THE CIRCULATION.
_The Blood--Its Plasma and Corpuscles--Coagulation of the Blood--The Uses
of the Blood--Transfusion--Change of Color--The Organs of the
Circulation--The Heart, Arteries, and Veins--The Cavities and Valves of the
Heart--Its Vital Energy--Passage of the Blood through the Heart--The
Frequency and Activity of its Movements--The Pulse--The Sphygmograph--The
Capillary Blood-vessels--The Rate of the
Circulation--Assimilation--Injuries to the Blood-vessels._
[Sidenote: 1. What is required by every living organism? In plants?
Insects? Reptiles? Man?]
1. THE BLOOD.--Every living organism of the higher sort, whether animal or
vegetable, requires for the maintenance of life and activity, a circulatory
fluid, by which nutriment is distributed to all its parts. In plants, this
fluid is the sap; in insects, it is a watery and colorless blood; in
reptiles and fishes, it is red but cold blood; while in the nobler animals
and man, it is the red and warm blood.
[Sidenote: 2. Importance and abundance of blood? Dependence of life? Abel?
Mosaic law? In what part of the body is blood not found? Quantity of blood
in the body?]
2. The blood is the most important, as it is the most abundant, fluid of
the body; and upon its presence, under certain definite conditions, life
depends. On this account it is frequently, and very properly, termed "the
vital fluid." The importance of the blood, as essential to life, was
recognized in the earliest writings. In the narration of the death of the
murdered Abel, it is written, "the voice of his _blood_ crieth from the
ground." In the Mosaic law, proclaimed over thirty centuries ago, the
Israelites were forbidden to eat food that contained blood, for the reason
that "the life of the flesh is in the blood." With the exception of a few
tissues, such as the hair, the nails, and the _cornea_ of the eye, blood
everywhere pervades the body, as may be proven by puncturing any part with
a {102} needle. The total quantity of blood in the body is estimated at
about one-eighth of its weight, or eighteen pounds.
[Sidenote: 3. Color of blood? Its consistence? Odor?]
3. The color of the blood, in man and the higher animals, as is well known,
is red; but it varies from a bright scarlet to a dark purple, according to
the part whence it is taken. "Blood is thicker than water," as the adage
truly states, and has a glutinous quality. It has a faint odor, resembling
that peculiar to the animal from which it is taken.
[Sidenote: 4. What is stated of the blood as viewed under the microscope?]
4. When examined under the microscope, the blood no longer appears a simple
fluid, and its color is no longer red. It is then seen to be made up of two
distinct parts: first, a clear, colorless fluid, called the _plasma_; and
secondly, of a multitude of minute solid bodies, or corpuscles, that float
in the watery plasma. The plasma, or nutritive liquid, is composed of water
richly charged with materials derived from the food, viz., albumen, which
gives it smoothness and swift motion; fibrin; certain fats; traces of
sugar; and various salts.
[Illustration: FIG 25.--THE BLOOD CORPUSCLES.]
[Sidenote: 5. State what you can of the little bodies called corpuscles.]
5. THE BLOOD CORPUSCLES.--In man, these remarkable "little bodies," as the
word _corpuscles_ signifies, are of a yellow color, but by their vast
numbers impart a red hue to the blood. They are very small, having a
diameter of about 1/3500 of an inch, and being one-fourth of that fraction
in thickness; so that if 3,500 of them were placed in line, side by side,
they would only extend one inch; or, if {103} piled one above another, it
would take at least 14,000 of them to stand an inch high. Although so small
in size, they are very regular in form. As seen under the microscope, they
are not globular or spherical, but flat, circular, and disc-like, with
central depressions on each side, somewhat like a pearl button that has not
been perforated. In freshly-drawn blood they show a disposition to arrange
themselves in little rolls like coins (Fig. 25).
[Illustration: FIG. 26.
_a_, Oval Corpuscles of a fowl. _b_, Corpuscles of a frog. _c_, Those of a
shark.
The five small ones at the upper part of the figure, represent the human
corpuscles magnified 400 times.]
[Sidenote: 6. The size and shape of corpuscles? Why is the fact important?]
6. The size and shape of the blood corpuscles vary in different animals, so
that it is possible to discriminate between those of man and the lower
animals (Fig. 26). This is a point of considerable practical importance.
