A Carrier must move. To enable the blood to carry food and oxygen _to_ the
cells and waste materials _from_ the cells, and also to distribute heat,
it is necessary to keep it moving, or circulating, in all parts of the
body. So closely related to the welfare of the body is the circulation(17)
of the blood, that its stoppage for only a brief interval of time results
in death.
*Discovery of the Circulation.*--The discovery of the circulation of the
blood was made about 1616 by an English physician named Harvey. In 1619 he
announced it in his public lectures and in 1628 he published a treatise in
Latin on the circulation. The chief arguments advanced in support of his
views were the presence of valves in the heart and veins, the continuous
movement of the blood in the same direction through the blood vessels, and
the fact that the blood comes from a cut artery in jets, or spurts, that
correspond to the contractions of the heart.
No other single discovery with reference to the human body has proved of
such great importance. A knowledge of the nature and purpose of the
circulation was the necessary first step in understanding the plan of the
body and the method of maintaining life, and physiology as a science dates
from the time of Harvey's discovery.
*Organs of Circulation.*--The organs of circulation, or blood vessels, are
of four kinds, named the heart, the arteries, the capillaries, and the
veins. They serve as contrivances both for holding the blood and for
keeping it in motion through the body. The heart, which is the chief organ
for propelling the blood, acts as a force pump, while the arteries and
veins serve as tubes for conveying the blood from place to place.
Moreover, the blood vessels are so connected that the blood moves through
them in a regular order, performing two well-defined circuits.
[Fig. 13]
Fig. 13--*Heart* in position in thoracic cavity. Dotted lines show positin
of diaphragm and of margins of lungs.
*The Heart.*--The human heart, roughly speaking, is about the size of the
clenched fist of the individual owner. It is situated very near the center
of the thoracic cavity and is almost completely surrounded by the lungs.
It is cone-shaped and is so suspended that the small end hangs downward,
forward, and a little to the left. When from excitement, or other cause,
one becomes conscious of the movements of the heart, these appear to be in
the left portion of the chest, a fact which accounts for the erroneous
impression that the heart is on the left side. The position of the heart
in the cavity of the chest is shown in Fig. 13.
*The Pericardium.*--Surrounding the heart is a protective covering, called
the pericardium. This consists of a closed membranous sac so arranged as
to form a double covering around the heart. The heart does not lie inside
of the pericardial sac, as seems at first glance to be the case, but its
relation to this space is like that of the hand to the inside of an empty
sack which is laid around it (Fig. 14). The inner layer of the pericardium
is closely attached to the heart muscle, forming for it an outside
covering. The outer layer hangs loosely around the heart and is continuous
with the inner layer at the top. The outer layer also connects at certain
places with the membranes surrounding the lungs and is attached below to
the diaphragm. Between the two layers of the pericardium is secreted a
liquid which prevents friction from the movements of the heart.
[Fig. 14]
Fig. 14--*Diagram of section of the pericardial sac*, heart removed. _A._
Place occupied by the heart. _B._ Space inside of pericardial sac. _a._
Inner layer of pericardium and outer lining of heart. _b._ Outer layer of
pericardium. _C._ Covering of lung. _D._ Diaphragm.
*Cavities of the Heart.*--The heart is a hollow, muscular organ which has
its interior divided by partitions into four distinct cavities. The main
partition extends from top to bottom and divides the heart into two
similar portions, named from their positions the right side and the left
side. On each side are two cavities, the one being directly above the
other. The upper cavities are called _auricles_ and the lower ones
_ventricles_. To distinguish these cavities further, they are named from
their positions the right auricle and the left auricle, and the right
ventricle and the left ventricle (Fig. 15). The auricles on each side
communicate with the ventricles below; but after birth there is no
communication between the cavities on the opposite sides of the heart. All
the cavities of the heart are lined with a smooth, delicate membrane,
called the _endocardium_.
[Fig. 15]
Fig. 15--*Diagram showing plan of the heart.* 1. Semilunar valves. 2.
Tricuspid valve. 3. Mitral valve. 4. Right auricle. 5. Left auricle. 6.
Right ventricle. 7. Left ventricle. 8. Chordae tendineae. 9. Inferior vena
cava. 10. Superior vena cava. 11. Pulmonary artery. 12. Aorta. 13.
Pulmonary veins.
*Valves of the Heart.*--Located at suitable places in the heart are four
gate-like contrivances, called valves. The purpose of these is _to give
the blood a definite direction_ in its movements. They consist of tough,
inelastic sheets of connective tissue, and are so placed that pressure on
one side causes them to come together and shut up the passageway, while
pressure on the opposite side causes them to open. A valve is found at the
opening of each auricle into the ventricle, and at the opening of each
ventricle into the artery with which it is connected.
