Enclosed in this bag, it is thoroughly mixed with weak hydrochloric
acid, secreted by numerous glands, and kept churning round and round by
the muscular action of its walls, that the contents may be kept well
mixed. The acid is just strong enough to kill protoplasm, and hence the
putrefactive bacteria which were necessary in the mouth, but would be
a very doubtful blessing in the interior of the body, are disposed of.
Other things are also killed. Not only does the stomach execute intruding
bacteria, but it also kills a good deal of our food. Fruit and salad
consist largely of still living cells, and occasionally there is bigger
game, _e.g._, oysters. One thing, however, the acid does not kill, and
that is the cells lining the stomach, and it may as well be said here
that the parts of the body exposed to ferments have the very necessary
power of resisting them, so that a normal animal does not digest itself.
The stomach, however, is a kitchen as well as a slaughter-house. The
gastric juice, or secretion of all the glands opening into it, contains,
besides the acid, two important ferments, both of which act on proteids.
Carbohydrates are absorbed, but not digested, in the stomach, as acid
destroys saliva. One of the ferments is rennet, an article familiar
to the culinary profession, which solidifies milk. The other acts on
proteids generally, converting them ultimately into a very simple form,
peptone, which is absorbed at once. How much of the proteid in the
stomach is converted into peptone is not known, for the action of acid
alone is sufficient to enable it to be absorbed. A solution of proteid,
_e.g._, white of egg, is quite altered if made slightly acid; it no
longer coagulates when boiled, but the change of the most practical
interest is that, if injected into the veins, it seems to become part of
the blood, while ordinary proteids act as poisons.
The peptonizing ferment, however, has one very important function: it
digests the collagen of the connective tissue, the substance which
becomes gelatin when boiled. The reason why this is so important is not
only that nothing else in the body affects it, but that fat is enclosed
in it, and if it were not thus set free would pass through the body
unabsorbed.
The final stage is the digestion by the pancreatic juice. After the
food has been exposed for some time to the gastric juice, it is allowed
to escape a little at a time from the stomach, and continues its way
along the alimentary tube. Before it has gone many inches it comes to
the openings of two ducts, those of the liver and the pancreas, and
immediately the acid stimulates them, and the glands pour out their
secretion. That of the liver is largely excretion or refuse from the
blood without direct action on the food, but it enables the pancreatic
juice to do its work by making the food again alkaline, and stimulates
the muscular coats of the intestine to force its contents along. That
of the pancreas is the most important digestive fluid in the body,
containing many ferments; it acts alike on proteids, carbohydrates, and
fats—in fact, digests everything—so that the rest of the long tube is
freed from any more laborious duty than absorbing them as they pass.
NOTE.—The digestive ferments are now prepared for examination by chopping
up the gland and placing it in glycerine; this extracts the ferment
and preserves it from the action of bacteria. The first experiments on
digestion, however, constitute one of the romances of physiology. A
Canadian named St. Martin got into trouble with Red Indians whilst in the
United States, America, and was shot through the body. The surgeon who
attended him was unable to make the wound close, and when it healed there
remained an opening in the man’s body communicating directly with his
stomach. The surgeon, Beaumont, saw possibilities in this, and, obtaining
gastric juice from his patient, made those classical experiments which
entitled him to a place among the fathers of physiology. Americans do
well to be proud of Beaumont, for it cost him many sacrifices, and his
patience and courage are above praise. Not only was he devoid of all
but the crudest appliances out in the backwoods, but his subject proved
intractable and mercenary. No sooner did he discover his value than he
crossed the border, and refused to return except upon exorbitant payment.
Even after this had been arranged, he repeated the performance whenever
he thought fresh extortion possible. In spite of these difficulties, the
investigations proved wonderfully accurate and complete.
IV.
Of the absorption of the materials thus prepared it is not necessary to
say much in a work of this compass, but the absorption of oxygen is too
important to be passed over.
Oxygen is required by the body pure, and, as it is uncombined with
anything in the air, it needs no digestion to free it. A special
organ, however, is necessary to absorb it. This is the lung. The lungs
originate, just like a gland, by a pouching of the alimentary canal
near its origin, but differ from a gland in their cells being very much
flattened, to offer a large surface to the air on one side and to the
bloodvessels on the other. Incessantly during life air is being drawn
into the lungs; that the cells to which it is there exposed may transfer
its oxygen to the blood; and then, after the cells have also transferred
the carbonic acid gas from the blood to the air, driven out again to be
replaced by fresh.
