The mechanical structure and properties of the organized fabric, which have occupied our attention in the preceding volume, are necessary for the maintenance of life, and the exercise of the vital powers. But however artificially that fabric may have been constructed, and however admirable the skill and the foresight that have been displayed in ensuring the safety of its elaborate mechanism, and in preserving the harmony of its complicated movements, it yet of necessity contains within itself the elements of its own dissolution. The animal machine, in common with every other mechanical contrivance, is subject to wear and deteriorate by constant use. Not only in the greater movements of the limbs, but also in the more delicate actions of the internal organs, we may trace the operation of many causes inevitably leading to their ultimate destruction. Continued friction must necessarily occasion a loss of substance in the harder parts of the frame, and evaporation is constantly tending to exhaust the fluids. The repeated actions of the muscles induce certain changes in these organs, both in their mechanical properties and chemical composition, which impair their powers of contraction, and which, if suffered to continue, would, in no long time, render them incapable of exercising their proper functions; and the same observation applies also to the nerves, and to all the other systems of organs.