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68 Burdon Sanderson has found, by a photographic method, not so liable to experimental errors as the one I have shown you, that in the muscles of the frog it is about the one-two-hundredth part of a second. Probably it is even shorter in the muscles of the higher animals. Their muscular substance is more unstable than that of a frog, and it goes off more rapidly under the nervous stimulus. Research also shows that probably in all living matter submitted to a stimulus there is a latent period, a period in which molecular changes are happening which precede, and possibly end in, the particular phenomenon manifested by the living matter. Thus when the nervous stimulus reaches the cell of a secreting gland, or a blood-vessel, or a nerve cell in the spinal cord or brain, it does not produce an immediate effect, but excites changes which occupy time.

When we next meet we shall study tetanus or cramp, and how a nerve probably acts on a muscle.