UPON THE CIRCULATION THROUGH THE VISCERA.]
VII.
All the movements as yet described are absolutely necessary to the
continuation of life; they are, moreover, independent of the efforts
of the will. But there remains yet another kind of movement without
which the body, left to itself, would die. This is the movement of the
limbs—organs by which the body is able to move from one place to another,
to capture its food and convey it, viâ the mouth, to its stomach—in a
word, to satisfy its chemical and physical needs.
To understand how the limbs work requires a knowledge of their anatomy,
for which we have not time or space here; but the principle throughout is
that of a system of levers, the bones, worked by the voluntary muscles.
Here, as before, a diagram will probably be found to convey more than
could ever be expressed in words. (See Diagram 39.)
The diagram represents very roughly, but it is hoped very plainly, the
main principle of the elbow-joint; but for an exact knowledge of the
mechanism of the joint, and the comparative strain upon, and therefore
strength of, each muscle, the reader must consult some work on anatomy.
He will there find, if he goes on to read the description of the hand,
what a wonderful precision, complexity, and amount of movement can be
obtained by variations of this simple device.
[Illustration: DIAGRAM 39.—DIAGRAM OF THE ARM.
1, Lifting; 2, pressing.]
Here, however, we must leave the study of the manner and object of the
bodily movements, and proceed to investigate the far more intricate
question of how they originate and are controlled.
ESSAY IV.
THE NERVOUS SYSTEM.
I.
Now comes the final problem. Protoplasm forms a structure always
changing, always making good its waste by chemical action upon raw
material, always capturing raw material or in search of it, always,
when it exists in large quantities, and the labour is therefore divided
between many cells, economically apportioning the work and the spoils.
How is it that all the actions, chemical as well as physical, of a vast
number of cells composing a large body are, no matter how complicated,
always harmonious, and always with purpose directed to the advantage of
the whole animal?
In the first essay in this book we discussed the phenomenon of life, and
described briefly the chemical and physical peculiarities of protoplasm.
These in the two succeeding essays we have gone into more fully; but
there is one characteristic of that interesting substance which yet
remains for us to examine in specialized cells, viz., its extreme
readiness to respond to changes in its environment.
In Essay I. we saw that chemical agents, light, heat, electricity,
etc.—had a definite effect upon protoplasm, and that, though they might
influence different kinds in different ways, the effect was nevertheless
invariable; in a word, the response of protoplasm to circumstances is
automatic. But the most remarkable thing about this is that the response
is not confined to the protoplasm actually affected, but is transmitted
to that nearest to the part stimulated, and again passed on to that
beyond, so that a wave of excitation passes through the whole mass, not
stopping till it has reached the extreme confines of the cell. It may
even pass beyond these and set up activity in neighbouring cells. The
power of conductivity once grasped, it may easily be seen that certain
cells, by specializing in this direction and adapting their shape to the
needs of the body, might by throwing out long threads to reach distant
parts set up an organic system of telegraphy.
The organs developed for the control of the body owe their origin to
the outer layer. (See Diagram 5.) This was only to be expected. In the
second essay, in which we treated of the chemistry of the body, we, of
course, touched upon all three layers from which the body is built up;
but the one which chiefly occupied our attention was the innermost layer,
which is so admirably arranged as a chemical laboratory. In the third
essay we dealt chiefly with the middle layer, which both by its position
and its bulk might have been guessed to be the foundation of most of
the motor organs. Now that we have come to the organs of perception and
transmission of impressions, it is only natural to expect that they
should be specialized from the cells already in contact with the external
world, and which, since they form the envelope of the animal, must allow
all such stimuli as reach the subjacent motor layer to pass through them.
Hitherto we have not dealt at great length with the development of the
organs whose functions we have been describing, either from the point
of view of the embryologist or the evolutionist. Nor have we spent much
time upon their gross anatomy. With the nervous system we must proceed
rather differently; for to understand how its higher functions can be
performed they must be traced from their origin step by step, while their
complexity is largely vested in the structure of special organs.
