_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.]
We are all well acquainted with levers. We apply them every day. A box
arrives with its lid nailed down; we take a chisel, use it as a lever,
pry the lid open, and see no marvel in what we have done (Fig. 1). And
yet we thereby did with ease what would have been impossible for us even
if we had put out the whole of our unaided strength. The use of levers
is an old discovery; more than 1500 years before Christ, Englishmen,
living on Salisbury Plain, applied the invention when they raised the
great stones at Stonehenge and at Avebury; more than 2000 years earlier
still, Egyptians employed it in raising the pyramids. Even at that time
men had made great progress; they were already reaping the rewards of
discoveries and inventions. But none, I am sure, surprised them more
than the discovery of the lever; by its use one man could exert the
strength of a hundred men. They soon observed that levers could be used
in three different ways. The instance already given, the prying open of
a lid by using a chisel as a lever, is an example of one way (Fig. 1);
it is then used as a lever of the first order. Now in the first order,
one end of the lever is applied to the point of resistance, which in the
case just mentioned was the lid of the box. At the other end we apply
our strength, force, or power. The edge of the box against which the
chisel is worked serves as a fulcrum and lies between the handle where
the power is applied and the bevelled edge which moves the resistance or
weight. A pair of ordinary weighing scales also exemplifies the first
order of levers. The knife edge on which the beam is balanced serves as
a fulcrum; it is placed exactly in the middle of the beam, which we
shall suppose to be 10 inches long. If we place a 1-lb. weight in one
scale to represent the resistance to be overcome, the weight will be
lifted the moment that a pound of sugar has been placed in the opposite
scale--the sugar thus representing the power. If, however, we move the
knife-edge or fulcrum so that it is only 1 inch from the sugar end of
the beam and 9 inches from the weight end, then we find that we have to
pour in 9 lb. of sugar to equalise the 1-lb. weight. The chisel used in
prying open the box lid was 10 inches long; it was pushed under the lid
for a distance of 1 inch, leaving 9 inches for use as a power lever. By
using a lever in this way, we increased our strength ninefold. The
longer we make the power arm, the nearer we push the fulcrum towards the
weight or resistance end, the greater becomes our power. This we shall
find is a discovery which Nature made use of many millions of years ago
in fashioning the body of man and of beast. When we apply our force to
the long end of a lever, we increase our power. We may also apply it, as
Nature has done in our bodies, for another purpose. We have just noted
that if the weight end of the beam of a pair of scales is nine times the
length of the sugar end, that a 1-lb. weight will counterpoise 9 lb. of
sugar. We also see that the weight scale moves at nine times the speed
of the sugar scale. Now it often happens that Nature wants to increase,
not the power, but the speed with which a load is lifted. In that case
the "sugar scale" is placed at the long end of the beam and the "weight
scale" at the short end; it then takes a 9-lb. weight to raise a single
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
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 joint for the
head, then, two different objects had to be secured: free mobility for
the head, and a safe transit for the medullary part of the brain stem.
How well these objects have been attained is known to all of us, for we
can move our heads in the freest manner and suffer no damage whatsoever.
Indeed, so strong and perfect is the joint that damage to it is one of
the most uncommon accidents of life.
Let us see, then, how this triumph in engineering has been secured. In
her inventive moods Nature always hits on the simplest plan possible. In
this case she adopted a ball-and-socket joint--the kind by which older
astronomers mounted their telescopes. By such a joint the telescope
becomes, just as the head is, a lever of the first order. The eyeglass
is placed at one end of the lever, while the object-glass, which can be
swept across the face of the heavens, is placed at the other or more
distant end. In the human body the first vertebra of the backbone--the
atlas--is trimmed to form a socket, while an adjacent part of the base
of the skull is shaped to play the part of ball. The kind of joint to be
used having been hit upon, the next point was to secure a safe passage
for the brain stem. That, too, was worked out in the simplest fashion.