For example, it is sometimes desirable to decide in a court of justice the
source, whether from man or an inferior animal, of blood stains upon the
clothing of an accused person, or upon some deadly weapon. This may be done
by a microscopical examination of a minute portion of the dried stain,
previously refreshed by means of gum-water. Certain celebrated cases are
recorded in which the guilt of criminals has been established, and they
have been condemned and punished upon the evidence which science rendered
on this single point, the detecting of the human from other blood.
[Sidenote: 7. The character of the blood of dead animals? Means of
detecting such blood?]
7. The character of the blood of dead, extinct, and even fossil animals,
such as the mastodon, has been ascertained by obtaining and examining
traces of it which had been shut up, perhaps for ages, in the circulatory
canals of bone. A means of detecting blood in minute quantities is found
{104} in the spectroscope, the same instrument by which the constitution of
the heavenly bodies has been studied. If a solution containing not more
than one-thousandth part of a grain of the coloring matter of the
corpuscle, be examined, this instrument will detect it.
[Sidenote: 8. White corpuscles? Total number of corpuscles in the body?]
8. The corpuscles, just described, are known as the red blood corpuscles.
Besides these, and floating along in the same plasma, are the white
corpuscles. These are fewer in number, but larger and globular in form.
They are colorless, and their motion is less rapid than that of the other
variety. The total number of both varieties of these little bodies in the
blood is enormous. It is calculated that in a cubic inch of that fluid
there are eighty-three millions, and at least five hundred times that
number in the whole body.
[Sidenote: 9. The blood in its natural condition in the body? Describe the
process by which the coagulation of blood takes place?]
9. COAGULATION.--The blood, in its natural condition in the body, remains
perfectly fluid; but, within a few minutes after its removal from its
proper vessels, whether by accident or design, a change takes place. It
begins to coagulate, or assume a semi-solid consistence. If allowed to
stand, after several hours it separates into two distinct parts, one of
them being a dark red jelly, the coagulum, or clot, which is heavy and
sinks; and the other, a clear, straw-colored liquid, called serum, which
covers the clot. This change is dependent upon the presence in the blood of
fibrin, which possesses the property of solidifying under certain
circumstances; one of these circumstances being when the blood is separated
from living tissues. The color of the clot is due to the entanglement of
the corpuscles with the fibrin.
[Sidenote: 10. If coagulation were impossible? How is it in fact?]
10. In this law of the coagulation of the blood is our safeguard against
death by hæmorrhage, or against undue loss of blood. If coagulation were
impossible, the {105} slightest injury in drawing blood would prove fatal.
Whereas now, in vastly the larger proportion of cases, bleeding ceases
spontaneously, because the blood, as it coagulates, stops the mouths of the
injured blood-vessels. In another class of cases, where larger vessels are
cut or torn, it is simply necessary to close them by a temporary pressure;
for in a few minutes the clot will form and seal them up. In still more
serious cases, where the blood-vessel is of large size, the surgeon is
obliged to tie a "ligature" about it, and thus prevent the force of the
blood-current from washing away the clots, which, forming within and around
the vessel, would close it effectually.
[Sidenote: 11. What is worthy of remark? Coagulation of the blood of
inferior animals? Of the blood of birds?]
11. It is worthy of remark that this peculiarity is early implanted in the
blood, even before birth, and in advance of any existing necessity for it;
thus anticipating and guarding against danger. But this is not all. Of most
of the inferior animals, which, as compared with man, are quite helpless,
the blood coagulates more rapidly, and in the case of the birds, almost
instantly. The relative composition of fluid and coagulated blood may be
thus represented:
_Fluid Blood._ _Coagulated Blood._
Plasma----------Serum---------Serum
\
\
--------Fibrin--------
\
\
Corpuscles------Corpuscles-------Clot.
[Sidenote: 12. The blood, as a provider and purifier? What uses does the
blood subserve? Experiments? Transfusion?]
12. THE USES OF THE BLOOD.--The blood is the great provider and purifier of
the body. It both carries new materials to all the tissues, and removes the
worn out particles of matter. This is effected by the plasma. It both
conveys oxygen and removes carbonic acid. This is done through the
corpuscles. Some singular experiments have {106} been tried to illustrate
the life-giving power of the blood. An animal that has bled so freely as to
be at the point of dying, is promptly brought back to life by an operation
called transfusion, by which fresh blood from a living animal is injected
into the blood-vessels of his body.
[Sidenote: 13. The case of the deaf and feeble dog? Horse? Dead dog?]
