THE PROCESS OF LIFE.
In the foregoing chapter, on "The Nature of Animal Life," we have seen
that animals breathe, feed, grow, are sensitive, exhibit various
activities, and reproduce their kind. These may be regarded as primary
life-processes, in virtue of which the animal characterized by them
is a living creature. We have now to consider some of these
life-processes--the sum of which we may term the process of life--a
little more fully and closely.
The substance that exhibits these life-processes is protoplasm, which
exists in minute separate masses termed cells. It seems probable,
however, that these cells, separate as they seem, are in some cases
united to each other by minute protoplasmic filaments. In the higher
animals the cells in different parts of the body take on different forms
and perform different functions. Like cells with like functions are also
aggregated together into tissues. Thus the surfaces of the body,
external and internal, are bounded by or lined with epithelial tissue;
the bones and framework of the body are composed of skeletal tissue;
nervous tissue goes to form the brain and nerves; contractile tissue is
found in the muscles; while the blood and lymph form a peculiar
nutritive tissue. The organs of the body are distinct parts performing
definite functions, such as the heart, stomach, or liver. An organ may
be composed of several tissues. Thus the heart has contractile tissue in
its muscular walls, epithelial tissue lining its cavities, and skeletal
tissue forming its framework. Still, notwithstanding their aggregation
into tissues and organs, it remains true that the body of one of the
higher animals is composed of cells, together with certain
cell-products, horny, calcareous, or other. The simplest animals, called
protozoa, are, however, unicellular, each organism being constituted by
a single cell.
We must notice that, even during periods of apparent inactivity--for
example, during sleep--many life-processes are still in activity, though
the vigour of action may be somewhat reduced. When we are fast asleep,
respiration, the heart-beat,[B] and the onward propulsion of food
through the alimentary canal, are still going on. Even at rest, the
living animal is a _going_ machine. In some cases, however, as during
the hibernating sleep of the dormouse or the bear, the vital activities
fall to the lowest possible ebb. Moreover, in some cases, the
life-processes may be temporarily arrested, but again taken up when the
special conditions giving rise to the temporary arrest are removed.
Frogs, for example, have been frozen, but have resumed their
life-activities when subsequently thawed.
Let us take the function of respiration as a starting-point in further
exemplification of the nature of the life-processes of animals.
The organs of respiration, in ourselves and all the mammalia, are the
lungs, which lie in the thoracic cavity of the chest, the walls of which
are bounded by the ribs and breast-bone, its floor being formed of a
muscular and movable partition, the diaphragm, which separates it from
the stomach and other alimentary viscera in the abdominal region. The
lungs fit closely, on either side of the heart, in this thoracic cavity;
and when the size of this cavity is altered by movements of the ribs and
diaphragm, air is either sucked into or expelled from the lungs through
the windpipe, which communicates with the exterior through the mouth or
nostrils. It is unnecessary to describe the minute structure of the
lungs; suffice it to say that, in the mammal, they contain a vast number
of tubes, all communicating eventually with the windpipe, and
terminating in little expanded sacs or bags. Around these little sacs
courses the blood in a network of minute capillary vessels, the walls of
which are so thin and delicate that the fluid they contain is only
separated from the gas within the sacs by a film of organic tissue.
The blood is a colourless fluid, containing a great number of round red
blood-discs, which, from their minute size and vast numbers, seem to
stain it red. They may be likened to a fleet of little boats, each
capable of being laden with a freight of oxygen gas, while the stream in
which they float is saturated with carbonic acid gas. This latter
escapes into the air-sacs as the fluid courses through the delicate
capillary tubes.
Whither goes the oxygen? Whence comes the carbonic acid gas? The answer
to these questions is found by following the course of the
blood-circulation. The propulsion of the blood throughout the body is
effected by the heart, an organ consisting, in mammals, of two receivers
(auricles) into which blood is poured, and two powerful force-pumps
(ventricles), supplied with blood from the receivers and driving it
through great arteries to various parts of the body. There are valves
between the receivers and the force-pumps and at the commencement of the
great arterial vessels, which ensure the passage of the blood in the
right direction. The two receivers lie side by side; the two force-pumps
form a single muscular mass; and all four are bound up into one organ;
but there is, during adult life, no direct communication between the
right and left receivers or the right and left force-pumps.
