A Book of Exposition | Page 3

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 being yoked to the pre-fulcral end of the lever,
very close to the cup on which the head rocks. However, if the greater
straight muscles lose power by working on a very short lever, they gain,
in speed; we set them quickly and easily into action when we give a
nod of recognition. All the strength or power is yoked to the
post-fulcral end of the head; the pre-fulcral end of its lever is poorly
guarded. Japanese wrestlers know this fact very well, and seek to gain
victory by pressing up the poorly guarded pre-fulcral lever of the head,
thus producing a deadly lock at the fulcral joint. Indeed, it will be
found that those who use the jiu-jitsu method of fighting have
discovered a great deal about the construction and weaknesses of the
levers of the human body.
Merely to poise the head on the atlas may seem to you as easy a matter
as balancing the beam of a pair of scales on an upright support. I am
now going to show that a great number of difficulties had to be
overcome before our heads could be safely poised on our necks. The
head had to be balanced in such a way that through the pivot or joint on
which it rests a safe passageway could be secured for one of the most
delicate and most important of all the parts or structures of the human
machine. We have never found a good English name for this structure,
so we use its clumsy Latin one--_Medulla oblongata_--or medulla for
short. In the medulla are placed offices or centres which regulate the
vital operations carried on by the heart and by the lungs. It has also to
serve as a passageway for thousands of delicate gossamer-like nerve
fibres passing from the brain, which fills the whole chamber of the
skull, to the spinal cord, situated in the canal of the backbone. By
means of these delicate fibres the brain dispatches messages which
control the muscular engines of the limbs and trunk. Through it, too,
ascend countless fibres along which messages pass from the limbs and

trunk to the brain. In creating a movable joint for the head, then, a safe
passage had to be obtained for the medulla--that part of the great nerve
stem which joins the brain to the spinal cord. The medulla is part of the
brain stem.
This was only one of the difficulties which had to be overcome. The
eyes are set on the pre-fulcral lever of the head. For our safety we must
be able to look in all directions--over this shoulder or that. We must
also be able to turn our heads so that our ears may discover in which
direction a sound is reaching us. In fashioning a fulcral