13. It is related that a dog, deaf and feeble from age, had hearing and
activity restored to him by the introduction into his veins of blood taken
from a young dog; and, that a horse, twenty-six years old, having received
the blood of four lambs acquired new vigor. And further, that a dog, just
dead from an acute disease, was so far revived by transfusion, as to be
able to stand and make a few movements.
[Sidenote: 14. Transfusion, as a fashionable remedy? What further of
transfusion?]
14. Transfusion has been practised upon man. At one time, shortly after
Harvey's discovery of the "Circulation of the Blood," it became quite a
fashionable remedy, it being thought possible by it to cure all forms of
disease, and even to make the old young again. But these claims were soon
found extravagant, and many unhappy accidents occurred in its practice; so
that being forbidden by government and interdicted by the Pope, it rapidly
fell into disuse. At the present time, however, it is sometimes resorted to
in extreme cases, when there has been a great and rapid loss of blood; and
there are upon record several instances where, other means having failed,
life has been restored or prolonged by the operation of transfusion.
[Sidenote: 15. The seat of the reviving power of the blood? What further is
related?]
15. This reviving power of the blood seems to reside in the corpuscles; for
transfusion, when attempted to be performed with the serum alone, has, in
every case, proved fruitless. Now, though so much depends upon the blood
and its corpuscles, it is a mistake to suppose that in them alone is the
seat of life, or that they are, in an exclusive manner, alive. All the
organs and parts of the body are mutually dependent one upon the other; and
the complete usefulness {107} of the blood, or of any other part, flows out
of the harmonious action of all the parts.
[Sidenote: 16. Changes in the blood? What further is stated?]
16. CHANGE OF COLOR.--The blood undergoes a variety of changes in its
journey through the system. As it visits the different organs it both gives
out and takes up materials. In one place it is enriched, in another it is
impoverished. By reason of these alterations in its composition, the blood
also changes its color. In one part of the body it is bright red, or
arterial; in another it is dark blue, or venous. In the former case it is
pure and fit for the support of the tissues; in the latter, it is impure
and charged with effete materials. (The details of the change from dark to
bright will be given in the chapter on Respiration.)
[Sidenote: 17. Motion of the blood? What is meant by the circulation of the
blood? How confined? Discovery made by Harvey?]
17. CIRCULATION.--The blood is in constant motion during life. From the
heart, as a centre, a current is always setting toward the different
organs; and from these organs a current is constantly returning to the
heart. In this way a ceaseless circular movement is kept up, which is
called the Circulation of the Blood. This stream of the vital fluid is
confined to certain fixed channels, the blood-vessels. Those branching from
the heart are the arteries; those converging to it are the veins. The true
course of the blood was unknown before the beginning of the seventeenth
century. In 1619 it was discovered by the illustrious William Harvey. Like
many other great discoverers, he suffered persecution and loss, but unlike
some of them, he was fortunate enough to conquer and survive opposition. He
lived long enough to see his discovery universally accepted, and himself
honored as a benefactor of mankind.
{108}
[Illustration: FIG. 27.--THE ORGANS OF CIRCULATION.]
{109} [Illustration: FIG. 28.--THE HEART AND LARGE VESSELS.
A, Right Ventricle. B, Left Ventricle. C, Right Auricle. D, Left Auricle.
E, Aorta. F, Pulmonary Artery.]
[Sidenote: 18. Office of the heart? Location of the heart? Its beat? Its
shape? Protection to the heart? What else is said in relation to the
heart?]
18. THE HEART.--The heart is the central engine of the circulation. In this
wonderful little organ, hardly larger than a man's fist, resides that
sleepless force by which, during the whole of life, the current of the
blood is kept in motion. It is placed in the middle and front part of the
chest, inclining to the left side. The heartbeat may be felt and heard
between the fifth and sixth ribs, near the breast-bone. The shape of the
heart is conical, with the apex or point downward and in front. The base,
which is upward, is attached so as to hold it securely in its place, while
the apex is freely moveable. In order that loss of power from friction may
be obviated, the heart is enclosed between two layers of serous membrane,
which forms a kind of sac. This membrane is as smooth as satin, and itself
secretes a fluid in sufficient quantities to keep it at all times well
lubricated. The lining membrane of the heart, likewise, is extremely
delicate and smooth.
{110} [Illustration: FIG. 29.--SECTION OF THE HEART.
A, Right Ventricle. B, Left Ventricle. C, Right Auricle. D, Left Auricle.
E, F, Inlets to the Ventricles. G, Pulmonary Artery. H, Aorta.]