The central parts of both ball and socket were cut away, or, to state
the matter more exactly, were never formed. Thus a passage was obtained
right through the centre of the fulcral joint of the head. The centre of
the joint was selected because when a lever is set in motion the part at
the fulcrum moves least, and the medulla, being placed at that point, is
least exposed to disturbance when we bend our heads backwards, forwards,
or from side to side. When we examine the base of the skull, all that we
see of the ball of the joint are two knuckles of bone (Fig. 3, A),
covered by smooth slippery cartilage or gristle, to which anatomists
give the name of occipital condyles. If we were to try to complete the
ball, of which they form a part, we should close up the great
opening--the _foramen magnum_--which provides a passageway for the brain
stem on its way to the spinal canal. All that is to be seen of the
socket or cup is two hollows on the upper surface of the atlas into
which the occipital condyles fit (Fig. 3, B). Merely two parts of the
brim of the cup have been preserved to provide a socket for the
condyles or ball.
[Illustration: Fig. 3.--A, The opening in the base of the skull, by
which the brain stem passes to the spinal canal. The two occipital
condyles represent part of the ball which fits into the cup formed by
the atlas. B, The parts of the socket on the ring of the atlas.]
As we bend our heads, the occipital condyles revolve or glide on the
sockets of the atlas. But what will happen if we roll our heads
backwards to such an extent that the bony edge of the opening in the
base of the skull is made to press hard against the brain stem and crush
it? That, of course, would mean instant death. Such an accident has been
made impossible (1) by making the opening in the base of the skull so
much larger than the brain stem that in extreme movements there can be
no scissors-like action; (2) the muscles which move the head on the
atlas arrest all movements long before the danger-point is reached; (3)
even if the muscles are caught off their guard, as they sometimes are,
certain strong ligaments--fastenings of tough fibres--are so set as
automatically to jam the joint before the edge of the foramen can come
in contact with the brain stem.
These are only some of the devices which Nature had to contrive in order
to secure a safe passageway for the brain stem. But in obtaining safety
for the brain stem, the movements of the head on the atlas had to be
limited to mere nodding or side-to-side bending. The movements which are
so necessary to us, that of turning our heads so that we can sweep our
eyes along the whole stretch of the skyline from right to left, and from
left to right, were rendered impossible. This defect was also overcome
in a simple manner. The joints between the first and second
vertebrae--the atlas and axis--were so modified that a turning movement
could take place between them instead of between the atlas and skull.
When we turn or rotate our heads, the atlas, carrying the skull upon it,
swings or 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 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 is only as 10 to 6.6; our power is increased by
only one-third. Now this has an important bearing on the problem we are
going to investigate, for the weight of our body falls on the foot, so
that only about one-third of the lever--that part of it which is formed
by the heel--projects behind the point on which the weight of the body
rests. The strength of the muscles which act on the heel will be
increased only by about one-third.
We have already seen that a double engine, made up of the
_gastrocnemius_ and _soleus_, is the power which is applied to the heel
when we walk, and that the pad of the foot, lying across the sole in
line with the ball of the great toe, serves as a fulcrum or rest. The
weight of the body falls on the foot between the fulcrum in front and
the power behind, as in a lever of the second order. We have explained
why the power of the muscles of the calf is increased the more the
weight of the body is shifted towards the toes, but it is also evident
that the speed and the extent to which the body is lifted are
diminished. If, however, the weight be shifted more towards the heel,
the muscles of the calf, although losing in power, can lift their load
more quickly and to a greater extent.
We must look closely at the foot lever if we are to understand it. It is
arched or bent; the front pillar of the arch stretches from the summit
or keystone, where the weight of the body is poised, to the pad of the
foot or fulcrum (Fig. 6); the posterior pillar, projecting as the heel,
extends from the summit to the point at which the muscular power is
applied. A foot with a short anterior pillar and a long posterior pillar
or heel is one designed for power, not speed. It is one which will serve
a hill-climber well or a heavy, corpulent man. The opposite kind, one
with a short heel and a long pillar in front, is well adapted for
running and sprinting--for speed. Now, we do find among the various
races of mankind that some have been given long heels, such as the
dark-skinned natives of Africa and of Australia, while other races have
been given relatively short, stumpy heels, of which sort the natives of
Europe and of China may be cited as examples. With long heels less
powerful muscular engines are required, and hence in dark races the calf
of the leg is but ill developed, because the muscles which move the heel
are small. We must admit, however, that the gait of dark-skinned races
is usually easy and graceful. We Europeans, on the other hand, having
short heels, need more powerful muscles to move them, and hence our
calves are usually well developed, but our gait is apt to be jerky.