The way in which the nervous system was evolved is shown in Diagram 5.
Originally, no doubt, the cells of the outer layer, when the latter was
in its simplest form—that is to say, only one cell thick, not several,
as it is in our skin—would, when influenced in any way directly call
forth the activity of the motor cells lying beneath them. (See Diagram
40, Fig. 1.) In Fig. 2, however, we see one cell of the outer layer
becoming specialized. It has thrown out a process above the surface of
the skin the more readily to catch impressions, and has sent another down
into the body the better to distribute them. Diagram 41, Fig. 1 shows the
nerve cell at a further stage. The principle is the same, but the cell
is removed to a safer place. In Fig. 2 it is not exposed to the outside
world at all, but by receiving its impulses second-hand from several
cells the same work is done with greater economy and uniformity. Some of
the special sense organs are still developed in this way.
[Illustration: DIAGRAM 40.—SHOWING ORIGIN OF A NERVE CELL.]
[Illustration: DIAGRAM 41.—SHOWING THE DEVELOPMENT OF A NERVE CELL.]
Once the nerve cell is developed and safely shifted into the interior of
the body, it is clothed with a protecting feltwork of connective tissue,
and the nerve fibres are also surrounded by connective-tissue cells which
secrete around them the fatty substance which makes nerves look white.
Such is the nerve cell or intermediary between the world and the muscles;
but thence to harmonious movement in a body with complex organs capable
of varied actions is a long step. To obtain precision and uniformity
throughout the body, all the impressions received must be collected and
balanced, and stimuli, the correct outcome of this balancing, must be
transmitted to the muscles, glands, etc., whose activity circumstances
require. The way in which cells of the outer layer become enclosed to
form a central nervous system is shown in Diagram 5; but its development
will be better seen in the figures of Diagram 42.
[Illustration: DIAGRAM 42.—TO ILLUSTRATE THE DEVELOPMENT OF THE NERVOUS
SYSTEM.]
[Illustration: DIAGRAM 43.—CROSS-SECTION OF THE SPINAL CORD, SHOWING HOW
IT GIVES OFF NERVES.]
[Illustration: DIAGRAM 44.]
This diagram shows how certain cells of the outer layer are budded off
and transferred to a safe place within the body. In this position the
cells are further developed, throwing out one long fibre, which goes to
some distant organ of the body, and short fibres, which, though they do
not join those of other cells and become continuous, closely interlace
and put them into communication. They are also separated from one another
by connective tissue, which supports them, holding them suspended with
only their fibres approaching one another (Diagram 43). Diagram 44 shows
how the bone which replaces the supporting rod (see Diagram 6) throws an
arch round the feltwork of connective tissue in which the nerve cells are
suspended, giving them still further protection.
It will be noticed in the figures of Diagram 42, which is fuller than
Diagram 5, that there are three of these buds—one central and two
lateral. The central one becomes a tube running the whole length of
the animal, while the lateral buds form solid clusters or ganglia,
arranged in pairs at intervals beside it (see Diagram 45). Fibres from
these ganglia go to the skin, and bring to the nerve cells information
from the outside world, which they duly pass on to the cells of the
central column. The cells of the central column, when set in motion by
the ganglion cells, send out impulses to the muscles, whose contraction
is necessary to perform the movement which circumstances indicate. A
movement brought about in this way is called reflex.
[Illustration: DIAGRAM 45.—CENTRAL NERVE TUBE AND GANGLIA.]
The reflex movements are, however, not quite the simplest. For instance,
the food is moved along the alimentary canal by the contraction of two
sets of muscle fibres—an outer longitudinal coat and an inner circular
one. Between these two coats are some nerve cells, which are thrown into
activity by the presence of food and the iron compounds of the bile
secreted by the liver in the tube. These sympathetic cells do not send
their impulses to any centre for examination, but at once stimulate the
muscle fibres between which they lie, thereby producing the peristaltic
movements we have already described. Yet it should be remembered that,
though these cells act independently of the central nervous system, they
are under its control, and can, if need be, have their action modified
for the benefit of the body as a whole.