[Sidenote: 19. Formation of the heart? Right and left heart?]
19. THE CAVITIES OF THE HEART.--The heart is hollow, and so partitioned as
to contain four chambers or cavities; two at the base, known as the
_auricles_, from a fancied resemblance to the ear of a dog, and two at the
apex or point, called _ventricles_. An auricle and a ventricle on the same
side, communicate with each other, but there is no opening from side to
side. It is customary to regard the heart as a double organ, and to speak
of its division into the right and left heart. For while both halves act
together in point of time, each half sustains an entirely distinct portion
of the labor of the circulation. Thus, the right heart always carries the
dark or venous blood, and the left always circulates the bright or arterial
blood.
[Sidenote: 20. Capacity of the chambers of the heart? What wise provision
is mentioned? The auricles?]
20. If we examine the heart, we at once notice that though its various
chambers have about the same capacity, the walls of the ventricles are
thicker and stronger than those of the auricles. This is a wise provision,
for it is by the powerful action of the former that the blood is forced to
the most remote regions of the body. The auricles, on the contrary, need
much less power, for they simply discharge their contents into the cavities
of the heart near at hand and below them--into the ventricles. {111}
[Sidenote: 21. Substance of the heart? Its fibres? Its movements? The
advantage of such movements? Action of the heart? Its period of repose?]
21. ACTION OF THE HEART.--The substance of the heart is of a deep red
color, and its fibres resemble those of the voluntary muscles by which we
move our bodies. But the heart's movements are entirely involuntary. The
advantage of this is evident; for if it depended upon us to will each
movement, our entire attention would be thus engaged, and we would find no
time for study, pleasure, or even sleep. The action of the heart consists
in alternate contractions and dilatations. During contraction the walls
come forcibly together, and thus drive out the blood. In dilatation, they
expand and receive a renewed supply. These movements are called _systole_
and _diastole_. The latter may be called the heart's period of repose; and
although it lasts only during two-fifths of a heart-beat, or about a third
of a second, yet during the day it amounts to more than nine hours of total
rest.
[Sidenote: 22. Remarkable property of the tissue of the heart? How shown?
How interesting? In cold-blooded animals? Heart of a turtle? Of a frog?
Alligator?]
22. A remarkable property of the tissue of the heart is its intense
vitality. For while it is more constantly active than any other organ of
the body, it is the last to part with its vital energy. This is especially
interesting in view of the fact that after life is apparently extinguished,
as from drowning, or poisoning by chloroform, there yet lingers a spark of
vitality in the heart, which, by continued effort, may be fanned into a
flame so as to revivify the whole body. In cold-blooded animals this
irritability of the heart is especially remarkable. The heart of a turtle
will pulsate, and the blood circulate for a week after its head has been
cut off; and the heart will throb regularly many hours after being cut out
from the creature's chest. The heart of a frog or serpent, separated
entirely from the body, will contract at the end of ten or twelve hours:
that of an alligator has been known to beat twenty-eight hours after the
death of the animal. {112}
[Sidenote: 23. Course of the blood through the heart? Course of
heart-currents?]
23. PASSAGE OF THE BLOOD THROUGH THE HEART.--Let us now trace the course of
the blood through the several cavities of the heart. In the first place,
the venous blood, rendered dark and impure by contact with the changing
tissues of the body, returns to the right heart by the veins. It enters and
fills the right auricle during its dilatation: the auricle then contracts
and fills the right ventricle. Almost instantly, the ventricle contracts
forcibly and hurries the blood along the great artery of the lungs, to be
purified in those organs. Secondly, having completed the circuit of the
lungs, the pure and bright arterial blood enters the left auricle. This now
contracts and fills the left ventricle, which cavity, in its turn,
contracts and sends the blood forth on its journey again through the
system. This general direction from right to left is the uniform and
undeviating course of heart-currents.
[Sidenote: 24. Openings of the ventricles? How guarded? How do the valves
operate? The consequence? Heart-sounds?]
24. The mechanism which enforces and regulates it, is as simple as it is
beautiful. Each ventricle has two openings, an inlet and an outlet, each of
which is guarded by strong curtains, or valves. These valves open freely to
admit the blood entering from the right, but close inflexibly against its
return. Thus, when the auricle contracts, the inlet valve opens; but as
soon as the ventricle begins to contract, it closes promptly. The contents
are then, so to speak, cornered, and have but one avenue of escape, that
through the outlet valve into the arteries beyond. As soon as the ventricle
begins to dilate again, this valve shuts tightly and obstructs the passage.
