A Book of Exposition | Page 5

turns on the axis. When we search for the
manner in which this has been accomplished, we see again that Nature
has made use of the simplest means at her disposal. When we examine
a vertebra in the course of construction within an unborn animal, we
see that it is really made up by the union of four parts (see Fig. 4): a
central block which becomes the "body" or supporting part; a right and
a left arch which enclose a passage for the spinal cord; and, lastly, a
fourth part in front of the central block which becomes big and strong
only in the first vertebra--the atlas. When we look at the atlas (Fig. 4),
we see that it is merely a ring made up of three of the parts--the right
and left arches and the fourth element,--but the body is missing. A
glance at Fig. 4, B, will show what has become of the body of the atlas.
It has been joined to the central block of the second vertebra--the
axis--and projects upwards within the front part of the ring of the atlas,
and thus forms a pivot round which rotatory movements of the head
can take place. Here we have in the atlas an approach to the formation
of a wheel--a wheel which has its axle or pivot placed at some distance
from its centre, and therefore a complete revolution of the atlas is
impossible. A battery of small muscles is attached to the lateral levers
of the atlas and can swing it freely, and the head which it carries, a
certain number of degrees to both right and left. The extent of the
movements is limited by stout check ligaments. Thus, by the simple
expedient of allowing the body of the atlas to be stolen by the axis, a
pivot was obtained round which the head could be turned on a
horizontal plane.
[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