obliquely across the body, and waves of contraction are continually
passing down its long axis, the whole body is affected by continual
electrical changes. By very delicate instruments it can be demonstrated
that with each beat the two hands alternately become electrically
positive and negative to each other.
Whilst dealing with the electrical phenomena of muscle, it may be as well
to state that nerve fibres, which are studied with very much the same
apparatus, show the same electrical changes, the point of injury or of
the greatest activity being negative to all the rest. Single cells are
less easily investigated, but in glands it is possible to show that the
same rule holds.
Undoubtedly the most curious fact about the generation of electricity by
protoplasm is that, by a modification of muscle and nerve, which causes
them to lose their ordinary properties, they are converted into a special
organ for giving electric shocks. Armed with powerful batteries of this
description, an otherwise rather helpless class of fish are enabled to
defend themselves from their enemies, and deal unexpected death to their
more agile prey.
* * * * *
Having now run over a few of the physical properties of protoplasm, we
may pass on to a brief investigation of the movements we find in the body
of man.
III.
In describing the movements of the body, we shall have to treat them as
several and distinct, as indeed they are; but the fact should not be
lost sight of that they cannot really be isolated: one idea embraces the
whole. Two kinds of movement may, however, be distinguished in the vital
functions: movement of the actual cells, such as muscles; and movement of
non-protoplasmic elements acted upon by the cells—_e.g._, lymph.
There is a parallel to this in the chemical side of life, where we
find some phenomena peculiar to the living elements, and others, like
digestion, going on in the living body, but outside the cells.
Taking the movements in the natural order—that is, proceeding from the
simpler to the more complex—the first to be considered is undoubtedly
that of the leucocytes, or general scavengers of the tissues. The body
consists, so far as we have defined its anatomy, of three layers of
cells, and its shape is that of a tube with hollow walls. (See Diagram
6.) Within the cavity of the body are various organs, such as the
muscles, which are formed from the middle layer; and its space is largely
reduced by glands, lungs, and other ramifications of the inner layer
which forms the alimentary canal.
These organs hang more or less freely in the body cavity, slung to its
walls by enveloping sheets of connective tissue, the whole being bathed
in lymph. Now, in such an arrangement the products of wear and tear must
accumulate. Cells here and there die for various reasons, and pieces
of cells become detached even in adult animals. The interior of a bone
is always being eaten away to decrease its weight, or in order that it
may be replaced by fresh bone of a closer texture, and in young animals
and embryos there are many structures which, useful for a time, have
eventually to be removed; as an instance, we may quote the tadpole’s
tail. In fact, if the tissues were left to themselves, the body would
soon be choked with débris, and to avoid this it is supplied with an army
of scavengers, the leucocytes.
The leucocytes are detached cells which owe their origin to the middle
layer. In size they are, of course, very small, quite invisible to the
naked eye. In appearance they resemble unicellular organisms of the
amœba type, which we have had occasion to mention several times already
(Essay II., Section II.; Essay III., Section I., Diagram 1). They are
of several different varieties, some being larger and more active than
others; but they all wander about in the lymph and blood like independent
animals, creeping in and out between the cells of the organs, and
devouring any foreign matter they come across. They sometimes multiply,
like independent animals, by division, especially in the presence of
inflammation, or when they have much work to do, and a rapid increase in
their numbers is needed; and they have been induced to live, and feed,
and multiply, outside the body (in which case they must be considered to
have become independent organisms), thanks to the careful attentions of
the experimenter.
