which it gets rid of its waste, are the lungs, the kidneys, and the
skin. Through the lungs, carbonic acid and water escape; through the
kidneys, water, ammonia in the shape of urea, and various salts; through
the skin, water and a few salts. As the blood passes through lung,
kidney, and skin, it throws off little by little the impurities which
clog it, one at one place, another at another, and returns from each
purer and fresher. The need to get rid of carbonic acid and to gain a
fresh supply of oxygen is more pressing than the need to get rid of
either ammonia or salts. Hence, while all the blood which leaves the
left ventricle has to pass through the lungs before it returns to the
left ventricle again, only a small part of it passes through the
kidneys, just enough to fill at each stroke the small arteries leading
to those organs. The blood craves for great draughts of oxygen, and
breathes out great mouthfuls of carbonic acid, but is quite content to
part with its ammonia and salts in little driblets, bit by bit.
The three channels manage between them to keep the blood pure and fresh,
working hard and clearing off much when much food or water is taken or
much work is done, and taking their ease and working slow when little
food is eaten or when the body is at rest.
THE WHOLE STORY SHORTLY TOLD. § IX.
=55.= And now you ought to be able to understand how it is that we live on
the food we eat.
Food, inasmuch as it can be burnt, is a source of power. In burning it
gives forth heat, and heat is power. If we so pleased, we might burn in
a furnace the things which we eat as food, and with them drive a
locomotive or work a mill; if we so pleased, we might convert them into
gunpowder, and with them fire cannon or blast rocks. Instead of doing
so, we burn them in our own bodies, and use their power in ourselves.
Food passing into the alimentary canal is there digested; the nourishing
food-stuffs are with very little change dissolved out from the
innutritious refuse; they pass into and become part and parcel of the
blood.
The blood, driven by the unresting stroke of the heart’s pump, courses
throughout the whole body, and in the narrow capillaries bathes every
smallest bit of almost every part. Kept continually rich in combustible
material by frequent supplies of food, the blood as well at every round
sucks up oxygen from the air of the lungs; and thus arterial blood is
ever carrying to all parts of the body, to muscle, brain, bone, nerve,
skin, and gland, stuff to burn and oxygen to burn it with.
Everywhere oxidation, burning, is going on, in some spots or at some
times fiercely, in other spots or at other times faintly, changing the
arterial blood rich in oxygen to venous blood poor in oxygen. From most
places where oxidation is going on, the venous blood goes away hotter
than the arterial which came; and all the hot blood mingling together
and rushing over the whole body keeps the whole body warm. Sweeping as
it continually does through innumerable little furnaces, the blood must
needs be warm. This is why =we= are warm. But from some places, as from
the skin, the venous blood goes away cooler than the arterial which
came, because while journeying through the capillaries of the skin it
has given up much of its heat to whatever is touching the skin, and has
also lost much heat in turning liquid perspiration into vapour. This is
why so long as we are in health we never get hotter than a certain
degree of temperature, the so-called blood-heat, 98° Fahr., and why we
make warm the clothes which we wear and the bed in which we sleep.
Everywhere oxidation is going on, oxidation either of the blood itself
or of the structures which it bathes, and whose losses it has to make
good. Everywhere change is going on. Little by little, bit by bit, every
part of the body, here quickly, there slowly, is continually mouldering
away and as continually being made anew by the blood. Made anew
according to its own nature. Though it is the same blood which is
rushing through all the capillaries, it makes different things in
different parts. In the muscle it makes muscle; in the nerve, nerve; in
the bone, bone; in the glands, juice. Though it is the same blood, it
gives different qualities to different parts: out of it one gland makes
saliva, another gastric juice: out of it the bone gets strength, the
brain power to feel, the muscle power to contract.
When the biceps muscle contracts and raises the arm, it does work. The
power to do that work, the muscle got from the blood, and the blood from
the food. All the work of which we are capable comes, then, from our
food, from the oxidation of our food, just as the power of the
steam-engine comes from the oxidation of its fuel. But you know that in
the steam-engine only a very small part of the power, or energy, as it
is called, of the fuel goes to move the wheel. By far the greater part
is lost in heat. So it is with our bodies: all the force we can exert
with our bodies is but a small part of the power of our food; all the
rest goes to keep us warm.
Visiting all parts of the body, rebuilding and refreshing every spot it
touches, the blood current also carries away from each organ the waste
matters of which that organ has no longer any use. Just as each part or
organ has different properties and different work, so also is the waste
of each not exactly the same, though all are alike inasmuch as they are
all the results of oxidation. The waste of the muscle is not exactly the
same as the waste of the brain or of the liver. Possibly the waste
things which the blood bears from one organ may be useful to another,
and so be made to do double work, just as the tar which the gasworks
throw away makes the fortune of the colour manufacturer.
Be this as it may, the waste products of all parts, travelling hither
and thither in the body, come at last to be brought down to very simple
things, with all their virtue gone out of them, with all, or all but
all, their power of burning lost, fit for nothing but to be cast away,
come at last to be urea or ammonia, carbonic acid, and salts. In this
shape, the food, after a longer or shorter sojourn in the body, having
done its work, having built up this or that part, having helped the
muscle to contract or the liver to secrete, having by its burning given
rise to work or to heat, goes back powerless to the earth and air from
which it came. And so the tale is told.
HOW WE FEEL AND WILL. § X.