(The mechanical means by which the lungs are filled and emptied come
under another heading.)
V.
Food having been absorbed by cells set apart for the purpose, the next
problem is, How is it distributed to those specialized for other work?
The medium for this distribution is a liquid called lymph. All the spaces
in the body are filled with lymph, all the organs bathed with it, every
cell moistened with it; yet it is comparatively stagnant, and the food
has to be conveyed from the walls of the alimentary canal to the lymph in
the neighbourhood of the cell requiring nourishment by a more expeditious
agent. This is done by the blood.
[Illustration: DIAGRAM 11.—PRINCIPLE OF THE SYSTEM OF BLOODVESSELS.]
[Illustration: DIAGRAM 12.—PRINCIPLE OF DOUBLE CIRCULATORY SYSTEM.]
The blood is a fluid akin to the lymph, but confined in a system of
tubes. Through these tubes it is driven at a considerable velocity, and
in the course it takes passes within a reasonable distance of every
cell in the body. As it passes the cells of the alimentary canal, they
discharge the nutriment they have absorbed into it; as it passes through
the other organs of the body, it discharges the requisite materials
into the lymph bathing the actual cells: these are then able to help
themselves.
The lymphatic system is very simple. Lymph is practically fluid which
has exuded through the walls of the bloodvessels, and is like the plasma
of the blood, a thin solution of proteids in water containing just
enough salt to hold them in solution. From different parts of the body
a series of tubes run towards the heart, going up with increase in size
and decrease in number as they near it. Into these tubes the lymph is
forced with every movement of the body. At a slow rate, but varying with
the activity of the animal, it is forced to flow along these tubes,
regurgitation being prevented by valves at intervals, until it reaches
the place where the lymphatic vessels join a large vein, and it is poured
back into the blood-stream, thus completing its cycle.
The blood is entirely confined in a closed system of tubes, along which
it moves always in the same direction. The main principle of the system
is that of a ring. One side of the ring is split into a vast number of
fine tubes to give a large surface for absorption and discharge of food
among the cells; the other side is a single tube, with an enlargement
in which the blood from different parts is mixed (see Diagram 11). This
enlargement, which is contractile and fitted with valves, rhythmically
draws the blood in from one direction and pumps it out in another. (The
mechanics of the process we shall study later.)
As a matter of fact, this system is twofold, as in Diagram 12. In passing
through one-half of its course the blood absorbs oxygen in the lungs; in
the other it yields oxygen to the tissues, and absorbs, whilst passing
over the alimentary canal, proteid, carbohydrate, water, and salts, which
are duly distributed to the other organs. Fat is absorbed by the lymph
direct, but poured into the blood for distribution.
The blood which passes over the alimentary canal on its way back to the
heart goes through the liver. In this gland it leaves the carbohydrate
which it has taken up, and a large store is laid down there after a meal,
to be doled out as it is wanted. Blood also passes through the liver from
the spleen, where it has been, so to speak, overhauled for repairs.
[Illustration: DIAGRAM 13.—SCHEME OF THE CIRCULATORY SYSTEM.
Blood system on the right, lymph system on the left.]
As the medium for chemical communication throughout the community of
cells, the blood has another all-important and obvious function, viz.,
that of clearing away the waste products of life. Of these there is, of
course, the same quantity as of new material introduced. Carbonic acid
gas is discharged into the lungs, but all the nitrogen and most of the
other elements in the new combinations which protoplasm has made them
assume leave by the kidneys, plus a little water by the skin as sweat and
a few items discharged into the last part of the alimentary canal amongst
the unabsorbed portions of the food.
In their constituents, blood and lymph resemble one another, being
both weak solutions of salts and proteid material; but the blood is
distinguished from the lymph by the presence of innumerable extremely
minute bodies, which give it its red colour. These corpuscles, to give
them their proper name, are the vehicle by which oxygen is transported
from the lungs to the tissues. They consist of an envelope of protoplasm
filled with a red fluid (hæmoglobin), which combines loosely and easily
with oxygen. In shape they are discoid, with a thickened rim and
biconcave sides, another device for increasing surface and reducing bulk.
(See Diagram 14.)
[Illustration: DIAGRAM 14.—A RED BLOOD CORPUSCLE.]