Apart from their duties of devouring the inside layers of bones and
clearing away dead tissue, they are supposed by some to assist in the
absorption of food by creeping between the cells lining the alimentary
canal, and, after throwing out arms to engulf particles of food,
returning with their spoils into the body. Perhaps, however, the most
interesting, or at any rate most romantic, of their many and important
functions are what may be called their emergency duties. Frequently
people, especially those who live in smoky towns, draw into their lungs
particles of dust and soot, which if left adhering to the walls of the
air cavities would cause dangerous irritation. As if by magic a leucocyte
will discover the presence of such a nuisance, and, crawling between
the cells forming the wall of the lung, in which, by the way, it is
outside the body proper, will engulf it and carry it away with him. This
exploit, however, pales beside the warfare which goes on in the body
between leucocytes and invading bacteria. A bacterium thrives in the
blood or lymph, since it finds itself in a warm alkaline fluid containing
complex organic substances, by breaking down which it can easily obtain
energy. Unfortunately, the products of such a process are frequently
virulent poisons, the effect of which upon neighbouring cells produces
the distressing symptoms which we associate with disease. No sooner,
however, has the bacterium begun to generate poisons, than leucocytes,
influenced by chemical attraction (Essay I.), swarm upon it. First come
leucocytes of a small kind, full of zymogen granules, which crowd round
the bacterium till they have covered it. After a time they creep away,
leaving it dead. They are now in an exhausted condition, and no longer
contain granules, having doubtless discharged them as a destructive
ferment upon their enemy. Then a leucocyte of another kind moves to
the attack, or, rather, to clear up the remains, for he is a large,
non-granular, active fellow, and eats up the dead bacterium by the simple
process of engulfing him whole. (See Diagram 31.)
A natural question arising out of the study of leucocytes is, What
becomes of them? Particles of soot and similar refuse can hardly be
considered nutritious, or even digestible, food, and one is rather
drawn to the conclusion that the leucocyte performs its functions for
the good of the body at large, not of itself, and that when its work is
done it must die. Many leucocytes, probably, loaded with unconsidered and
undesirable trifles, cast themselves into the alimentary canal, and are
got rid of with the useless portions of the food; but they do not always
have the luck or energy to get to a natural outlet. An unpleasantly
familiar phenomenon is the boil. Here we have some irritating substance
under the skin setting up inflammation, and leucocytes swarm up to remove
the cause of the trouble. Before, however, this is done, many have
perished in the fray, and they have collected in numbers to the formation
of what is commonly known as pus, or matter. Their dispersal into the
body is now neither easy nor desirable, and the surgeon usually lets them
escape from the surface by a touch of the lancet.
[Illustration: DIAGRAM 31.
A, Eosinophile leucocyte; B, bacterium; C, leucocytes killing bacterium
with their enzyme; D, leucocytes leaving bacterium dead; E, hyaline
leucocyte devouring dead bacterium.]
Such, then, is very briefly the story of the leucocyte, neglecting such
problems as the differences between those found in the blood, called
white corpuscles to distinguish them from the red corpuscles, with
which they have no sort of connection; those found in the lymph, called
lymphocytes to distinguish them from those found in the blood; those
caught in the act of devouring bone, called osteoclasts; and those found
with bacteria inside them, therefore known as phagocytes; and without
speculating on how long an individual lives, and whether the different
varieties differ in origin or are merely at progressive stages of
development. The study of leucocytes is one of the most fascinating in
physiology, but we have many other things calling for our attention, and
we have said enough about the part they play in the life of the body to
justify our passing on to consider another essential movement.
IV.
Next in natural order for consideration come the movements of the
alimentary canal.
So far we have considered this structure as a chemical laboratory, a tube
consisting of a single layer of cells which secrete ferments into the
lumen, where digestion takes place, and then absorb the products, and we
have not yet accounted for the food travelling along the tube, without
which its functions, as described in the earlier part of the book, could
not be performed. That the passage of the food is not due to gravitation
is obvious from the many directions of the tube’s coils—not to quote the
old instance of a horse drinking, in which case the liquid first travels
upwards. One must therefore conclude in favour of some muscular method of
propulsion.
We have so far described the alimentary canal as a single layer of cells,
but it must be obvious that these soft secreting portions of the tube are
not capable of vigorous movement. The canal proper is surrounded by a
tough sheath of connective tissue which prevents its being overdistended
or ruptured, and, by means of a layer—or, rather, two layers—of
non-striped muscle which it contains, produces the movements which result
in the passage of its contents along the tube. These two layers lie well
to the outside of the connective-tissue sheath. The fibres of the inner
layer are arranged circularly, so as to form rings round the tube; those
of the outer have a longitudinal direction, running, therefore, parallel
with its long axis. When the former contract, the diameter of the tube is
reduced, while contraction of the latter has the effect of enlarging it.