For convenience’ sake, we had better here specify the chief kinds of
nervous action. First there is what we may call the immediate nerve
action, such as that we have just been describing; secondly there is
reflex action, the centres for which are in the spinal cord and the base
of the brain; and thirdly there is voluntary movement, which arises out
of the interaction of centres in the hemisphere of the brain, where the
most complex machinery of all is kept.
II.
Of the first kind we need say no more. The instance of peristaltic
movement illustrates it sufficiently; so we can at once begin a more
careful examination of reflex action.
The simplest instance of reflex action may be taken from the schoolroom.
If a boy suddenly sticks a pin into an unsuspecting schoolfellow, the
latter invariably starts, and frequently lets fall an exclamation also.
In this case the presence of an injurious agency is reported to the
nearest motor centre, which is in the spinal cord, and this automatically
convulses the body, jerking the limb out of danger.
This is reflex movement; the nerve fibre, which conveys an intimation of
the injurious influence, is a prolongation, or really two prolongations,
of a spinal ganglion cell. (See Diagram 46.) The near end of this fibre,
which enters the cord, has several branches. Some run a little way up the
cord, and some a little way down, so as to communicate with several motor
cells; but one branch runs right up the cord, and sends the message on to
the brain. Our outraged schoolboy starts a fraction of a second before
he is conscious of the pain of being pricked, and this first response
is involuntary and unvarying; the sensation, however, is reported to
his brain, and the workings of that wonderful organ are less easy to
predict. It leads to his taking stock of the aggressor, on the strength
of which he decides whether it is safe to attempt a reprisal, and, if so,
in what form it will be most effective and least likely to attract the
master’s attention. This knotty point settled, the motor cells of the
brain send down messages to the motor cells of different parts of the
spinal cord, and these in turn set the necessary muscles in motion for
delivering a surreptitious kick or aiming a splash of ink, as the case
may be. This is voluntary movement.
[Illustration: DIAGRAM 46.—EVOLUTION OF A SPINAL GANGLION CELL.]
[Illustration: DIAGRAM 47.—SCHEME OF THE CENTRAL NERVOUS SYSTEM.
→ shows the path taken by an impulse in reflex action.
↣ shows the path for a voluntary action.]
The difference between reflex and voluntary movement is, as may be seen
from the above instances, very much a matter of degree; but we had better
leave a comparison between them, and any discussion as to the extent to
which the manifestations of consciousness are automatic, until we have
finished describing reflex movement, and set forth the little we know
about voluntary movement.
Time and space forbid a complete list of reflex movements. The following
are, however, a few typical examples of how the body is automatically
made to perform such acts as are necessary, and of how such as do not
require deliberation are brought about without taxing the intellect.
A reflex action which is unpleasantly familiar is the cough, also the
somewhat similar phenomenon of the sneeze. In this case, a foreign body
which obstructs the windpipe, or causes irritation to the membrane lining
the nose, is, on being reported at the spinal cord, incontinently blown
out by an explosive blast of air from the lungs.
An organ which is very important, and at the same time very
sensitive—viz., the eye—has many protective reflexes. The external
surface of the eye is covered by a very delicate membrane, which must be
kept moist and scrupulously clean. Whenever this membrane gets in the
least dry, or any dust falls on it, the eyelids are closed for a moment,
thereby bathing it with the secretion of the tear glands. Few people are
aware, I think, that they blink their eyes on an average twice every
minute. The eyes are also closed quite involuntarily by a reflex when any
danger threatens them—for instance, a sudden dazzling light, a strong
wind, or a blow aimed at the face; and if any foreign substance—say
a fly—does get into one of them, the secretion of the tear glands is
enormously increased to wash it out.
The size of the pupil, again, is quite involuntarily, _i.e._, reflexly,
altered in proportion to the strength of the light.