With one more fact we may now conclude the chemical survey of the body.
The blood has to pass through certain glands, or it becomes poisoned, and
this quite apart from whether the gland secretes healthily or not.
Disease of the thyroid (a ductless gland in the Adam’s apple) causes
goitre; of the suprarenal, Addison’s disease; of the pancreas, diabetes.
Whether these organs secrete some substance into the blood which
counteracts poisons formed in it, or whether they remove injurious
elements from it, is not certain, but they are necessary to keep the
great means of chemical communication in order.
NOTE.—The thyroid gland no longer secretes anything into the alimentary
canal, and its duct disappears at an early age. If, however, it become
diseased or is surgically removed, the distressing symptoms of goitre
supervene. Such a patient may be completely cured by grafting a thyroid,
excised from another animal, anywhere in his body. Doctors usually,
however, give the patient extract of sheep’s thyroid either in pills or
injections.
ESSAY III.
THE MECHANICS AND PHYSICS OF THE BODY.
I.
In the preceding essay we regarded protoplasm as a chemical factor in the
universe.
We have seen how it is always changing, always taking in food, always
giving off waste materials. We have seen, too, that it grows and that it
does work, and that in a large mass the cells which compose it share the
labour instead of each component cell performing all the vital functions.
We have now to consider the work which protoplasm does—in a word, the
mechanical effect of the chemical actions just described.
The simplest movement of protoplasm is to be seen by the aid of the
microscope in certain vegetable cells, where granules seem always
streaming about in different directions. A step higher, and we find
this streaming movement converted into movements of the whole cell.
In the simplest unicellular animals the fluid protoplasm is contained
in a membrane, or denser bounding layer, to which are attached fine
filaments springing from a minute body known as the centrosome. These
centrosomes—for there are sometimes several in a cell—seem to control the
mechanical department, just as the nucleus does the chemical. Along the
fibrils at intervals are minute globules, and by watching the distance
between them it is seen that the fibrils undergo changes in length,
pulling in the membrane when they shorten, and letting the cell flow out
in any direction when they relax. By adjusting these two movements to
balance one another, the cell can move in any direction, surround and
engulf particles of food, and assume a strange variety of shapes. (See
Diagram 1.)
[Illustration: DIAGRAM 15.—CELL DIVISION.]
In some cells, probably in all, the centrosome presides over division.
Cells, however, do not always divide in the same way. Some simply
lengthen, the nucleus also lengthening inside, become constricted in the
middle like a dumb-bell, and separate. (See Diagram 15.)
[Illustration: DIAGRAM 16.—CELL DIVISION.]
Others manage differently. In them the nucleus simply bursts, and turns
its essential elements, a number—always a constant number—of coarse
threads, adrift. Meanwhile, two centrosomes have moved to opposite ends
of the cell, and there anchored themselves by fibrils; other fibrils
springing from them become attached to the nuclear threads, and when all
is ready pull them apart, equally divided, to their respective ends,
where they re-form into two fresh nuclei. (See Diagram 16.)
Unicellular animals, which are constant in shape and swim instead of
flowing when they want to get anywhere, have at first sight nothing
in common with those which do the latter. From their surfaces spring
fringes of free protoplasmic threads, called cilia, from their fancied
resemblance to eyelashes, which serve as motor organs, and beat the water
like oars. (See Diagram 2.) Waves of movement, as they lash one after
another, all in the same direction, seem to pass over the cell, and it
is propelled through the water; while others, which are situated in the
neighbourhood of the cell’s mouth, stir the water into eddies, and drive
food particles into it.
[Illustration: DIAGRAM 17.—CILIA OF AN EPITHELIAL CELL.]
These cilia are important, as they are adapted for many purposes in
large animals. The cells which line the cavity at the back of the nose,
the tubes of the lungs, and other parts of the body, have a few cilia
on their free surface, and it is in them that the structure of these
organs can best be made out. At the foot of each cilium is a minute
globule, from which a fine fibril passes into the cell, and the fibrils,
collectively forming a leash, are attached to its opposite end. (See
Diagram 17.) It seems highly probable that the globule is a centrosome
giving rise to two fibrils, one attached as described, the other passing
up one side of the cilium, and fast to its apex. The result of this
arrangement is that when the fibrils contract the cilium is bent over
with a jerk to the side up which the fibril runs, and when they relax it
slowly straightens itself. There is, therefore, no fundamental difference
between this and the other mode of progression; both are dependent upon
the centrosome.
