The text-book machinery has been sequestered in the Biographical and Critical Notes at the end of the book. Their character and position are 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 motor cycle. They are more simple, and it is easy to find in our bodies examples of all the three orders of levers. The joints at which bony levers meet and move on each other are very different from those we find in motor cycles. Indeed, I must confess they are not nearly so simple. And, lastly, I must not forget to mention another difference. These levers we are going to study are living--at least, are so densely inhabited by myriads of minute bone builders that we must speak of them as living. I want to lay emphasis on that fact because I did not insist enough on the living nature of muscular engines. [Illustration: Fig. 1.--Showing a chisel 10 inches long used as a lever of the first order.]