=56.= One other matter we have to note before we have given the full
answer to the question why we move.
We have seen that we move by reason of our muscles contracting, and that
in a general way a muscle contracts because a something started in the
brain by our will passes down from the brain through more or less of the
spinal cord, along certain nerves till it reaches the muscle. It is this
something, which we may call a =nervous impulse=, which causes the muscle
to contract.
But what leads us to exercise our wills? What starts the nervous
impulse?
All the nerves in the body do not end in muscles. Many of them end, for
instance, in the skin, in those papillæ of which I spoke a little while
ago. These nerves cannot be used for carrying nervous impulses from the
brain to the skin. By an effort of the will you can make your muscles
contract; but try as much as you can, you cannot produce any change in
your skin.
What purpose do these nerves serve, then? If you prick or touch your
finger, you feel the prick or touch; you say you have =sensation= in your
finger. Suppose you were to cut across the nerves which lead from the
skin of your finger along your arm up to your brain. What would happen?
If you pricked or touched your finger, you would not feel either prick
or touch. You would say you had lost all sensation in your finger. These
nerves ending in the finger then, have a different use from those ending
in the muscle. =The latter carry impulses from the brain to the muscle,
and so, being instruments for causing movements, are called motor
nerves. The former, carrying impulses from the skin to the brain, and
being instruments for bringing about sensations, are called sensory
nerves.= All parts of the skin are provided with these sensory nerves,
but not to the same extent. The parts where they abound, as the fingers,
are said to be very sensitive; the parts where they are scanty, as the
back of the trunk, are said to be less sensitive. Other parts besides
the skin have also sensory nerves.
Motor nerves are of one kind only; they all have one kind of work to
do--to make a muscle contract. But there are several kinds of sensory
nerves, each kind having a special work to do. The several works which
these different kinds of sensory nerves have to do are called =the
senses=.
The work of the nerves of the skin, all over the body, is called the
=sense of touch=. By touch you can learn whether a body is rough or
smooth, wet or dry, hot or cold, and so on.
You cannot, however, by touch distinguish between salt and sugar. Yet
directly you place either salt or sugar on your tongue you can recognize
it, because you then employ sensory nerves of another kind, the nerves
which give us the =sense of taste=. So also we have nerves of =smell=,
nerves of =hearing=, and nerves of =sight=.
The nerves of touch, where they end, or rather where they begin in the
skin, sometimes have and sometimes have not, little peculiar structures
attached to them, little =organs of touch=. So also the nerves of taste,
and smell, end or rather begin in a peculiar way. When we come to the
nerves of hearing and of seeing, we find these beginning in most
elaborate and complicated organs, the ear and the eye.
Of all these =organs of the senses= you will learn more hereafter;
meanwhile, I want you to understand that by means of these various
sensory nerves, we are, so long as we are alive and awake, receiving
impressions from the external world, sensations of touch, sensations of
roughness and smoothness, of heat and cold, sensations of good and bad
odours, sensations of tastes of various kinds, sensations of all manner
of sounds, sensations of the colours and forms of things.
By our skin, by our nose, by our tongue and palate, by our ears, and
above all by our eyes, impressions caused by the external world are for
ever travelling up sensory nerves to the brain; thither come also
impressions from within ourselves, telling us where our limbs are and
what our muscles are doing. Within the brain these impressions become
sensations. They stir the brain to action; and the brain, working on
them and by them, through ways we know not of, governs the body as a
conscious intelligent will.
* * * * *
NICHOLSON’S GEOLOGY.
_Text-Book of Geology, for Schools and Colleges._
By H. ALLEYNE NICHOLSON, M. D., D. SC., M. A., PH. D.,
F. R. S. E., F. G. S., etc., Professor of Natural History
and Botany in University College, Toronto.
_12mo. 266 pages. Price, $1.50._
This work is thoroughly adapted for the use of beginners. At the same
time the subject is treated with such fulness as to render the work
suitable for advanced classes, while it is intended to serve as an
introduction to a larger work which is in course of preparation by the
author.
* * * * *
NICHOLSON’S ZOOLOGY.
_Text-Book of Zoology, for Schools and Colleges._
BY SAME AUTHOR AS ABOVE.
_12mo. 353 pages. Price, $1.75._
In this volume much more space has been devoted, comparatively speaking,
to the Invertebrate Animals, than has usually been the case in works of
this nature: upon the belief that all teachings of Zoology should, where
possible, be accompanied by practical work, while the young student is
much more likely to busy himself practically with shells, insects,
corals, and the like, than with the larger and less attainable
Vertebrate Animals.
Considerable space has been devoted to the discussion of the principles
of Zoological classification, and the body of the work is prefaced by a
synoptical view of the chief divisions of the animal kingdom.
⁂ A copy of any of the above works, for examination, will be sent by
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FOOTNOTES:
[1] It is unusual for muscles to have two tendons at the same end.
Hence the name =biceps=, or “two-headed.”
[2] From _pulmo_, =lung=; the artery of the lung.
[3] From _hepar_, =liver=; the vein of the liver.
Typographical errors corrected by the etext transcriber:
would be unvailing=> would be unavailing {pg 34}
cordæ tendineæ=> chordæ tendineæ {pg 70}
the triscuspid valve between=> the tricuspid valve between {pg 72}
the body may may=> the body may {pg 103}