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8 ON THE CHANGES OF DIGESTION year witnesses remarkable variations, for the leaves, having fulfilled their purpose, their circulation becoming occluded, they fade and fall; but on returning spring the old stock is not in the same condition as before, for the past year had left its trace; so with each yearly cycle, till at length more general decay occurs, and the old weather-beaten stock, that has withstood the stormy blasts of many a winter, succumbs and dies.

With equal distinctness of demarcation do we find that human life has its stages; we have infancy and youth, succeeded by manhood in its strength and prime, and then the gradual fading of the powers, first the physical, and then the intellectual; but the differences impressed upon the whole organism at these respective periods are accompanied with a physical state also changing, and the one is dependent upon the other. A child, with its freshness of thought, the wildness of its imagination, and the quickness of its new powers, has a brain structurally differing from that of the old man, with his maturer thoughts, and his calmer reason, whose brain is acted upon by the impressions stored up in the memory, rather than by new objective observa- tions. The elasticity of the youthful step, and the enjoyment of vigorous exercise, are marks of strength of lung, and power of circulation, which an octogenarian does not possess; and not less apparent are the functional peculiarities of digestion during the different periods of life.