centres are ready to discharge, another and more powerful irritation reach the centre--say by pinching the skin of the back--there will be no discharge on the legs. If the vagus be irritated, the heart is arrested; but this does not take place if at the same time, or immediately before, the foot has been sharply pinched. A few gentle taps on the abdomen suffice to stop the heart; but if a drop of acid be previously placed on the skin, we tap in vain, the heart continues to beat. Brown Séquard cites several cases in which convulsions were arrested by irritation of sensitive surfaces;[199] and Dr. Crichton Browne records a case of a patient in whom there was abolition of spinal reflex, due to cerebral irritation: tickling the soles of the feet, or pricking the toes, which normally excites reflex movements, in this case excited none whatever. “This seems to prove that nerve currents, set in motion by irritation of the brain, or some of its convolutions, transmitted down the cord, may inhibit reflex action.”[200] Examples might indefinitely be multiplied. Pinch the skin of a rabbit between the eyes, and you will observe that pulse and respiration are slackened; but if the tail, which is very sensitive, be pinched, this slackening is only momentary, and is succeeded by a quickening--unless the pain be great. Even the effect of intense pain may be neutralized by stimulating the vagus--just as the effect of stimulating the vagus may be neutralized by pain. Claude Bernard found that having dropped ammonia on the eyelid of a dog, the pain caused a convulsive closure of the lid; but on galvanizing the vagus, the lid