pound of sugar, but the sugar scale moves with nine times the speed of
the weight scale. Nature often sacrifices power to obtain speed. The arm
is used as a lever of this kind when a cricket ball is thrown.

Nothing could look less like a pair of scales than a man's head or
skull, and yet when we watch how it is poised and the manner in which it
is moved, we find that it, too, acts as a lever of the first order. The
fulcrum on which it moves is the atlas--the first vertebra of the spine
(Fig. 2). When a man stands quite erect, with the head well thrown back,
the ear passages are almost directly over the fulcrum. It will be
convenient to call that part of the head which is behind the ear
passages the _post-fulcral,_ and the part which is in front the
_pre-fulcral._ Now the face is attached to the pre-fulcral part of the
lever and represents the weight or load to be moved, while the muscles
of the neck, which represent the power, are yoked to the post-fulcral
end of the lever. The hinder part of the head serves as a crank-pin for
seven pairs of neck muscles, but in Fig. 2 only the chief pair is drawn,
known as the _complex_ muscles. When that pair is set in action, the
post-fulcral end of the head lever is tilted downwards, while the
pre-fulcral end, on which the face is set, is turned upwards.

[Illustration: Fig. 2.--The skull as a lever of the first order.]

The complex muscles thus tilt the head backwards and the face upwards,
but where are the muscles which serve as their opponents or antagonists
and reverse the movement? In a previous chapter it has been shown that
every muscle has to work against an opponent or antagonist muscle. Here
we seem to come across a defect in the human machine, for the _greater
straight_ muscles in the front of the neck, which serve as opposing
muscles, are not only much smaller but at a further disadvantage by