Page:Philosophical Review Volume 1.djvu/501

] overwhelming agitation intelligence remains unmodified. But soon external objects are no longer perceived; vision is disturbed, ideas become more and more confused, then consciousness is lost. The convulsive reflex movements still continue but become less and less frequent and at the end of the fourth minute they also cease. (3) Nutrition. The cerebral cells are renewed by the aliment contained in the blood. Hunger and thirst are psychic notifications of its changed condition. Although consciousness continues long after the supply of aliment has been withdrawn, delirium at last sets in, ideas become incoherent, sensibility is modified, and at last, death ensues. It seems as if the separate centres, and even the separate cells, have a life of their own which endures after the bond that unites them in rational co-ordination has been broken. (4) Temperature. Consciousness is manifested only within certain very narrow limits of heat. For the warm-blooded animals the normal temperature of the blood does not vary much. For man it is about 98° F. At a few degrees above this, delirium appears and, if the temperature still rises, consciousness is soon lost. Excepting in the hibernating animals, which endure a low degree of cold, still not without the loss of consciousness, life itself ceases when the temperature of the blood falls much below the normal. But long before life becomes extinct, consciousness disappears and all spontaneous movements cease. The daily variation in the temperature of human blood, in health, is only about five degrees. (5) Age. All living tissues are subject to a common law. They are born, grow old and die. This process is constantly going on in every living man. The nervous tissues are no exception. The brain, like a tree withered at the top, may lose its vitality long before the other parts of the organism have yielded to the law of death. Or it may put forth its signs of vitality when trunk and roots give no promise of longer life, like the pine-tree whose evergreens wave triumphantly on the breeze above the scars of the lightning's stroke. (6) Sleep. One of the conditions of the existence of the nervous system is the intermittence of its action. No living creature can continue always