THE MAINTAINING OF LIFE
Equal in importance to being alive is the power to go on living;
therefore, having described the signs of life, our next task is to
consider how that life is maintained. When the primary fact of life was
given as continuous power development, the foundation was laid for this
topic, for life cannot fail to go on if continuous power development is
maintained.
Power development in living animals as in locomotives depends on fuel
and oxygen; evidently continuous supplies of these must be provided if
life is to go on. The living animal differs from the locomotive in this:
that while some one attends to supplying the locomotive with fuel, most
living animals, except the very young, have to attend to providing
themselves. There are exceptions to this rule. The tapeworms that
inhabit the intestines of animals, and sometimes of men, live in a
stream of food; they are put to no trouble to obtain it. The same is
true of many kinds of parasites. Except for these, however, it holds
true that animals must attend to their own wants. We shall now begin to
see the utility of the most conspicuous sign of life spoken of in the
first chapter, namely, motion, for food must be gotten where it is; only
tapeworms and similar animals swim in it. All the rest, including
ourselves, must go to where the food is. Even animals like oysters,
that are anchored to the rocks, have to use motion in getting food. In
their case the motion consists in setting up a current in the sea water
into and through their bodies, from which current they sift out the tiny
food particles which abound in the ocean.
If an animal happens to live in the ocean, where every drop furnishes
its particle or two of foodstuff, and especially if the animal is small,
or sluggish, like the oyster, almost any kind of motion will serve to
bring the animal all the food it needs. The simplest of the one-celled
animals, that must be watched through the microscope to see how they
behave, blunder about aimlessly, and in the course of their blundering
bump up against food particles often enough to keep themselves fed. If
an animal happens to live where food is scarcer, or if it is big and
active, and so must have large quantities of food, aimless blundering
about will never get it enough to keep it alive. It must have some means
of finding out where the food is. Since we ourselves come under the head
of animals whose food needs are so large that we must locate food
supplies, and not depend on happening onto them, we can identify in
ourselves the means which are used for doing this. We all know that our
sense organs, the eyes, ears, nose, and finger tips are what we depend
on for telling us where food is to be found. The same is true of all
animals that are able to hunt for food; they have some sort of sense
organs to help in guiding them to where the food is.
The story of the machinery for finding food is not yet quite complete,
for the muscles which actually make the movements by which the animal
gets about are in one part of the body, while the sense organs which
are to furnish the information by which the movements are guided are in
quite a different part, and in animals as large as ourselves, some
distance away. From our eyes to our leg muscles is quite a space, and it
is evident that this space must be bridged somehow if our legs are to
move in obedience to information which our eyes bring in. In ourselves
and in almost all other animals this space is bridged by means of
special machinery for the purpose. We are familiar with it under the
name of the nervous system. We may not have been in the habit of
thinking of the nervous system in just this way, but at bottom this is
exactly what the nervous system does for us, namely, guides our muscles
according to the information brought in by our sense organs. There is
more to nervous activity than just this, but this is the starting point
and groundwork for all the rest, as we shall try to show presently.
Continuous food supplies are the main necessity for continued life, but
there are some other things that have to be looked out for in addition.
The favorite food for large numbers of animals, and, indeed, in many
cases the only food, consists of the bodies of other animals. All the
flesh-eating sorts prey on other animals for their food. This places the
other animals on the defensive, so that a large part of their activity
consists in escaping the attacks of the beasts that wish to eat them.
For most kinds of animals the greater part of their waking life is taken
up with movements which serve either to get them food or to prevent them
from becoming food for others. If we add to these the movements that are
necessary to preserve the animals against other kinds of danger than the
danger of being eaten, and those connected with the propagation and
care of the young, we shall have about covered the list of what we may
call the serious activities of animals, and of men as well. Many kinds
are active besides in play. This is particularly true of young animals,
although grown-ups, both among animals and men, find play both agreeable
and beneficial when not overindulged.
Protective motions need to be even more accurately made than those whose
purpose is the getting of food, for if the food is missed at one effort
another trial may be more successful, but if an attempt to escape fails
there will probably be no more chances to try. The sense organs and the
nervous system are just as deeply concerned, therefore, in avoiding harm
as in finding food materials, and it is as important for them to do
their work well in the one case as in the other. When we think of the
activities of animals, for whatever purpose they are carried on, we must
think of them as made up of the combined actions of the muscles, the
nerves, and the sense organs, and not of any of them working by
themselves.
These parts of us that are so closely concerned in the maintaining of
our life by getting us food and keeping us safe from harm make up, also,
the only parts of us which really share in what we may call conscious
living. When we come right down to it we could spare our other
organs--heart, lungs, stomach, and the rest--and never miss them so far
as adding anything to our happiness is concerned. In fact, the less
these organs intrude themselves into our attention the better off we
are; only when we are ailing do we begin to think about them. Of course,
they are absolutely necessary to us, and we should die instantly if one
of the more important of them were to stop working, but the part they
play is not one which enters actively into our consciousness, as do the
muscles, nerves, and sense organs.
Naturally, we will ask what all these other organs are for if they do
not share in our conscious life. Why can we not get along with just
those that we use for getting food, for avoiding harm, for play, and for
the other activities which they carry on? The answer to this question is
found in the fact emphasized above that continuous power development is
necessary to continued life. By themselves the muscles, nerves, and
sense organs cannot carry on power development; they require the aid of
a great many other organs to do this. Just how these other organs work
will be described later; at present it will be enough to recall that
every muscle, every nerve, and every sense organ is actually made up of
a great many of the tiny construction units--the cells about which we
were talking a few pages back--and that every one of these cells must be
developing power all the time if it is to go on living. In order to be
able to do this they must, every one, be able to oxidize fuel
continuously, and this means that they must receive constant supplies,
both of the fuel itself and of the oxygen with which it combines. Some
system of delivering these materials must be in operation, and in case
the materials have to be prepared for use beforehand this must be
provided for also. The heart, the lungs, the stomach, and the various
other organs that are useful but not conspicuous, are concerned in these
necessary jobs. In an automobile factory we have a similar situation;
the men that stand at the machines actually make the parts that go into
the finished automobiles, but unless other men are hard at work
preparing the castings, and bringing them to where the machine operators
can get them, not many automobiles will be turned out. So in the body,
unless the various organs are carrying on their work of preparing and
delivering materials to the muscles, the nerves, and the sense organs,
these latter cannot perform their tasks of getting the food for the
whole body and of securing the body against harm, nor can they carry on
the pleasant, but not absolutely necessary, activities of play and
recreation.