Finally we have muscle cells. These are only found in a fairly
complicated animal, since they are a product of the division of labour
principle, and their sole business is movement. There are two varieties
of muscle, but the principle is the same in both—a long thin cell, with
fibrils traversing its length whose contraction causes the cell to
shorten and thicken, thus reducing the distance between its two ends. At
present the development of muscle and the way in which it ‘contracts,’
to use the word accepted in this case for describing a redistribution of
bulk, are little understood, and there are accordingly many opinions; but
I think careful study will eventually show that some modification of the
centrosome, with its contractile fibrils, is responsible for the movement.
[Illustration: DIAGRAM 18.—MUSCLE.]
The two varieties are: the smooth, or involuntary, and the striped, or
voluntary, muscle. Smooth muscle consists of spindle-shaped cells with
one elongated nucleus. (See Diagram 18, Fig. 1.) It only contracts very
slowly, and is not under control of the will; but it is very abundant
in the body, since it effects practically all the movements of the
alimentary canal and bloodvessels. Voluntary or striped muscle, so called
from its appearance under the low power of a microscope, consists of
long fibres, each containing many nuclei. (See Diagram 18, Fig. 2.) Its
protoplasm is rich in hæmoglobin, and in it, under powerful microscopes,
can be made out two kinds of fibrils: Rutherford’s fibrils, the
complicated structure of which gives muscle its striped appearance; and
Marshall’s fibrils, which are much finer and more difficult to see. The
muscle of the heart, though not under control of the will, is striped;
but it differs from ordinary striped muscle in being made up of small
branched cells with only one nucleus.
The way in which the three elements of striped muscle contribute to a
contraction is practically unknown, and the subject of much dispute. In
fact, one could hardly wish for a better soil for theories, and some
which grow in it are very wonderful indeed. We have reason for supposing
that there are two contractile substances—one which gives a sharp twitch,
the other a slow, hard pull; and on the whole there seems good reason
to believe that Rutherford’s fibrils give the sudden movements, while
Marshall’s give the more forcible ones; and that the ordinary protoplasm
of the cell is restricted to the duty of nourishing the fibrils.
[Illustration: DIAGRAM 19.—STRIPED MUSCLE FIBRE, MORE HIGHLY MAGNIFIED
THAN IN DIAGRAM 18.]
The muscle cells are modified from among those of the bud forming the
middle layers of the embryo. (See Diagram 5.) Other cells of this bud
form connective tissue, by, so to speak, spinning long fibres of the
substance called collagen, which turns to gelatin when boiled. (See
Diagram 20.) This connective tissue permeates the whole body, affording a
firm foundation for the many layers of cells which form the skin and the
single layer of digestive cells; supporting the other organs throughout,
and keeping the different parts of the body in their places, in doing
which, however, it is assisted by other fibres which are not collagenous,
but elastic. It also forms tracts which become lymph and blood vessels.
In parts of the animal which require special support it forms solid
rods, the collagen combining with calcium salts to form a clear, hard
substance—cartilage. At one period in the development of an animal or
animals we find the only solid support is cartilage, but cartilage is not
sufficiently rigid for a very large beast, especially on land, so is only
used for outlying parts, the main framework being bone.
[Illustration: DIAGRAM 20.—A CONNECTIVE-TISSUE CELL GIVING RISE TO LONG
COLLAGENOUS FIBRES.]
Bone is formed very much as if Nature were rectifying a mistake. When a
rod of cartilage is unequal to its work it is eaten hollow, and fresh
connective-tissue cells immigrate and fill up the cavity, eventually
laying down a fine network of cells in its place, the meshes of which
are filled with inorganic calcium salts, chiefly phosphate of lime.
Nature then benefits by experience, and the last bones to be formed are
not preceded by any makeshift cartilage, but built up straight away in
ordinary connective tissue.
This brings us back again to muscle, for the object of nearly all the
voluntary muscle is to cause movement among the bones. For this purpose
the muscle cells or fibres are arranged parallel to one another, and
bound up together by connective tissue, the whole bundle being known
as ‘a muscle.’ The two ends of a muscle are attached to two bones by
connective tissue, which sometimes forms a short cord, or tendon. Then,
when the muscle contracts, the two places of its attachment are pulled
towards one another, and something has to move. But before saying more
about the way in which the bones are jointed and muscles attached—in
fact, what movements are possible in the human body—it would be as well
here to describe the chief properties of muscle and the way in which they
are studied.
