intended to permit instructors freedom of treatment. Some may wish to
test a student's ability in the use of reference books by having him
report on allusions. Some may wish to explain these themselves. A few
may find my experience helpful. For them suggestions are included in the
Critical Notes. In general, I have assumed that instructors will prefer
their own methods and have tried to leave them unhampered.




THE EXPOSITION OF A MECHANISM

THE LEVERS OF THE HUMAN BODY[1]

_Sir Arthur Keith_


In all the foregoing chapters we have been considering only the muscular
engines of the human machine, counting them over and comparing their
construction and their mechanism with those of the internal-combustion
engine of a motor cycle. But of the levers or crank-pins through which
muscular engines exert their power we have said nothing hitherto. Nor
shall we get any help by now spending time on the levers of a motor
cycle. We have already confessed that they are arranged in a way which
is quite different from that which we find in the human machine. In the
motor cycle all the levers are of that complex kind which are called
wheels, and the joints at which these levers work are also circular, for
the joints of a motor cycle are the surfaces between the axle and the
bushes, which have to be kept constantly oiled. No, we freely admit that
the systems of levers in the human machine are quite unlike those of a