The closing of these valves occasions the two heart-sounds, which we hear
at the front of the chest.
[Sidenote: 25. Heart-beats? The heart as a susceptible organ? Heat,
exercise, etc.? Posture?]
25. FREQUENCY OF THE HEART'S ACTION.--The alternation of contraction and
dilation constitutes the {113} heartbeats. These follow each other not only
with great regularity, but with great rapidity. The average number in an
adult man is about seventy-two in a minute. But the heart is a susceptible
organ, and many circumstances affect its rate of action. Heat, exercise,
and food will increase its action, as cold, fasting, and sleep will
decrease it. Posture, too, has a curious influence; for if while sitting,
the beats of the heart number seventy-one; standing erect will increase
them to eighty-one, and lying down will lower them to sixty-six.
[Sidenote: 26. Mental emotions? Sudden excitement? Excessive joy? The
heart-beat rate? Bonaparte and Wellington?]
26. The modifying influence of mental emotions is very powerful. Sudden
excitement of feeling will cause the heart to palpitate, or throb
violently. Depressing emotions sometimes temporarily interrupt its
movements, and the person faints in consequence. Excessive joy, grief, or
fear, has occasionally suspended the heart's action entirely, and thus
caused death. The rate of the heart-beat may be naturally above or below
seventy-two. Thus it is stated that the pulse of the savage is always
slower than that of the civilized man. Bonaparte and Wellington were very
much alike in their heart's pulsations, which were less than fifty in the
case of each.
[Sidenote: 27. Average number of heart-beats? In one hour? Year? Lifetime?]
27. ACTIVITY OF THE HEART.--The average number of heart-beats during a
lifetime may be considered as at the rate of seventy-two per minute,
although this estimate is probably low; for during several years of early
life the rate is above one hundred a minute. In one hour, then, the heart
pulsates four thousand times; in a day, one hundred thousand times; and in
a year, nearly thirty-eight million times. If we compute the number during
a lifetime, thirty-nine years being the present average longevity of
civilized mankind, we obtain as the vast aggregate, fourteen hundred
millions of pulsations. {114}
[Sidenote: 28. Amount of blood expelled? Theories of the ancients?]
28. Again, if we estimate the amount of blood expelled by each contraction
of the ventricles, at four ounces, then the weight of the blood moved
during one minute will amount to eighteen pounds. In a day it will be about
twelve tons; in a year, four thousand tons; and in the course of a
lifetime, over one hundred and fifty thousand tons. These large figures
indicate, in some measure, the immense labor necessary to carry on the
interior and vital operations of our bodies. In this connection, we call to
mind the fanciful theories of the ancients in reference to the uses of the
heart. They regarded it as the abode of the soul, and the source of the
nobler emotions--bravery, generosity, mercy, and love. The words courage
and cordiality are derived from a Latin word signifying heart. Many other
words and phrases, as hearty, heart-felt, to learn by heart, and
large-hearted, show how tenaciously these exploded opinions have fastened
themselves upon our language.
[Sidenote: 29. The tendency at the present time? Why is this view
inadequate?]
29. At the present time the tendency is to ascribe purely mechanical
functions to the heart. This view, like the older one, is inadequate; for
it expresses only a small part of our knowledge of this organ. The heart is
unlike a simple machine, because its motive power is not applied from
without, but resides in its own substance. Moreover, it repairs its own
waste, it lubricates its own action, and it modifies its movements
according to the varying needs of the system. It is more than a mere
force-pump, just as the stomach is something more than a crucible, and the
eye something more than an optical instrument.
[Sidenote: 30. What are the arteries? Their walls? Their membrane?]
30. THE ARTERIES.--The tube-like canals which carry the blood away from the
heart are the arteries. Their walls are made of tough, fibrous materials,
so that they sustain the mighty impulse of the heart, and are not ruptured.
In common with the heart, the arteries have a {115} delicately smooth
lining membrane. They are also elastic, and thus re-enforce the action of
the heart: they always remain open when cut across, and after death are
always found empty.
[Sidenote: 31. Early anatomists? The service of the illustration?]
31. The early anatomists observed this phenomenon, and supposing that the
same condition existed during life, came to the conclusion that these tubes
were designed to act as air-vessels, hence the name artery, from a Greek
word which signifies containing air. This circumstance affords us an
illustration of the confused notions of the ancients in reference to the
internal operations of the body. Cicero speaks of the arteries as
"conveying the breath to all parts of the body."