Let us now follow the purified stream, with its oxygen-laden
blood-discs, as it leaves the capillary tubes of the lungs. It generally
collects, augmented by blood from other similar vessels, into large
veins, which pour their contents into the left receiver. Thence it
passes on into the left force-pump, by which it is propelled, through a
great arterial vessel and the numerous branches it gives off, to the
head and brain, to the body and limbs, to the abdominal viscera; in
short, to all parts of the body except the lungs. In all the parts thus
supplied, the vessels at length break up into a delicate capillary
network, so that the blood-fluid is separated from the tissue-cells only
by the delicate organic film of the capillary walls. Then the blood
begins to re-collect into larger and larger veins. But a change has
taken place; the blood-discs have delivered up to the tissues their
freight of oxygen; the stream in which they float has been charged with
carbonic acid gas. The veins leading from various parts of the body
converge upon the heart and pour their contents into the right receiver;
thence the blood passes into the right force-pump, by which it is
propelled, by arteries, to the lungs. There the blood-discs are again
laden with oxygen, the stream is again purified of its carbonic acid
gas, and the blood proceeds on its course, to renew the cycle of its
circulation.
[Illustration: Fig. 6.--Diagram of circulation.
L.A., left auricle of the heart; L.V., left ventricle; H., capillary
plexus of the head; B., capillary plexus of the body; A.C., alimentary
canal; Lr., liver; R.A., right auricle of the heart; R.V., right
ventricle; Lu., lungs.]
Now, if we study the process of respiration and that of circulation,
with which it is so closely associated, in other forms of life, we shall
find many differences in detail. In the bird, for example, the mechanism
of respiration is different. There is no diaphragm, and the lungs are
scarcely distensible. There are, however, large air-sacs in the abdomen,
in the thoracic region, in the fork of the merry-thought, and elsewhere.
These are distensible, and to reach them the air has to pass through the
lungs, and as it thus passes through the delicate tubes of the lungs, it
supplies the blood with oxygen and takes away carbonic acid gas. In the
frog there is no diaphragm, and there are no ribs. The lungs are hollow
sacs with honey-combed sides, and they are inflated from the mouth,
which is used as a force-pump for this purpose. In the fish there are no
lungs, respiration being effected by means of gills. In these organs the
blood is separated from the water which passes over them (being gulped
in by the mouth and forced out between the gill-covers) by only a thin
organic film, so that it can take up the oxygen dissolved in the water,
and give up to the water the carbonic acid it contains. In fishes, too,
we have only one receiver and one force-pump, the blood passing through
the gills on its way to the various parts of the body. In the lobster,
again, there are gills, but the mechanism by which the water is drawn
over them is quite different, and the blood passes through them on its
way to the heart, after passing through the various organs of the body,
not on its way from the heart, as in vertebrate fishes. The blood, too,
has no red blood-discs. In the air-breathing insects the mechanism is,
again, altogether different. The air, which obtains access to the body
by spiracles in the sides (see Fig. 1, p. 3), is distributed by delicate
and beautiful tubes to all parts of the organs; so that the oxygen is
supplied to the tissues directly, and not through the intervention of a
blood-stream. In the earthworm, on the other hand, there is a
distributing blood-stream, but there is no mechanism for introducing the
air within the body; while in some of the lowliest forms of life there
is neither any introduction of air within the body nor any distribution
by means of a circulating fluid. Beginning, therefore, with the surface
of the body simply absorbent of oxygen, we have the concentration of the
absorbent parts in special regions, and an increase in the absorbent
surface, either (1) by the pushing out of processes into the surrounding
medium, as in gills; or (2) by the formation of internal cavities,
tubes, or branching passages, as in lungs and the tracheal air-system of
insects.
What, then, is the essential nature of the respiratory process thus so
differently manifested? Clearly the supply of oxygen to the cellular
tissue-elements, and, generally closely associated with this, the
getting rid of carbonic acid gas.
Let us now glance at the life-processes which minister to nutrition,
beginning, as before, with the mode in which these processes are
effected in ourselves.
The alimentary canal is a long tube running through the body from the
mouth to the vent. In the abdominal region it is coiled upon itself, so
that its great length may be conveniently packed away. Opening into this
tube are the ducts of certain glands, which secrete fluids which aid in
the digestion of the food. Into the mouth there open the ducts of the
salivary glands, which secrete the saliva; in the stomach there are a
vast number of minute gastric glands; in the intestine, besides some
minute tubular glands, there are the ducts of the large liver (which
secretes the bile) and the pancreas, or sweetbread. Since, with the
exception of the openings of these ducts, the alimentary canal is a
closed tube, its contents, though lying within the body, are in a sense
outside it, just as the fuel in a tubular boiler, though within the
boiler, is really outside it. The organic problem, therefore, is how to
get the nutritive materials through the walls of the tube and thus into
the body.