Reflex actions are, however, by no means only protective. The act of
swallowing is reflex. So is the secretion of the digestive glands when
the lining membranes of the stomach are stimulated by the presence of
food. The very act of standing depends on the reflex principle, the
tendency of the body to collapse and fall being unconsciously perceived
and corrected by the spinal cord. Walking is also a reflex action. It may
be objected that we think about walking, and do so with intention; but
it is of common experience that we can walk along ‘thinking of something
else,’ and the way in which an intellectual though absent-minded man will
run into people, charge lamp-posts, trip over steps, and tread upon dogs,
is sufficient to absolve the organ of thought and intention from any
share in the performance.
The blood-pressure is also automatically regulated, both the diameter
of the bloodvessels and the frequency of the heart-beat being under
reflex control; and we may, as a final instance of reflex action,
describe one of Nature’s most perfect and merciful contrivances—fainting.
Suppose a man receives a severe wound—say, has his hand struck off by
a sword—the shock to his system causes an immediate dilatation of the
large bloodvessels of the abdomen; this results in a great fall of
blood-pressure, and the heart, finding that it has much less resistance
to overcome, slackens its beats so that soon the flow of blood is very
slow indeed. Hence, it has time to clot over the wound, and the man
does not bleed to death. Incidentally, the feeble current of blood is
insufficient to keep the most delicate organ of the body, the brain, in
its normal state of activity, and the man is relieved from his pain by
unconsciousness, which passes off when the heart again quickens its beat.
It is perhaps needless to remark that fainting fits are not always and
only caused by flesh wounds; they may be due to weakness or other causes.
Now, if we consider the instances quoted above, we are able to deduce a
few general principles from them. In the first place, it may be noticed
that reflex action compels us to perform the movements necessary to our
existence whether we like it or no. It is not for us to decide whether we
will breathe or not. We must. The strongest-willed man who ever lived,
no matter how much a philosopher, could not commit suicide by holding
his breath, as Cato boasted he could. Directly he lost consciousness,
supposing he managed to hold out till then, the tainted blood bathing
the respiratory centre would awake it to activity, and he would start
breathing afresh. Again, it is noticeable that many of these actions
could not possibly be performed by a voluntary effort. We can, to a
certain extent, regulate the depth and frequency of our breathing, and
we can blink our eyes voluntarily; but an average man would be quite
at a loss what to do if asked to make the pupil of his eye dilate and
contract, the glands of his stomach secrete, or his heart alter its
rhythm.
It is a familiar fact that some reflex actions can be altered by an
effort of the will; in other words, an impulse from a brain cell will
prevent a nerve cell in the spinal cord from discharging. But it is an
equally familiar fact that with continuous stimulation the impulses
accumulate and ultimately overcome this resistance. Most people have at
some time or other striven to resist the inclination to cough, consequent
upon a tickling sensation in the throat, and know that there comes a
time when they can restrain themselves no longer. This is because the
accumulated stimuli from the throat, having reached a greater strength
than the prohibitive impulse from the brain, succeed in compelling the
cells in the cord to discharge.
Lastly, reflexes can be learnt. When a young child first endeavours to
stand upright, the sensation of falling is doubtless conveyed to the
brain, and thought taken of how the erect position can be maintained. But
it is not until after many experiments and failures that the brain-cells
can send messages to the right cells in the cord, and these set the
necessary muscles in motion. Experience teaches what must be done, and
constant practice eventually enables the spinal cord to act for itself
without referring for orders to the brain. It is on the same principle
that we learn to ride the bicycle. At first we have to devote our whole
attention to keeping our balance, but in a short time we find we are
doing it with our mind free to contemplate the scenery.