[Sidenote: 32. The arterial system? The branches and sub-branches of the
arteries?]
32. The arterial system springs from the heart by a single trunk, like a
minute and hollow tree, with numberless branches. As these branches leave
the heart they divide and subdivide, continually growing smaller and
smaller, until they can no longer be traced with the naked eye. If, then,
we continue the examination by the aid of a microscope, we see these small
branches sending off still smaller ones, until all the organs of the body
are penetrated by arteries.
[Sidenote: 33. Successive undulations from the heart? Course of the
arteries? Protection of the arteries? General location of the arteries?]
33. THE PULSE.--With each contraction of the left heart, the impulse causes
a wave-like motion to traverse the entire arterial system. If the arteries
were exposed to view, we might see successive undulations speeding from the
heart to the smallest of the branches, in about one-sixth part of a second.
The general course of the arteries is as far as possible from the surface.
This arrangement is certainly wise, as it renders them less liable to
injury, the wounding of an artery being especially dangerous. It also
protects the arteries from external and unequal pressure, by which the
force of the heart would be {116} counteracted and wasted. Accordingly, we
generally find these vessels hugging close to the bones, or hiding behind
the muscles and within the cavities of the body.
[Sidenote: 34. Where do the arteries lie? If we apply the finger? Pulse?
Where felt?]
34. In a few situations, however, the arteries lie near the surface; and if
we apply the finger to any of these parts, we will distinctly feel the
movement described, taking place in harmony with the heart-beat. This is
part of the wave-motion just mentioned, and is known as the pulse. All are
more familiar with the pulse at the wrist, in the _radial_ artery; but the
pulse is not peculiar to that position, for it may be felt in the _carotid_
of the neck, in the _temporal_ at the temple, and elsewhere, especially
near the joints.
[Illustration: FIG. 30.--THE FORM OF THE PULSE.]
[Sidenote: 35. The pulse as an index? Of what does it inform the physician?
Instrument for recording pulsation?]
35. Since the heart-beat makes the pulse, whatever affects the former
affects the latter also. Accordingly, the pulse is a good index of the
state of the health, so far as the health depends upon the action of the
heart. It informs the physician of the condition of the circulation in four
particulars: its rate, regularity, force, and fullness; and nearly every
disease modifies in some respect the condition of the pulse. A very
ingenious instrument, known as the sphygmograph, or pulse-writer, has
recently been invented, by the aid of which the pulse is made to write upon
paper its own signature, or rather to sketch its own profile. This
instrument shows with great accuracy the difference between the pulses of
health and those of disease. In Fig. 30 is traced the form of the pulse in
health, which should be read from left to right. That part of the trace
{117} which is nearly perpendicular coincides with the contraction of the
ventricles; while the wavy portion marks their dilatation.
[Sidenote: 36. What are the veins? How do they form? What do they
resemble?]
36. THE VEINS.--The vessels which convey the blood on its return to the
heart are the veins. They begin in the several organs of the body, and at
first are extremely small; but uniting together as they advance, they
constantly increase in size, reminding us of the way in which the fine
rootlets of the plant join together to form the large roots, or of the
rills and rivulets that flow together to form the large streams and rivers.
In structure, the veins resemble the arteries, but their walls are
comparatively inelastic. They are more numerous, and communicate with each
other freely in their course, by means of interlacing branches.
[Illustration: FIG. 31.--THE VALVES OF THE VEINS.]
[Sidenote: 37. Valves in the veins? What are they? Their position?
Experiment with the cord?]
37. But the chief point of distinction is in the presence of the valves in
the veins. These are little folds of membrane, disposed in such a way, that
they only open to receive blood flowing toward the heart, and close against
a current in the opposite direction. Their position in the veins on the
back of the hand may be readily observed, if we first obstruct the return
of blood by a cord tied around the forearm or wrist. In a few minutes the
veins will appear swollen, and upon them will be seen certain prominences,
about an inch apart. These latter indicate the location of the valves, or,
rather, they show that the vessels in front of the valves are distended by
the blood, which cannot force a passage back through them.
[Sidenote: 38. What will be proved by the experiment? What inference is
drawn?]