At an ordinary meal we are in the habit of consuming a certain amount of
meat, with some fat, together with bread and potatoes, and perhaps some
peas or beans and a little salt. This is followed by, say, milky
rice-pudding, with which we take some sugar; and a cheese course may,
perhaps, be added. The whole is washed down with water more or less
medicated with other fluid materials. Grouping these substances, there
are (1) water and salts, including calcium phosphate in the milk; (2)
meat, peas, milk, and cheese, all of which contain albuminous or allied
materials; (3) bread, potatoes, and rice, which contain starchy matters;
and here we may place the sugar; (4) fat, associated with the meat or
contained in the cream of the milk. Now, of all the materials thus
consumed, only the water, salts, and sugar are capable, in their
unaltered condition, of passing through the lining membrane of the
alimentary canal, and thus of entering the body. The albuminous
materials, the starchy matter, and the fat--that is to say, the main
elements of the food--are, in their raw state, absolutely useless for
nutritive purposes.
The preparation of the food begins in the mouth. The saliva here acts
upon some of the starchy matter, and converts it into a kind of sugar,
which _can_ pass through the lining membrane of the alimentary canal,
and thus enter the body. The fats and albuminous matters here remain
unaltered, though they are torn to pieces by the mastication effected by
the teeth. In the stomach the albuminous constituents of the meat are
attacked by the gastric juice and converted into peptones; and in this
new condition they, too, can soak through the lining membrane of the
alimentary canal, and thus can enter the body. In the stomach all action
on starch is arrested; but in the intestine, through the effect of a
ferment contained in the pancreatic juice, this action is resumed, and
the rest of the starch is converted into absorbable sugar. Another
principle contained in pancreatic juice takes effect on the albuminous
matters, and converts them into absorbable peptones. The pancreatic
juice also acts on the fats, converting them into an emulsion, that is
to say, causing them to break up into exceedingly minute globules, like
the butter globules in milk. It furthermore contains a ferment which
splits up the fats into fatty acids and glycerine; and these fatty
acids, with an alkaline carbonate contained in small quantities in
pancreatic juice, form soluble soaps, which further aid in emulsifying
fats. The bile also aids in emulsifying fats.
The effect, then, of the various digestive fluids upon the food is to
convert the starch, albuminous material, and fat into sugar, peptones,
glycerine, and soap, and thus render them capable of passing through the
lining membrane of the canal into the body.
The materials thus absorbed are either taken up into the blood-stream or
pass into a separate system of vessels called lacteals. All the blood
which comes away from the alimentary canal passes into the liver, and
there undergoes a good deal of elaboration in that great chemical
laboratory of the body. The fluid in the lacteals passes through
lymphatic glands, in which it too undergoes some elaboration before it
passes into the blood-stream by a large vessel or duct.
Thus the blood, which we have seen to be enriched with oxygen in the
lungs, is also enriched with prepared nutritive material through the
processes of digestion and absorption in the alimentary organs and
elaboration in the liver and lymphatic glands.
Here let us again notice that the details of the process of nutrition
vary very much in different forms of life. In some mammals the organs of
digestion are specially fitted to deal with a flesh diet; in others they
are suited for a diet of herbs. In the graminivorous birds the grain is
swallowed whole, and pounded up in the gizzard. The leech swallows
nothing but blood. The earthworm pours out a secretion on the leaves, by
which they are partially digested before they enter the body. Many
parasitic organisms have no digestive canal, the nutritive juices of
their host being absorbed by the general external surface of the body.
But the essential life-process is in all cases the same--the absorption
of nutritive matter to be supplied to the cell or cells of which the
organism is built up.
Thus in the mammal the blood, enriched with oxygen in the lungs, and
enriched also with nutritive fluids, is brought, in the course of its
circulation, into direct or indirect contact with all the myriads of
living cells in the body.