(See Diagram 32.)
The movements of the intestine are what is known as peristaltic.
Contraction of the muscle fibres is not simultaneous in all parts, but
passes in waves along it. Just in front of the food the longitudinal
fibres contract, and thus offer less resistance, while just behind the
circular fibres reduce the size of the tube, and so get up a pressure.
The result of a number of successive waves of contraction passing down
the alimentary canal is that the food is propelled along it.
[Illustration: DIAGRAM 32.—TO ILLUSTRATE THE PASSAGE OF FOOD ALONG THE
INTESTINE.]
The arrangement of the muscle varies in places to suit special needs.
Where the tube suddenly enlarges to form the stomach, and where the
stomach suddenly narrows to the intestine, there are two strong rings
of muscle, whose constricting influence converts the enlargement into a
closed chamber during gastric digestion; while the coats which actually
clothe it here run obliquely, and their activity causes the contents to
be slowly churned about inside.
Thus it will be seen that it is not only the voluntary muscles which
give the alimentary system its opportunities; without these unobtrusive
non-striped cells we should toil for our bread and swallow it in vain.
V.
Our next step, after having surveyed the principle of movement by which
the chemical necessities of the body are exposed to its absorbing
surface, must be to see how the fluid which transports them is made to
pass along the tubes containing it. We have already had occasion to
describe how these blood and lymph vessels ramify through all the organs,
when we were dealing with the chemical influence of the blood and lymph.
The tubes through which the lymph is brought back to the blood-stream
have thin walls, and no muscle of their own. They are subjected, however,
to a constantly varying pressure by the movements of the limbs and
trunk, and as, owing to valves inside them, the lymph can only escape in
one direction, there is a constant flow towards the junction with the
bloodvessels.
The bloodvessels are quite different. A far more certain and expeditious
current is necessary—hence the steady circulation through a system of
closed tubes.
In order to understand this passage of the blood, it is necessary to keep
in mind the great principle with which hydrostatics supplies us, viz.,
that a liquid always flows from a region of high to a region of lower
pressure. The problem of the vascular system is, therefore: How can the
pressure within a ring of tube be so arranged as to maintain a regular
flow always in the same direction?
Let us begin with the structure of the system. The tube through which
the blood first passes on leaving the heart is composed of four distinct
and essential elements: A lining of endothelial cells, which we need not
discuss at length; a main substance of tough white fibrous connective
tissue; elastic fibres and muscle fibres, the two last arranged in the
substance of the connective tissue. All these parts are present in
the main arteries which leave the heart, but in the fine meshwork of
capillaries to which the arteries give rise by repeated branchings there
is nothing left of the outer coats, only the lining of endothelial cells
separating the blood from the organ traversed. In the veins which these
capillaries unite to form, the connective-tissue sheath reappears, and
also some muscle; but the elastic coat is quite absent. The heart is
really a double coil of the tube (see Diagram 12), in which the muscular
coat is predominant, and is divided into four chambers by the valves,
which insure the blood flowing in the right direction when it contracts.
(See Diagram 33.)
[Illustration: DIAGRAM 33.—SCHEME OF CIRCULATION]
The way in which these structures work is as follows: Two of the chambers
of the heart (the auricles) receive blood from the veins, and when full
suddenly contract, driving their contents into the other two chambers
(the ventricles). The blood does not run back into the veins, although
the pressure in them is very low and there are no valves to prevent
it, because there is still less pressure in the ventricles, and also
because the veins enter the auricles obliquely, and the tendency of the
increasing pressure is to close their orifices. Having discharged the
blood into the ventricles, the auricles relax, and the pressure within
being a minus quantity, they are speedily filled with blood from the
veins, blood not being able to return after entering the ventricles, as
valves close automatically to prevent it.
