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86 removed, it returns at once to its former length. But may we not say that the contractility develops an elastic force different from the mere extensibility and retractility we have already studied, and that this elastic force does the work of producing movement? A contracted muscle, according to this view, is like a strong band of india-rubber pulling on two pieces of wood and drawing them together. In like manner, when a muscle contracts, a similar elastic power is developed that pulls upon the bones and causes movement. When the contraction ceases, the muscle returns to its original length, again by elasticity, but acting in the opposite way.

What is this property of irritability by virtue of which the muscle responds to a stimulus and contracts? Is it something possessed by muscles alone, or do we find it anywhere else? Upon this point, at one time, there was a keen controversy. In 1760, a great Swiss physiologist, Haller, wrote upon muscular irritability. Before this date, it was commonly held that the property of irritability was derived from the nerves, and