[Illustration: Fig. 6.--The bones forming the arch of the foot, seen
from the inner side.]
If we had the power to make our heels longer or shorter at will, we
should be able, as is the case in a motor cycle, to alter our
"speed-gear" according to the needs of the road. With a steep hill in
front of us, we should adopt a long, slow, powerful heel; while going
down an incline a short one would best suit our needs. With its
four-change speed-gear a motor cycle seems better adapted for easy and
economical travelling than the human machine. If, however, the human
machine has no change of gear, it has one very marvellous
mechanism--which we may call a _compensatory_ mechanism, for want of a
short, easy name. The more we walk, the more we go hill-climbing, the
more powerful do the muscular engines of the heel become. It is quite
different with the engine of a motor cycle; the more it is used, the
more does it become worn out. It is because a muscular engine is living
that it can respond to work by growing stronger and quicker.
I have no wish to extol the human machine unduly, nor to run down the
motor cycle because of certain defects. There is one defect, however,
which is inherent in all motor machines which man has invented, but from
which the human machine is almost completely free. We can illustrate the
defect best by comparing the movements of the heel with those of the
crank-pin of an engine. One serves as the lever by which the
gastrocnemius helps to propel the body; the other serves the same
purpose in the propulsion of a motor cycle. On referring to Fig. 7, A,
the reader will see that the piston-rod and the crank-pin are in a
straight line; in such a position the engine is powerless to move the
crank-pin until the flywheel is started, thus setting the crank-pin in
motion. Once started, the leverage increases, until the crank-pin stands
at right angles to the piston-rod--a point of maximum power which is
reached when the piston is in the position shown in Fig. 7, B. Then the
leverage decreases until the second dead centre is reached (Fig. 7, C);
from that point the leverage is increased until the second maximum is
reached (Fig. 7, D), whereafter it decreases until the arrival at the
first position completes the cycle. Thus, in each revolution there are
two points where all leverage or power is lost, points which are
surmounted because of the momentum given by the flywheel. Clearly we
should get most out of an engine if it could be kept working near the
points of maximum leverage--with the lever as nearly as possible at
right angles to the crank-pin.
[Illustration: Fig. 7.--Showing the crank-pin of an engine at: A, First
dead centre. B, First maximum leverage. C, Second dead centre. D, Second
maximum leverage.]
Now, we have seen that the tendon of Achilles is the piston cord, and
the heel the crank-pin, of the muscular engine represented by the
gastrocnemius and soleus. In the standing posture the heel slopes
downwards and backwards, and is thus in a position, as regards its
piston cord, considerably beyond the point of maximum leverage. As the
heel is lifted by the muscles, it gradually becomes horizontal and at
right angles to its tendon or piston cord. As the heel rises, then, it
becomes a more effective lever; the muscles gain in power. The more the
foot is arched, the more obliquely is the heel set and the greater is
the strength needed to start it moving. Hence, races like the European
and Mongolian, which have short as well as steeply set heels, need large
calf muscles. It is at the end of the upward stroke that the heel
becomes most effective as a lever, and it is just then that we most need
power to propel our bodies in a forward direction. It will be noted that
the heel, unlike the crank-pin of an engine, never reaches, never even
approaches, that point of powerlessness known to engineers as a dead
centre. Work is always performed within the limits of the most effective
working radius of the lever. It is a law for all the levers of the body;
they are set and moved in such a way as to avoid the occurrence of dead
centres. Think what our condition would have been were this not so; why,
we should require revolving fly-wheels set in all our joints!
[Illustration: Fig. 8.--The arch of the foot from the inner side,
showing some of the muscles which maintain it.]