Stimulated by the blood distending them, the ventricles then contract
simultaneously like the auricles, only with much greater force: for the
right ventricle has to drive the blood all through the vessels pervading
the lungs back to the left auricle; whilst the left ventricle, which is
proportionately stronger than the right, has to send its contents to
the furthest extremities of the body. They then relax, in order that
conditions of their internal pressure may favour another inflow from the
auricles, return of blood from the arteries being, as in the preceding
case, prevented by valves.
The pressure in the arteries during life is always fairly high; indeed,
the ventricles have to get up a considerable force before the valves
leading from them will open. The result of this is not only that the
blood is driven along them with a rush, but also that they are slightly
distended at each beat; and so, owing to the elasticity of their walls,
the blood continues to flow forwards even between the beats of the heart.
The rest of the journey is quite simple; the pressure in the capillaries
is lower than in the arteries, and the pressure in the veins lower than
in the capillaries, and lower in the veins, too, as they approach the
heart, till, where they join the auricle, it is actually minus, and the
blood has no other course open to it but to return to the auricle. It
looks as though accidents might happen in the veins owing to there being
so low a pressure there to direct the current, but this is prevented by
the presence of valves at intervals, to stop any return.
The rate at which the blood travels is another point which has an
important bearing on the nutrition. It does its work—_i.e._, gives out
nutriment and picks up refuse—whilst flowing through the capillaries; so
here one finds that it moves slowly. On the other hand, the sooner it
reaches them the better, so it races fast through the arteries. Finally,
its return to the heart need not be delayed, so it is quickened up again
through the veins. The principle by which this variation in the rate
of flow is obtained is simple and inevitable. If a tube through which
liquid is flowing is not the same size all the way along, the liquid
will be found to flow faster in the narrow parts than in the wider ones.
Now, in branching, the arteries do not keep becoming smaller in regular
proportion, and the result is that the capillaries have collectively a
diameter five hundred times larger than the aorta; hence the blood flows
through them only one-five-hundredth of the pace at which it leaves the
heart. But in uniting again to form the veins their cross-section is
reduced once more, so that that of the large veins near the heart is only
two and a half times larger than that of the aorta, and hence a flow only
two and a half times slower results.
The pace of the blood-stream must depend, obviously, on the pressure of
the blood in the arteries. This pressure is altered either by changing
the rapidity of the heart-beat or the diameter of the arteries, which
are capable of considerable variation owing to their muscular coat. The
regulation of the blood-pressure is managed by the nervous system, so
does not belong here, and we may leave it after mentioning one or two
facts. High pressure is due to a large quantity of blood being in the
arteries, and this may be due either to the rapidity with which it is
injected by the heart or to the reduced capacity of the bloodvessels
themselves. High pressure, due to the latter cause, throws a great strain
upon the heart, owing to the hard work it has in pumping blood into
the arteries; with a low pressure the heart beats feebly, having less
resistance to overcome.
Blood-pressure can be raised by stimulating the muscular coat and
reducing the capacity of the bloodvessels, and lowered by causing the
heart to beat more slowly or by removing blood from the body. This
latter operation was a favourite way with doctors of the old school; but
as our knowledge of physiology, and with it our control over the vital
functions, increases, such crude and heroic remedies are able to be
replaced by others which are less dangerous.
VI.
Comparisons are rightly regarded as objectionable, so it would hardly be
safe to say that the group of movements whose primary object is filling
the lungs, and which we must study next, is the most important in the
body, especially when we have just been speaking of the circulation,
which, however, would be of but little use if the blood could not
be oxidized; but we can at least say that its importance cannot be
overrated, so far-reaching are its effects.