The valve between the right auricle and the right ventricle is called the
_tricuspid_ valve. It is suspended from a thin ring of connective tissue
which surrounds the opening, and its free margins extend into the
ventricle (Fig. 16). It consists of three parts, as its name implies,
which are thrown together in closing the opening. Joined to the free edges
of this valve are many small, tendinous cords which connect at their lower
ends with muscular pillars in the walls of the ventricle. These are known
as the _chordae tendineae_, or heart tendons. Their purpose is to serve as
_valve stops_, to prevent the valve from being thrown, by the force of the
blood stream, back into the auricle.
The _mitral_, or bicuspid, valve is suspended around the opening between
the left auricle and the left ventricle, with the free margins extending
into the ventricle. It is exactly similar in structure and arrangement to
the tricuspid valve, except that it is stronger and is composed of two
parts instead of three.
[Fig. 16]
Fig. 16--*Right side of heart* dissected to show cavities and valves. _B._
Right semilunar valve. The tricuspid valve and the chordae tendineae shown
in the ventricle.
The _right semilunar_ valve is situated around the opening of the right
ventricle into the pulmonary artery. It consists of three pocket-shaped
strips of connective tissue which hang loosely from the walls when there
is no pressure from above; but upon receiving pressure, the pockets fill
and project into the opening, closing it completely (Fig. 16). The _left
semilunar_ valve is around the opening of the left ventricle into the
aorta, and is similar in all respects to the right semilunar valve.
*Differences in the Parts of the Heart.*--Marked differences are found in
the walls surrounding the different cavities of the heart. The walls of
the ventricles are much thicker and stronger than those of the auricles,
while the walls of the left ventricle are two or three times thicker than
those of the right. A less marked but similar difference exists between
the auricles and also between the valves on the two sides of the heart.
These differences in structure are all accounted for by the work done by
the different portions of the heart. The greater the work, the heavier the
structures that perform the work.
[Fig. 17]
Fig. 17--*Diagram of the circulation*, showing in general the work done by
each part of the heart. The right ventricle forces the blood through the
lungs and into the left auricle. The left ventricle forces blood through
all parts of the body and back to the auricle. The auricles force blood
into the ventricles.
*Connection with Arteries and Veins.*--Though the heart is in communication
with all parts of the circulatory system, it makes actual connection with
only a few of the blood tubes. These enter the heart at its upper portion
(Fig. 15), but connect with its different cavities as follows:
1. _With the right auricle_, the superior and the inferior venae cavae and
the coronary veins. The superior vena cava receives blood from the head
and the upper extremities; the inferior vena cava, from the trunk and the
lower extremities; and the coronary veins, from the heart itself.
2. _With the left auricle_, the four pulmonary veins. These receive blood
from the lungs and empty it into the left auricle.
3. _With the right ventricle_, the pulmonary artery. This receives blood
from the heart and by its branches distributes it to all parts of the
lungs.
4. _With the left ventricle_, the aorta. The aorta receives blood from the
heart and through its branches delivers it to all parts of the body.
*How the Heart does its Work.*--The heart is a muscular pump(18) and does
its work through the contracting and relaxing of its walls. During
contraction the cavities are closed and the blood is forced out of them.
During relaxation the cavities open and are refilled. The valves direct
the flow of the blood, being so arranged as to keep it moving always in
the same direction (Fig. 17).
The heart, however, is not a single or a simple pump, but consists in
reality of _four_ pumps which correspond to its different cavities. These
connect with each other and with the blood vessels over the body in such a
manner that each aids in the general movement of the blood.
[Fig. 18]
Fig. 18--Diagram illustrating the "cardiac cycle."
*Work of Auricles and Ventricles Compared.*--In the work of the heart the
two auricles contract at the same time--their contraction being followed
immediately by the contraction of both ventricles. After the contraction
of the ventricles comes a period of rest, or relaxation, about equal in
time to the period of contraction of both the auricles and the
ventricles.(19) On account of the work which they perform, the auricles
have been called the "feed pumps" of the heart; and the ventricles, the
"force pumps."(20) It is the function of the auricles to collect the blood
from the veins, to let this run slowly into the ventricles when both the
auricles and ventricles are relaxed, and finally, by contracting, _to
force an excess of blood into the ventricles_, thereby distending their
walls. The ventricles, having in this way been fully charged by the
auricles, now contract and force their contents into the large arteries.
*Sounds of the Heart.*--Two distinct sounds are given out by the heart as
it pumps the blood. One of them is a dull and rather heavy sound, while
the other is a short, sharp sound. The short sound follows quickly after
the dull sound and the two are fairly imitated by the words "lubb, dup."
While the cause of the first sound is not fully understood, most
authorities believe it to be due to the contraction of the heart muscle
and the sudden tension on the valve flaps. The second sound is due to the
closing of the semilunar valves. These sounds are easily heard by placing
an ear against the chest wall. They are of great value to the physician in
determining the condition of the heart.