Another property is essential in a lever: it must be rigid; otherwise it
will bend, and power will be lost. Now, if the foot were a rigid lever,
there would be missing two of its most useful qualities. It could no
longer act as a spring or buffer to the body, nor could it adapt its
sole to the various kinds of surfaces on which we have to tread or
stand. Nature, with her usual ingenuity, has succeeded in combining
those opposing qualities--rigidity, suppleness, and elasticity or
springiness--by resorting to her favorite device, the use of muscular
engines. The arch is necessarily constructed of a number of bones which
can move on each other to a certain extent, so that the foot may adapt
itself to all kinds of roads and paths. It is true that the bones of the
arch are loosely bound together by passive ties or ligaments, but as
these cannot be lengthened or shortened at will, Nature had to fall back
on the use of muscular engines for the maintenance of the foot as an
arched lever. Some of these are shown in Fig. 8. The foot, then, is a
lever of a very remarkable kind; all the time we stand or walk, its
rigidity, its power to serve as a lever, has to be maintained by an
elaborate battery of muscular engines all kept constantly at work. No
wonder our feet and legs become tired when we have to stand a great
deal. Some of these engines, the larger ones, are kept in the leg, but
their tendons or piston cords descend below the ankle-joint to be fixed
to various parts of the arch, and thus help to keep it up (Fig. 8).
Within the sole of the foot has been placed an installation of seventeen
small engines, all of them springing into action when we stand up, thus
helping to maintain the foot as a rigid yet flexible lever.
We have already seen why our muscles are so easily exhausted when we
stand stock-still; they then get no rest at all. Now, it sometimes
happens in people who have to stand for long periods at a stretch that
these muscular engines which maintain the arch are overtaxed; the arch
of the foot gives way. The foot becomes flat and flexible, and can no
longer serve as a lever. Many men and women thus become permanently
crippled; they cannot step off their toes, but must shuffle along on the
inner sides of their feet. But if the case of the overworked muscles
which maintain the arch is hard in grown-up people, it is even harder in
boys and girls who have to stand quite still for a long time, or who
have to carry such burdens as are beyond their strength. When we are
young, the bony levers and muscular engines of our feet have not only
their daily work to do, but they have continually to effect those
wonderful alterations which we call growth. Hence, the muscular engines
of young people need special care; they must be given plenty of work to
do, but that kind of active action which gives them alternate strokes of
work and rest. Even the engine of a motor cycle has three strokes of
play for one of work. Our engines, too, must have a liberal supply of
the right kind of fuel. But even with all those precautions, we have to
confess that the muscular engines of the foot do sometimes break down,
and the leverage of the foot becomes threatened. Nor have we succeeded
in finding out why they are so liable to break down in some boys and
girls and not in others. Some day we shall discover this too.
We are now to look at another part of the human machine so that we may
study a lever of the third order. The lever formed by the forearm and
hand will suit our purpose very well. It is pivoted or jointed at the
elbow; the elbow is its fulcrum (Fig. 9 B). At the opposite end of the
lever, in the, upturned palm of the hand, we shall place a weight of 1
lb. to represent the load to be moved. The power which we are to yoke to
the lever is a strong muscular engine we have not mentioned before,
called the _brachialis anticus_, or front brachial muscle. It lies in
the upper arm, where it is fixed to the bone of that part--the humerus.
It is attached to one of the bones of the forearm--the ulna--just beyond
the elbow.
In the second order of lever, we have seen that the muscle worked on one
end, while the weight rested on the lever somewhere between the muscular
attachment and the fulcrum. In levers of the third order, the load is
placed at the end of the lever, and the muscle is attached somewhere
between the load and the fulcrum (Fig. 9 A). In the example we are
considering, the brachial muscle is attached about half an inch beyond
the fulcrum at the elbow, while the total length of the lever, measured
from the elbow to the palm, is 12 inches. Now, it is very evident that
the muscle or power being attached so close to the elbow, works under a
great disadvantage as regards strength. It could lift a 24-lb. weight
placed on the forearm directly over its attachment as easily as a single
pound weight placed on the palm. But, then, there is this advantage: the
1-lb. weight placed in the hand moves with twenty-four times the speed
of the 24-lb. weight situated near the elbow. What is lost in strength
is gained in speed. Whenever Nature wishes to move a light load quickly,
she employs levers of the third order.
