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134 but an insufficient means of accomplishing this end, were it not for the muscles of the back, which, when properly developed by exercise, neutralize the action of those of the chest, and keep the shoulders pro­perly expanded. We lay a stress upon the necessity of exercise, for it must be remarked, that the greater its amount the greater the waste of substance, and the greater the quantity of blood carried to the wasting organ for its repair; while, provided the exercise has been moderately taken and properly conducted, the greater will be the strength of the muscles thus thrown into action. For the preservation of the erect posi­tion no less than five layers of powerful muscles are placed along the spine, many of them taking their origin from the back of the head, and being inserted into various processes of the vertebral column; and it is by their action, combined with the elasticity of the spine and its cartilages, that we are enabled to stand upright, and to perform all the varied move­ments of which the human form is capable. The power of the muscles is so accurately balanced, that, provided all be in a normal condition, no difficulty is experienced in maintaining the erect attitude for a very considerable time. The spine has also natural curves, which act as springs to diminish the con­cussions which the brain would receive in walking