In the first place, the material thus supplied is utilized for and
ministers to the growth of the organs and tissues. This growth is
effected by the multiplication of the constituent cells. The cells
themselves have a very limited power of growth. But, especially in the
early stages of the life of the organism, when well supplied with
nutriment, the cells multiply rapidly, by a process of fission, or the
division of each cell into two daughter cells. The first part of the
cell to divide is the nucleus, the protoplasmic network of which shows,
during the process, curious and interesting arrangements and groupings
of the fibres. When the nucleus has divided, the surrounding protoplasm
is constricted, and separates into two portions, each of which contains
a daughter nucleus.
In addition to the multiplication of cells, there is the formation,
especially during periods of growth, of certain products of cell-life
and cell-activity. Bone, for example, is a more or less permanent
product of the activity of certain specialized cells.
There is, perhaps, no more wonderful instance of rapid and vigorous
growth than the formation of the antlers of deer. These splendid weapons
and adornments are shed and renewed every year. In the spring, when they
are growing, they are covered over with a dark skin provided with short,
fine, close-set hair, and technically termed "the velvet." If you lay
your hand on the growing antler, you will feel that it is hot with the
nutrient blood that is coursing beneath it. It is, too, exceedingly
sensitive and tender. An army of tens of thousands of busy living cells
is at work beneath that velvet surface, building the bony antlers,
preparing for the battles of autumn. Each minute cell knows its work,
and does it for the general good--so perfectly is the body knit into an
organic whole. It takes up from the nutrient blood the special materials
it requires; out of them it elaborates the crude bone-stuff, at first
soft as wax, but ere long to become as hard as stone; and then, having
done its work, having added its special morsel to the fabric of the
antler, it remains embedded and immured, buried beneath the
bone-products of its successors or descendants. No hive of bees is
busier or more replete with active life than the antler of a stag as it
grows beneath the soft, warm velvet. And thus are built up in the course
of a few weeks those splendid "beams," with their "tynes" and "snags,"
which, in the case of the wapiti, even in the confinement of our
Zoological Gardens, may reach a weight of thirty-two pounds, and which,
in the freedom of the Rocky Mountains, may reach such a size that a man
may walk, without stooping, beneath the archway made by setting up upon
their points the shed antlers. When the antler has reached its full
size, a circular ridge makes its appearance at a short distance from the
base. This is the "burr," which divides the antler into a short
"pedicel" next the skull, and the "beam" with its branches above. The
circulation in the blood-vessels of the beam now begins to languish, and
the velvet dies and peels off, leaving the hard, dead, bony substance
exposed. Then is the time for fighting, when the stags challenge each
other to single combat, while the hinds stand timidly by. But when the
period of battle is over, and the wars and loves of the year are past,
the bone beneath the burr begins to be eaten away and absorbed, through
the activity of certain large bone-eating cells, and, the base of
attachment being thus weakened, the beautiful antlers are shed; the
scarred surface skins over and heals, and only the hair-covered pedicel
of the antler is left.[C]
Not only are there these more or less permanent products of
cell-activity which are built up into the framework of the body; there
are other products of a less enduring, but, in the case of some of them,
not less useful character. The secretions, for example, which, as we
have seen, minister in such an important manner to nutrition, are of
this class. The salivary fluids, the gastric juice, the pancreatic
products, and the bile,--all of these are products of cell-life and
cell-activity. And then there are certain products of cell-life which
must be cast out from the body as soon as possible. These are got rid of
in the excretions, of which the carbonic acid gas expelled in the lungs
and the waste-products eliminated through the kidneys are examples. They
are the ultimate organic products of the combustion that takes place in
the muscular, nervous, and other tissues.
The animal organism has sometimes been likened to a steam-engine, in
which the food is the fuel which enters into combustion with the oxygen
taken in through the lungs. It may be worth while to modify and
modernize this analogy--always remembering, however, that it is an
analogy, and that it must not be pushed too far.
In the ordinary steam-engine the fuel is placed in the fire-box, to
which the oxygen of the air gains access; the heat produced by the
combustion converts the water in the boiler into steam, which is made to
act upon the piston, and thus set the machinery in motion. But there is
another kind of engine, now extensively used, which works on a different
principle. In the gas-engine the fuel is gaseous, and it can thus be
introduced in a state of intimate mixture with the oxygen with which it
is to unite in combustion. This is a great advantage. The two can unite
rapidly and explosively. In gunpowder the same end is effected by mixing
the carbon and sulphur with nitre, which contains the oxygen necessary
for their explosive combustion. And this is carried still further in
dynamite and gun-cotton, where the elements necessary for explosive
combustion are not merely mechanically mixed, but are chemically
combined in a highly unstable compound.