[Illustration: Fig. 9A.--A chisel used as a lever of the third order. W,
weight; P, power; F, fulcrum.]
We have often to move our forearm very quickly, sometimes to save our
lives. The difference of one-hundredth of a second may mean life or
death to us on the face of a cliff when we clutch at a branch or jutting
rock to save a fall. The quickness of a blow we give or fend depends on
the length of our reach. A long forearm and hand are ill adapted for
lifting heavy burdens; strength is sacrificed if they are too long.
Hence, we find that the laboring peoples of the world--Europeans and
Mongolians--have usually short forearms and hands, while the peoples who
live on such bounties as Nature may provide for them have relatively
long forearms and hands.
[Illustration: Fig. 9B.--The forearm and hand as a lever of the third
order.]
Now, man differs from anthropoid apes, which are distant cousins of his,
in having a forearm which is considerably shorter than the upper arm;
whereas in anthropoid apes the forearm is much the longer. That fact
surprises us at first, especially when we remember that anthropoids
spend most of their lives amongst trees and use their arms much more
than their legs in swinging the weight of their heavy bodies from branch
to branch and from tree to tree. A long forearm and hand give them a
long and quick reach, so that they can seize distant branches and swing
themselves along safely and at a good pace. Our first thought is to
suppose that a long forearm, being a weak lever, will be ill adapted
for climbing. But when you look at Fig. 10, the explanation becomes
plain. When a branch is seized by the hand, and the whole weight of the
body is supported from it, the entire machinery of the arm changes its
action. The forearm is no longer the lever which the brachial muscle
moves (Fig. 10), but now becomes the base from which it acts. The part
which was its piston cord now serves as its base of fixation, and what
was its base of fixation to the humerus becomes its piston cord. The
humerus has become a lever of the third order; its fulcrum is at the
elbow; the weight of the body is attached to it at the shoulder and
represents the load which has to be lifted. We also notice that the
brachial muscle is attached a long way up the humerus, thus increasing
its power very greatly, although the rate at which it helps in lifting
the body is diminished. We can see, then, why the humerus is short and
the forearm long in anthropoid apes; shortening the humerus makes it
more powerful as a lever for lifting the body. That is why anthropoids
are strong and agile tree-climbers. But then watch them use those long
hands and forearms for the varied and precise movements we have to
perform in our daily lives, and you will see how clumsy they are.
[Illustration: Fig. 10.--Showing the action of the brachialis anticus in
the arm of an anthropoid ape.]
In the human machine the levers of the arm have been fashioned, not for
climbing, but for work of another kind--the kind which brings us a
livelihood. We must have perfect control over our hands; the longer the
lever of the forearm is made, the more difficult does control of the
hand become. Hence, in the human machine the forearm is made relatively
short and the upper arm long.
We have just seen that the brachial muscle could at one time move the
forearm and hand, but that when they are fixed it could then use the
humerus as a lever and thereby lift the weight of the body. What should
we think of a metal engine which could reverse its action so that it
could act through its piston-rod at one time and through its cylinder at
another? Yet that is what a great number of the muscular engines of the
human machine do every day.
There is another little point, but an important one, which I must
mention before this chapter is finished. I have spoken of the forearm
and hand as if they formed a single solid lever. Of course that is not
so; there are joints at the wrist where the hand can be moved on the
forearm. But when a weight is placed in the hand, these joints became
fixed by the action of muscles. The fixing muscles are placed in the
forearm, both in front and behind, and are set in action the moment the
hand is loaded. The wrist joint is fixed just in the same way as the
joints of the foot are made rigid by muscles when it has to serve as a
lever. Even when we take a pen in our hand and write, these engines
which balance and fix the wrist have to be in action all the time. The
steadiness of our writing depends on how delicately they are balanced.
Like the muscles of the foot, the fixers of the wrist may become
overworked and exhausted, as occasionally happens in men and women who
do not hold their pens correctly and write for long spells day after
day. The break-down which happens in them is called "writer's cramp,"
but it is a disaster of the same kind as that which overtakes the foot
when its arch collapses, and its utility as a lever is lost.