The blood-pressure is also automatically regulated, both the diameter of the bloodvessels and the frequency of the heart-beat being under reflex control; and we may, as a final instance of reflex action, describe one of Nature’s most perfect and merciful contrivances—fainting. Suppose a man receives a severe wound—say, has his hand struck off by a sword—the shock to his system causes an immediate dilatation of the large bloodvessels of the abdomen; this results in a great fall of blood-pressure, and the heart, finding that it has much less resistance to overcome, slackens its beats so that soon the flow of blood is very slow indeed. Hence, it has time to clot over the wound, and the man does not bleed to death. Incidentally, the feeble current of blood is insufficient to keep the most delicate organ of the body, the brain, in its normal state of activity, and the man is relieved from his pain by unconsciousness, which passes off when the heart again quickens its beat. It is perhaps needless to remark that fainting fits are not always and only caused by flesh wounds; they may be due to weakness or other causes. Now, if we consider the instances quoted above, we are able to deduce a few general principles from them. In the first place, it may be noticed that reflex action compels us to perform the movements necessary to our existence whether we like it or no. It is not for us to decide whether we will breathe or not. We must. The strongest-willed man who ever lived, no matter how much a philosopher, could not commit suicide by holding his breath, as Cato boasted he could. Directly he lost consciousness, supposing he managed to hold out till then, the tainted blood bathing the respiratory centre would awake it to activity, and he would start breathing afresh. Again, it is noticeable that many of these actions could not possibly be performed by a voluntary effort. We can, to a certain extent, regulate the depth and frequency of our breathing, and we can blink our eyes voluntarily; but an average man would be quite at a loss what to do if asked to make the pupil of his eye dilate and contract, the glands of his stomach secrete, or his heart alter its rhythm. It is a familiar fact that some reflex actions can be altered by an effort of the will; in other words, an impulse from a brain cell will prevent a nerve cell in the spinal cord from discharging. But it is an equally familiar fact that with continuous stimulation the impulses accumulate and ultimately overcome this resistance. Most people have at some time or other striven to resist the inclination to cough, consequent upon a tickling sensation in the throat, and know that there comes a time when they can restrain themselves no longer. This is because the accumulated stimuli from the throat, having reached a greater strength than the prohibitive impulse from the brain, succeed in compelling the cells in the cord to discharge.