38. This simple experiment proves that the true direction of the venous
blood is toward the heart. That the color {118} of the blood is dark, will
be evident, if we compare the hand thus bound by a cord with the hand not
so bound. It also proves that the veins lie superficially, while the
arteries are beneath the muscles, well protected from pressure; and that
free communication exists from one vein to another. If now we test the
temperature of the constricted member by means of a thermometer, we will
find that it is colder than natural, although the amount of blood is larger
than usual. From this fact we infer, that whatever impedes the venous
circulation tends to diminish vitality; and hence, articles of clothing or
constrained postures, that confine the body or limbs, and hinder the
circulation of the blood, are to be avoided as injurious to the health.
[Sidenote: 39. Capillaries? How regarded? Harvey?]
39. THE CAPILLARIES.--A third set of vessels completes the list of the
organs of the circulation, namely, the _capillary_ vessels, so called (from
the Latin word _capillaris_, hair-like), because of their extreme fineness.
They are, however, smaller than any hair, having a diameter of about 1/3000
of an inch, and can only be observed by the use of the microscope. These
vessels may be regarded as the connecting link between the last of the
arteries and the first of the veins. The existence of these vessels was
unknown to Harvey, and was the one step wanting to complete his great work.
The capillaries were not discovered until 1661, a short time after the
invention of the microscope.
[Sidenote: 40. The circulation of the blood in the web of a frog's foot?
Describe it. How general is the existence of the tissues?]
40. The circulation of the blood, as seen under the microscope, in the
transparent web of a frog's foot, is a spectacle of rare beauty, possessing
more than ordinary interest, when we consider that something very similar
is taking place in our own bodies, on a most magnificent scale. It is like
opening a secret page in the history of our own frames. We there see
distinctly the three classes of vessels with their moving contents; first,
the artery, {119} with its torrent of blood rushing down from the heart,
secondly, the vein, with its slow, steady stream flowing in the opposite
direction; and between them lies the network of capillaries, so fine that
the corpuscles can only pass through "in single file." The current has here
an uncertain or swaying motion, hurrying first in one direction, then
hesitating, and then turning back in the opposite direction, and sometimes
the capillaries contract so as to be entirely empty. Certain of the tissues
are destitute of capillaries; such are cartilage, hair, and a few others on
the exterior of the body. In all other structures, networks of these
vessels are spread out in countless numbers: so abundant is the supply,
that it is almost impossible to puncture any part with the point of a
needle without lacerating tens, or even hundreds of these vessels.
[Illustration: FIG. 32.--WEB OF A FROG'S FOOT, slightly magnified.]
[Illustration: FIG. 33.--MARGIN OF FROG'S WEB magnified 30 diameters.]
[Sidenote: 41. Elasticity of the capillaries? Grain of sand in the eye?
Blush? Other cases?]
41. The capillaries are elastic, and may so expand as to produce an effect
visible to the naked eye. If a grain of sand, or some other foreign
particle, lodge in the eye, it will become irritated, and in a short time
the white of the eye will be "blood-shot." This appearance is due to an
{120} increase in the size of these vessels. A blush is another example of
this, but the excitement comes through the nervous system, and the cause is
some transient emotion, either of pleasure or pain. Another example is
sometimes seen in purplish faces of men addicted to drinking brandy; in
them the condition is a congestion of the capillary circulation, and is
permanent, the vessels having lost their power of elastic contraction.
[Sidenote: 42. Show what time is required for a given portion of blood to
travel once around the body.]
42. RAPIDITY OF THE CIRCULATION.--That the blood moves with great rapidity
is evident from the almost instant effects of certain poisons, as prussic
acid, which act through the blood. Experiments upon the horse, dog, and
other inferior animals, have been made to measure its velocity. If a
substance, which is capable of a distinct chemical reaction (as _potassium
ferrocyanide_, or _barium nitrate_), be introduced into a vein of a horse
on one side, and blood be taken from a distant vein on the other side, its
presence may be detected at the end of twenty or thirty-two seconds. In
man, the blood moves with greater speed, and the circuit is completed in
twenty-four seconds.
[Sidenote: 43. Time required for all the blood to circulate completely
around?]
43. What length of time is required for all the blood of the body to make a
complete round of the circulation? This question cannot be answered with
absolute accuracy, since the amount of the blood is subject to continual
variations. But, if we assume this to be one-eighth of the weight of the
body, about eighteen pounds, it will be sufficiently correct for our
purpose. Now to complete the circuit, this blood must pass once through the
left ventricle, the capacity of which is two ounces. Accordingly, we find
that, under ordinary circumstances, all the blood makes one complete
rotation every two minutes; passing successively through the heart, the
capillaries of the lungs, the arteries, the capillaries of the extremities,
and through the veins. {121}
[Sidenote: 44. What is meant by assimilation? What can you say of its use,
etc.? Time?]
