[Illustration:  Fig. 4.—­A, The original parts of the first or atlas vertebra.  B, Showing the “body” of the first vertebra fixed to the second, thus forming the pivot on which the head turns.]

Nature thus set up a double joint for the movements of the head, one between the atlas and axis for rotatory movements, another between the atlas and skull for nodding and side-to-side movements.  And all these she increased by giving flexibility to the whole length of the neck.  Makers of modern telescopes have imitated the method Nature invented when fixing the human head to the spine.  Their instruments are mounted with a double joint—­one for movements in a horizontal plane, the other for movements in a vertical plane.  We thus see that the young engineer, as well as the student of medicine, can learn something from the construction of the human body.

In low forms of vertebrate animals like the fish and frog, the head is joined directly to the body, there being no neck.

No matter what part of the human body we examine, we shall find that its mechanical work is performed by means of bony levers.  Having seen how the head is moved as a lever of the first order, we are now to choose a part which will show us the plan on which levers of the second order work, and there are many reasons why we should select the foot.  It is a part which we are all familiar with; every day we can see it at rest and in action.  The foot, as we have already noted, serves as a lever in walking.  It is a bent or arched lever (Fig. 6); when we stand on one foot, the whole weight of our body rests on the summit of the arch.  We are thus going to deal with a lever of a complex kind.

[Illustration:  Fig. 5.—­Showing a chisel used as a lever of the second order.]

In using a chisel to pry open the lid of a box, we may use it as a lever either of the first or of the second order.  We have already seen (Fig. 1) that, in using it as a lever of the first order, we pushed the handle downwards, while the bevelled end was raised, forcing open the lid.  The edge of the box served as a rest or fulcrum for the chisel.  If, however, after inserting the bevelled edge under the lid, we raise the handle instead of depressing it, we change the chisel into a lever of the second order.  The lid is not now forced up on the bevelled edge, but is raised on the side of the chisel, some distance from the bevelled edge, which thus comes to represent the fulcrum.  By using a chisel in this way, we reverse the positions of the weight and fulcrum and turn it into a lever of the second order.  Suppose we push the side of the chisel—­which is 10 inches long—­under the lid to the extent of 1 inch, then the advantage we gain in power is as 1 to 10; we thereby increase our strength tenfold.  If we push the chisel under the lid for half its length, then our advantage stands as 10 to 5; our strength is only doubled.  If we push it still further for two-thirds of its length, then our gain in strength