II.
[Illustration: DIAGRAM 21.—APPARATUS FOR RECORDING A MUSCULAR
CONTRACTION.]
The way in which voluntary muscle is studied is very simple. A frog is
killed by thrusting a probe into the brain and down the spinal cord, and
a muscle is then dissected out and attached to a piece of apparatus (see
Diagram 21) in such a way that on its contracting it raises a lever,
and draws a line on a moving surface. The rate at which the surface
is moving is ascertained, so that the nature of the curve, which is a
graphic record of the contraction, can be analyzed. (See Diagram 22.) For
instance, when an electric shock is used to make the muscle contract, we
find that a slight shock causes a small contraction, as shown by a low
curve, while a stronger one, up to a certain point, causes an increase.
[Illustration: DIAGRAM 22.—GRAPHIC RECORD OF A RESPONSE TO A SINGLE
STIMULUS APPLIED AT A.
Lower line = tuning-fork records of ⅟₁₀₀″.]
But having described how muscle is studied, it is only necessary to
state a few facts concerning it; to discuss muscle, fully describing the
experiments by which its more obscure properties have been elucidated,
and the devices by which causes of error have been eliminated, would fill
volumes.
[Illustration: DIAGRAM 23.—CONTRACTIONS WITH TWO STIMULI AT DIFFERENT
INTERVALS OF TIME.]
Muscle is thrown into a state of contraction by an impulse reaching it
from a nerve, but it contracts quite as readily if excited directly
by a mechanical or electrical shock. A second shock causes a second
contraction, or, if the muscle is still in a state of contraction owing
to the first, causes it to contract still more. (See Diagram 23.) If a
number of stimuli are applied to a muscle in such rapid succession that
the effect of the preceding one has not passed off by the time the next
arrives, it will contract as far as possible, and remain contracted—a
state known as tetanus. (See Diagram 24.) A muscle is therefore kept in
a state of contraction by a continuous nervous effort, not arranged and
then left contracted.
[Illustration: DIAGRAM 24.—TETANUS.]
[Illustration: DIAGRAM 25.—FATIGUE CURVES.
Fast drum: a, point of stimulation. Every tenth contraction recorded.]
[Illustration: DIAGRAM 26.—EFFECT OF FATIGUE ON MUSCULAR CONTRACTION.
Slow drum. Every contraction recorded.]
Various conditions alter the character of a muscular response. With
repeated stimuli at short intervals a muscle fatigues, and each
contraction becomes smaller in extent and longer in duration. (See
Diagrams 25 and 26.) If the muscle has to lift a load it has a certain
check on its contraction, and its relaxation time is shortened.
Temperature also affects muscular contraction, moderate increase causing
a sharper, and moderate cooling a slower, rise and fall of the lever
on stimulation. (See Diagram 27.) Lastly, we have drugs which exert an
influence, but the only one of these which it is necessary to mention
here is veratria, which makes the slowly contracting fibrils continue
their activity after the quick ones have subsided. (See Diagram 28.)
[Illustration: DIAGRAM 27.—EFFECT OF TEMPERATURE.]
[Illustration: DIAGRAM 28.—VERATRIA CURVE.]
Finally, there are the electrical changes in muscle. These, again, may be
passed over briefly, since they are not easily understood or described.
To put the facts in a nutshell, the part of a muscle which is in activity
is negative to all other parts. Thus, if a muscle be dissected out and
cut across, the activity at the seat of the injury, while it lasts,
causes a current to pass through a galvanometer from uninjured parts
to the wounded. (See Diagram 29.) Again, if a muscle be dissected out
without injury, connected at two points with a galvanometer, and then
stimulated at one end, as the wave of contraction passes along it, first
one, then the other, contact becomes negative. (See Diagram 30.) S,
Stimulating electrodes; N, contraction which marks the wave of excitation
passing along the muscle; G, galvanometer which shows that the seat of
activity (N) is negative to the rest of the muscle.
[Illustration: DIAGRAM 29.—INJURY CURRENT: CROSS-SECTION OF MUSCLE
NEGATIVE TO REST.]
[Illustration: DIAGRAM 30.—ACTION CURRENT.]