44. ASSIMILATION.--The crowning act of the circulation, the furnishing of
supplies to the different parts of the body, is effected by means of the
capillaries. The organs have been wasted by use; the blood has been
enriched by the products of digestion. Here, within the meshes of the
capillary network, the needy tissues and the needed nutriment are brought
together. By some mysterious chemistry, each tissue selects and withdraws
from the blood the materials it requires, and converts them into a
substance like itself. This conversion of lifeless food into living tissue
is called assimilation. The process probably takes place at all times, but
the period especially favorable for it is during sleep. Then the
circulation is slower, and more regular, and most of the functions are at
rest. The body is then like some trusty ship, which after a long voyage is
"hauled up for repairs."
[Sidenote: 45. What is stated of the injuries to the blood-vessels?]
45. INJURIES TO THE BLOOD-VESSELS.--It is important to be able to
discriminate between an artery and a vein, in the case of a wound, and if
we remember the physiology of the circulation we may readily do so. For, as
we have already seen, hæmorrhage from an artery is much more dangerous than
that from a vein. The latter tends to cease spontaneously after a short
time. The arterial blood flows away from the heart with considerable force,
in jets; its color being bright scarlet. The venous blood flows toward the
heart from that side of the wound furthest from the heart; its stream being
continuous and sluggish; its color dark. In an injury to an artery,
pressure should be made between the heart and the wound; and in the case of
a vein that persistently bleeds, it should be made upon the vessel beyond
its point of injury. {122}
QUESTIONS FOR TOPICAL REVIEW.
PAGE
1. In what organisms is the so-called circulatory fluid found? 101
2. How is it designated in the different organisms? 101
3. What can you state of the importance of blood to the body? 101, 105
4. Of its great abundance, color, and composition? 101, 102, 107
5. Describe the corpuscles of the human blood. 102, 103, 104
6. What is said of them in comparison with those of the lower
animals? 103
7. Of the importance of sometimes detecting human from other blood? 103
8. What means have we of detecting blood in spots or stains? 103, 104
9. What is meant by coagulation of the blood? 104
10. What wisdom is there in the law of the blood's coagulation? 104, 105
11. How is this wisdom made manifest? 105
12. In what cases is the aid of the surgeon required? 105
13. What are the two great uses of the blood? 105
14. Through what mediums is the blood provided with new material and
relieved of the old material? 105
15. What do you understand by the operation called transfusion? 106
16. What cases of transfusion are reported of the lower animals? 106
17. What can you state of transfusion as practised upon man? 106
18. What further can you say on the subject? 106, 107
19. What changes take place in the color of the blood in its journey
through the system? 107
20. State all you can in relation to the circulation of the blood. 107
21. All, in relation to the size, shape, and location of the
heart. 107, 109
22. How is the loss of power in the heart movements obviated? 109
23. Give a description of the formation of the heart. 109, 110, 111
24. What can you state of the ventricles and auricles of the heart? 110
25. Describe the action of the heart. 111
26. What special vitality does the tissue of the heart possess? 111
27. State all you can on the subject. 111
28. Describe the course of the blood through the cavities of the
heart. 112
29. Describe the mechanism that regulates the heart-currents. 112
30. How do you account for the two heart-sounds at the front of
the chest? 112
31. State what you can of the frequency of the heart's action. 112, 113
32. Of the activity of the heart. 113, 114
33. What do you understand by the arteries? 114, 115
34. State what you can of the arteries and the arterial system. 114, 115
35. What do you understand by the pulse? 115, 116
36. In what part of the body may the pulse be felt? 116
37. What further can you state of the pulse? 116, 117
38. What are the veins? 117
39. Where do they exist, and how are they formed? 117
40. Describe the valves of the veins and their uses. 117
41. Now give a full description of the construction of the veins. 117
42. What further can you state of the veins? 117, 118
43. What do you understand by the capillaries? 118, 119
44. What service do the capillaries perform? 118, 119, 121
45. Describe the circulation of the blood in the region of
the heart. 118, 119
46. What can you state of the rapidity of the blood's circulation? 120
47. Of the process known as assimilation? 121
48. Of injuries to the blood-vessels? 121
* * * * *
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