The simplest unicellular animals, which we have mentioned so often already, spend their lives in alternate spells of activity and rest. In the third essay we mentioned briefly the weakening of each successive response when a muscle, in which tissue fatigue has chiefly been studied, is stimulated. Before the muscle contracted it contained a form of sugar; when it is tired the sugar is gone, and has been replaced by the products of the chemical action by which the energy was evolved. A period of rest must then follow, for the muscle to be cleansed and replenished. The case of glands, described in Essay II., is somewhat similar. After the gland cell has discharged its ferment, it must spend some time secreting a fresh stock before it is ready to discharge again. In fact, a cell seems to load itself up with supplies, like a locomotive with coal, and, after working till the fuel is nearly exhausted, it has to stop to take in more. All the cells in the body rest at times; even the cells of the heart, carefully as they are nourished and incessant as their work seems, rest between each beat, and the cells of the nervous system form no exception. The brain no less than the body requires periodic rests to renew its chemical stores, and these rests have to be all the longer, as during the waking hours the brain works harder and less intermittently than any other organ. It is only because the brain is the seat of consciousness and the source of voluntary movements that these phenomena are suspended during sleep. Death may seem at first sight a very simple affair, the breaking up of protoplasm into simpler non-living compounds; but the death of the body is anything but simple—in fact, it is not always easy to say when the body is dead. Usually, however, it is considered dead when the central nervous system has succumbed, though the muscles may continue to live for several hours. Death may begin in many ways. The loss of some organs will bring death only after a considerable time, while the failure of others disturbs its economy fatally, and causes an almost immediate cessation of the vital functions. Any interference with the normal conditions of the brain, heart, or lungs is very dangerous, and it is injury or disease of one of the three which puts an end to most men’s troubles. If the brain weakens so that it no longer keeps the heart beating or the muscles, which fill and empty the lungs, to their duties, the body, for obvious reasons, can no longer keep up the cycle of changes we call life. On the other hand, if the lungs cannot oxidize the blood, or the heart drive fresh pabulum to the brain, that organ collapses immediately, and, if a stream of pure blood is not quickly restored to it, dies. No return of the circulation can then restore it; the death of the rest of the animal must follow.