The lungs are, as we have described them above, a pair of delicate
membranous sacs connected by a tube, the trachea, with the alimentary
canal, from which they originally budded out. They are subdivided,
though how we need not describe in detail, into a vast number of small
compartments, so as to give the maximum surface in the space accorded
them, and the whole somewhat resembles a cluster of grapes, the stalks
being the branches of the trachea. The membranous parts are pervaded by
an elastic network, enveloping the compartments in such a way that it
would reduce them permanently to the resemblance of a bunch of raisins
rather than grapes, were it not that they are enclosed in an airtight
box—the thorax—from the walls of which they cannot shrink without causing
a vacuum. Owing, however, to the latter arrangement and the trachea being
open to the external world, they are always more or less distended with
air.
The thorax, which they thus must always exactly fill, is a conical-shaped
box, its walls being the ribs, and its floor a sheet of muscle known as
the diaphragm. It contains, besides the lungs, only the heart and large
bloodvessels. The problem, therefore, of drawing air into the lungs and
(after the gaseous interchange described in Essay II., Section IV., has
taken place) of expelling it again, becomes solely a matter of increasing
and decreasing the capacity of the thorax. (See Diagram 34.) This can
be done in two ways: the diameter through the ribs can be increased, or
the diaphragm can be pulled down, increasing its depth. Actually, both
these methods come into play together. Diagram 35 will probably give a
better idea of how this is done than could easily be conveyed by a verbal
description. An attempt is here made to show the action of the ribs and
the diaphragm—first, of each separately, then of the two combined. The
elasticity of the lungs themselves is sufficient to drive out the tidal
air if the diaphragm and the muscles of the ribs are relaxed, though in
hard breathing a muscular movement may depress the ribs and a contraction
of the abdominal muscles force up the diaphragm.
[Illustration: DIAGRAM 34.—MODEL (ADAPTED FROM RUTHERFORD) FOR SHOWING
HOW THE LUNGS ARE FILLED WITH AIR BY ALTERING THE SIZE OF THE THORAX.]
But though the primary object of raising the ribs and depressing the
diaphragm may be to fill the lungs, its secondary influence upon
the trunk as a whole is hardly less important. The effect upon the
circulation is profound. The compartments of the lungs are enveloped in
innumerable capillary bloodvessels, and, as these lie around and between
them in the cavity of the thorax, they must, when breath is drawn in, be
subjected to a negative pressure before the lung itself, and be the first
to experience a positive pressure when the air is expelled. Here, again,
a diagram is the best explanation. (See Diagram 36.)
The pulmonary vessels, moreover, are not the only ones influenced. The
reader who attentively examined Diagram 13 must have been struck by the
peculiarities of the circulation through the spleen, intestine and liver,
and the obstacles which this repeated breaking up into fine vessels must
offer to the flow of blood, as described in Section V. of this essay.
[Illustration: DIAGRAM 35.—SHOWING HOW THE CAPACITY OF THE THORAX IS
INCREASED BY RAISING THE RIBS AND DEPRESSING THE DIAPHRAGM.]
[Illustration: DIAGRAM 36.—MODEL FOR SHOWING EFFECT OF MOVEMENTS OF THE
THORAX ON THE PULMONARY CIRCULATION.]
The liver forms the crux of the situation. (See Diagram 37.) A vein
carrying blood from the intestine and spleen is broken up into fine
capillaries to pass through that organ, and the pressure in this vein
is extremely low. How is a sufficiently rapid flow of blood to be
maintained? The answer to this riddle is best given by Diagram 38, which
shows how, by the contraction of the diaphragm at each breath, the large
veins entering the heart are subjected to a negative pressure which draws
blood out of the liver, while, simultaneously, that organ is squeezed and
the blood it contains forced out. Obviously this natural pump influences
not only the flow of blood, but also that of the lymph, and what was
said about the hepatic vessels also holds good for the thoracic duct, up
which the lymph, rich with fat absorbed from the intestine, passes to be
emptied into the large veins near the heart. So, though vigour in the
action of the diaphragm is more favourable to health than necessary to
life, deep breathing is an essential factor in the well-being of the body.
[Illustration: DIAGRAM 37.—A DIAGRAMMATIC VIEW OF THE CIRCULATION THROUGH
THE ORGANS UPON WHICH THE DIAPHRAGM PRESSES WHEN IT DESCENDS.]