Page:Popular Science Monthly Volume 42.djvu/782

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as the nerves pass out from the spinal cord to reach the muscles they again become separated and, as Ferrier and Yeo have discovered, the motor roots which enter into the brachial plexus are arranged with a view to definite co-ordinated movements. Now, this plexus has been to me, and I think to many others, a perfect perplexity, both in its anatomy and physiology. Yet, if we takePSM V42 D782 Diagram of the brachial plexus.jpgFig. 13.—Diagram of the Brachial Plexus. it from the same point of view as we have taken the motor centers, it becomes comparatively simple. We must not forget that, although the monkey is so much like man that we can draw most useful deductions regarding human physiology from experiments on these animals, we must not transfer without more ado the results of these experiments to man in their entirety. We must remember that man, although formerly a frugivorous and probably more or less arboreal animal, is now very different from a monkey, and experiments in the laboratory must be compared with and corrected by those experiments which disease makes upon man in producing localized palsies. I think it very probable that many here would find it difficult to answer the question, What are the movements which result in the monkey from stimulation of the fifth cervical nerve?—nor might he be able to remember them six months hence even if he learned to-night that they consist in the shoulder and arm being raised upward and backward, the humerus rotated outward, the forearm flexed and supinated, the wrist extended, and the tips of the fingers flexed. But he would find it easy enough to remember them, not for six months only, but for the rest of his life, if he were told that they were simply those required to raise the hand in such a way as to grasp an apple hanging