But in the gas-engine, not only is the fuel and the oxygen thus
intimately mixed, but the controlled explosions and the resulting
condensation are caused to act directly on the piston, and not through
the intervention of water in a boiler. Whereas, therefore, in the
steam-engine the combustion is to some extent external to the working of
the machine, in the gas-engine it is to a large extent internal and
direct.
Now, instead of likening the organism as a whole to a steam-engine, it
is more satisfactory to liken each cell to a gas-engine. We have seen
that the cell-substance around the nucleus is composed of a network of
protoplasm, the plasmogen, enclosing within its meshes a more fluid
material, the plasm. It is probable that this more fluid material is an
explosive, elaborated through the vital activity of the protoplasmic
network. During the period of repose which intervenes between periods of
activity, the protoplasmic network is busy in construction, taking from
the blood-discs oxygen, and from the blood-fluid carbonaceous and
nitrogenous materials, and knitting these together into relatively
unstable explosive compounds. These explosive compounds are like the
mixed air and gas of the gas-engine. A rested muscle may be likened to a
complex and well-organized battery of gas-engines. On the stimulus
supplied through a nerve-channel a series of co-ordinated explosions
takes place: the gas-engines are set to work; the muscular fibres
contract; the products of the explosions (one of which is carbonic acid
gas) are taken up and hurried away by the blood-stream; and the
protoplasm sets to work to form a fresh supply of explosive material.
Long before the invention of the gas-engine, long before gun-cotton or
dynamite were dreamt of, long before some Chinese or other inventor
first mixed the ingredients of gunpowder, organic nature had utilized
the principle of controlled explosions in the protoplasmic cell.
Certain cells are, however, more delicately explosive than others.
Those, for example, on or near the external surface of the body--those,
that is to say, which constitute the end organs of the special
senses--contain explosive material which may be fired by a touch, a
sound, an odour, the contact with a sapid fluid or a ray of light. The
effects of the explosions in these delicate cells, reinforced in certain
neighbouring nerve-knots (ganglionic cells), are transmitted down the
nerves as along a fired train of gunpowder, and thus reach that
wonderful aggregation of organized and co-ordinated explosive cells, the
brain. Here it is again reinforced and directed (who, at present, can
say how?) along fresh nerve-channels to muscles, or glands, or other
organized groups of explosives. And in the brain, somehow associated
with the explosion of its cells, consciousness and the mind-element
emerges; of which we need only notice here that it belongs to a _wholly
different order of being_ from the physical activities and products with
which we are at present concerned.
No analogies between mechanical contrivances and organic processes can
be pushed very far. To liken the organic cell to a gas-engine is better
than to liken the organism to a steam-engine, because it serves to
indicate the fact that the fuel does not simply combine with the oxygen
in combustion, but that an unstable or explosive combination of "fuel"
and oxygen is first formed; and again, because the effect of this is
direct, and not through the intervention of any substance to which the
combustion merely supplies the necessary heat. But beyond the fact that
a kind of explosive is formed which, like a fulminating compound, can be
fired by a touch, there is no very close analogy to be drawn. Nor must
we press the explosion analogy too far. The essential thing would seem
to be this--which, perhaps, the analogy may have served to lead up
to--that the vital protoplasmic network of the cell has the power of
building up complex and unstable chemical compounds, which are probably
stored in the plasm within the spaces between the threads of the
network; and that these unstable compounds, under the influence of a
stimulus (or, possibly, sometimes spontaneously) break down into simpler
and more stable compounds.[D] In the case of muscle-cells, this latter
change is accompanied by an alteration in length of the fibres and
consequent movements in the organism, the products of the disruptive
change being useless or harmful, and being, therefore, got rid of as
soon as possible. But very frequently the products of explosive activity
are made use of. In the case of bone-cells, one of the products of
disruption is of permanent use to the organism, and constitutes the
solid framework of the skeleton. In the case of the secreting cells of
the salivary and other digestive glands, one of the disruptive products
is of temporary value for the preparation of the food. It is exceedingly
probable that these useful products of disruption, permanent or
temporary, took their origin in waste products for which natural
selection has found a use, and which have been, through natural
selection, rendered more and more efficacious. This, however, is a
question we are not at present in a position to discuss.