What can be done by reflex action can only be appreciated by observing
an animal from which the brain has been removed. A frog which has been
treated in this way—the operation, it should be said, if performed under
an anæsthetic can cause no pain, either at the moment or afterwards—will
live for weeks—in fact, almost indefinitely—if proper precautions be
taken. But it is an automaton pure and simple. Unless touched it sits
absolutely still. If touched it hops once or twice straight ahead
regardless of obstacles. If placed in water it swims, equally regardless
of obstacles. If turned on its back it immediately resumes its normal
position. If small chips of wood are placed on its back it kicks them
off. If the table on which it is sitting be tilted it will crawl up
the incline until it reaches a level. But it will starve in the midst
of plenty, having lost all power of thought, memory and perception. If
diligently fed by hand a frog, a fish, or a bird will live for a long
time without any brain, since their repertoire of movements is small and
mostly reflex, and their occasions for deliberated action comparatively
few. But the higher we get in the scale of life the more the brain takes
over the duties of the cord, the less automatic become the greater number
of the actions, and hence the more open does the animal’s conduct lie to
moral criticism.
III.
We have now seen how protoplasm exists in a large body, sharing the work
of living amongst specialized cells, and how it responds as a whole to
the influences exerted upon it by its surroundings. The next thing to
consider is how it is situated with regard to matter which does not form
part of its own body; how protected from, and how put into communication
with, the rest of the universe.
With regard to the former, we have seen that in the single cells,
constituting unicellular organisms, there is always a bounding membrane
of denser texture than the rest of the protoplasm. As the cell develops
its capabilities, we have a shell or case of non-living matter secreted
around it, with apertures for communication with the outside world, and
increasingly effective protection is provided as protoplasm, whether
in the single cell or the body, leaves the water, and has to face the
inclemencies of terrestrial life.
[Illustration: DIAGRAM 48.—SHOWING THE FORMATION OF THE SKIN.]
[Illustration: DIAGRAM 49.—STRUCTURE OF SKIN.]
In the schematic embryo (Diagram 6) and other diagrams contained in
this volume, the skin has so far been represented as consisting of a
single layer of living cells; but we must now admit that the skin of man
is quite different. Such a covering would be no protection from heat,
cold, or irritating chemicals, while, in order to prevent its drying
up, it would have to be kept moist with slime, and we should look very
like frogs. In order that an adequate defence may be provided for the
body, this layer of cells divides tangentially, forming two layers. The
inner of these two then divides tangentially again, and a second layer
is interposed between the innermost and that first formed. The skin
now consists of three layers, and so the process is repeated until
it is several layers thick. (See Diagram 48.) It is the innermost and
best-nourished layer which keeps dividing; the other layers, as they
get pushed outwards, are only reached by a little lymph which filters
between the cells, and are eventually starved even of that. As they
get pushed away from the dividing layer, however, they set to work to
surround themselves with a horny wall, which thickens and thickens, until
eventually there is hardly any cell left. (See Diagram 49.) Finally
the cells die and the horny envelopes form a dead cuticle, protecting
the living layers beneath, and are ultimately sloughed off when their
successors are ready to replace them.
[Illustration: DIAGRAM 50.—SHOWING THE DEVELOPMENT OF HAIR.]
Not even a horny layer of dead cells is, however, always sufficient
protection, and the growing layer has sometimes to supplement it by
hair or feathers. How hair is developed is shown in the accompanying
diagram (50). The growing layer sends a strand straight downwards into
the connective tissue, which forms the basement of the skin. The cells
in the middle of this strand, which behaves like ordinary skin, are the
least well nourished, and accordingly die and leave a tube. This tube,
if no further development took place, might become a sweat gland; but if
it is to give rise to a hair it becomes cup-shaped at the base, enclosing
a small loop of bloodvessel. The cells just above the capillary, being
better nourished than the rest, grow more rapidly than their neighbours,
and the result is that a column of cells which we know as a hair pushes
its way up through the tube. (See Diagram 50.)
This outer layer comes everywhere between the main bulk of the body and
the outer world. Hair and sweat glands do not by any means represent its
only modifications. Teeth are formed from it in somewhat the same manner
as hair, while we have already seen that it gives rise to the whole
nervous system.