In the busy hive of cells which constitutes what we call the animal
body, there is thus ceaseless activity. During periods of apparent rest
the protogen filaments of the cell-net are engaged in constructive work,
building up fresh supplies of complex and unstable materials, which,
during periods of apparent activity, break up into simpler and more
stable substances, some of which are useful to the organism while others
must be got rid of as soon as possible. From another point of view, the
cells during apparent rest are storing up energy which is utilized by
the organism during its periods of activity. The storing up of available
energy may be likened to the winding up of a watch or clock; it is
during apparent rest that the cell is winding itself up; and thus we
have the apparent paradox that the cell is most active and doing most
work when it is at rest. During the repose of an organ, in fact, the
cells are busily working in preparation for the manifestation of
energetic action that is to follow. Just as the brilliant display of
intellectual activity in a great orator is the result of the silent work
of a lifetime, so is the physical manifestation of muscular power the
result of the silent preparatory work of the muscle-cells.[E]
One point to be specially noted is the varied activity of the cells.
While they are all working for the general good of the organism, they
are divided into companies, each with a distinct and definite kind of
work. This is known as the physiological division of labour. It is
accompanied by a morphological differentiation of structure. By the form
of a cell, therefore, we can generally recognize the kind of work it has
to perform. The unstable compounds produced by the various cells must
also be different, though not much is known at present on this subject.
The unstable compound which forms bone and that which forms the salivary
ferment, the unstable matter elaborated by nerve-cells and that built up
by muscle-cells, are in all probability different in their chemical
nature. Whether the formative plasmogen from which these different
substances originate is in all cases the same or in different cases
different, we do not know.
It may, perhaps, seem strange that the products of cellular life should
be reached by the roundabout process of first producing a very complex
substance out of which is then formed a less complex substance, useful
for permanent purposes, as in bone, or temporary purposes, as in the
digestive fluids. It seems a waste of power to build up substances
unnecessarily complex and stored with an unnecessarily abundant supply
of energy. Still, though we do not know that this course is adopted in
all cases, there is no doubt that it is adopted in a great number of
instances. And the reason probably is that by this method the organs are
enabled to act under the influence of stimuli. They are thus like
charged batteries ready to discharge under the influence of the
slightest organic touch. In this way, too, is afforded a means by which
the organ is not dependent only upon the products of the immediate
activity of the protoplasm at the time of action, but can utilize the
store laid up during a considerable preceding period.
Sufficient has now been said to illustrate the nature of the process of
life. The fact that I wish to stand out clearly is that the animal body
is stored with large quantities of available energy resident in highly
complex and unstable chemical compounds, elaborated by the constructive
energy of the formative protoplasm of its constituent cells. These
unstable compounds, eminently explosive according to our analogy, are
built up of materials derived from two different sources--from the
nutritive matter (containing carbon, hydrogen, and nitrogen) absorbed in
the digestive organs, and from oxygen taken up from the air in the
lungs. The cells thus become charged with energy that can be set free on
the application of the appropriate stimulus, which may be likened to the
spark that fires the explosive.
Let us note, in conclusion, that it is through the blood-system,
ramifying to all parts of the body, and the nerve-system, the
ramifications of which are not less perfect, that the larger and higher
organisms are knit together into an organic whole. The former carries to
the cell the raw materials for the elaboration of its explosive
products, and, after the explosions, carries off the waste products
which result therefrom. The nerve-fibres carry the stimuli by which the
explosive is fired, while the central nervous system organizes,
co-ordinates, and controls the explosions, and directs the process of
reconstruction of the explosive compounds.
NOTES
[B] It has before been noticed that the organs themselves have their
periods of rest. The rhythm of rest and repose in the heart is not
that of the activity and sleep of the organism, but that of the
contraction and relaxation of the organ itself.
[C] From a popular article of the author's on "Horns and Antlers," in
_Atalanta_.
[D] It will be well here to introduce the technical terms for these
changes. The general term for chemical actions occurring in the
tissues of a living creature is _metabolism_; where the change is
of such a nature that complex and unstable compounds are built up
and stored for a while, it is called _anabolism_; where complex
unstable compounds break up into less complex and relatively
stable compounds, the term _katabolism_ is applied. We shall speak
of anabolic changes as _constructive_; katabolic, as _disruptive_,
or sometimes, _explosive_.
[E] I do not mean, of course, to imply that there is no reconstruction
during activity, but that it is then distinctly outbalanced by
disruptive changes.