*Arteries and Veins.*--These form two systems of tubes which reach from the
heart to all parts of the body. The arteries receive blood from the heart
and distribute it to the capillaries. The veins receive the blood from the
capillaries and return it to the heart. The arteries and veins are similar
in structure, both having the form of tubes and both having three distinct
layers, or coats, in their walls. The corresponding coats in the arteries
and veins are made up of similar materials, as follows:
1. _The inner coat_ consists of a delicate lining of flat cells resting
upon a thin layer of connective tissue. The inner coat is continuous with
the lining of the heart and provides a smooth surface over which the blood
glides with little friction.
2. _The middle coat_ consists mainly of non-striated, or involuntary,
muscular fibers. This coat is quite thin in the veins, but in the arteries
it is rather thick and strong.
3. _The outer coat_ is made up of a variety of connective tissue and is
also much thicker and stronger in the arteries than in the veins.
[Fig. 19]
Fig. 19--Artery dissected to show the coats.
Marked differences exist between the arteries and the veins, and these
vessels are readily distinguished from each other. The walls of the
arteries are much thicker and heavier than those of the veins (Fig. 19).
As a result these tubes stand open when empty, whereas the veins collapse.
The arteries also are highly elastic, while the veins are but slightly
elastic. On the other hand, many of the veins contain valves, formed by
folds in the inner coat (Fig. 20), while the arteries have no valves. The
blood flows more rapidly through the arteries than through the veins, the
difference being due to the fact that the system of veins has a greater
capacity than the system of arteries.
[Fig. 20]
Fig. 20--Vein split open to show the valves.
*Why the Arteries are Elastic.*--The elasticity of the arteries serves a
twofold purpose. It keeps the arteries from bursting when the blood is
forced into them from the ventricles, and it is a means of _supplying
pressure to the blood while the ventricles are in a condition of
relaxation._ The latter purpose is accomplished as follows:
Contraction of the ventricles fills the arteries overfull, causing them to
swell out and make room for the excess of blood. Then while the ventricles
are resting and filling, the stretched arteries press upon the blood to
keep it flowing into the capillaries. In this way _they cause the
intermittent flow from, the heart to become a steady stream in the
capillaries_.
The swelling of the arteries at each contraction of the ventricle is
easily felt at certain places in the body, such as the wrist. This
expansion, known as the "pulse," is the chief means employed by the
physician in determining the force and rapidity of the heart's action.
*Purpose of the Valves in the Veins.*--The valves in the veins are not used
for directing the _general_ flow of the blood, the valves of the heart
being sufficient for this purpose. Their presence is necessary because of
the pressure to which the veins are subjected in different parts of the
body. The contraction of a muscle will, for example, close the small veins
in its vicinity and diminish the capacity of the larger ones. The natural
tendency of such pressure is to empty the veins in two directions--one in
the same direction as the regular movement of the blood, but the other in
the opposite direction. The valves by closing cause the contracting muscle
to push the blood in one direction only--toward the heart. The valves in
the veins are, therefore, an economical device for _enabling variable
pressure_ in different parts of the body _to assist in the circulation_.
Veins like the inferior vena cava and the veins of the brain, which are
not compressed by movements of the body, do not have valves.
*Purposes of the Muscular Coat.*--The muscular coat, which is thicker in
the arteries than in the veins and is more marked in small arteries than
in large ones, serves two important purposes. In the first place it,
together with the elastic tissue, keeps the capacity of the blood vessels
_equal to the volume of the blood_. Since the blood vessels are capable of
holding more blood than may be present at a given time in the body, there
is a liability of empty spaces occurring in these tubes. Such spaces would
seriously interfere with the circulation, since the heart pressure could
not then reach all parts of the blood stream. This is prevented by the
contracted state, or "tone," of the blood vessels, due to the muscular
coat.
In the second place, the muscular coat serves the purpose of _regulating_
the amount of blood which any given organ or part of the body receives.
This it does by varying the caliber of the arteries going to the organ in
question. To increase the blood supply, the muscular coat relaxes. The
arteries are then dilated by the blood pressure from within so as to let
through a larger quantity of blood. To diminish the supply, the muscle
contracts, making the caliber of the arteries less, so that less blood can
flow to this part of the body. Since the need of organs for blood varies
with their activity, the muscular coat serves in this way a very necessary
purpose.
[Fig. 21]
Fig. 21--Diagram of network of capillaries between a very small artery and
a very small vein. Shading indicates the change of color of the blood as
it passes through the capillaries. _S._ Places between capillaries
occupied by the cells.
*Capillaries.*--The capillaries consist of a network of minute blood
vessels which connect the terminations of the smallest arteries with the
beginnings of the smallest veins (Fig. 21). They have an average diameter
of less than one two-thousandth of an inch (12 mu) and an average length of
less than one twenty-fifth of an inch (1 millimeter). Their walls consist
of a single coat which is continuous with the lining of the arteries and
veins. This coat is formed of a single layer of thin, flat cells placed
edge to edge (Fig. 22). With a few exceptions, the capillaries are found
in great abundance in all parts of the body.
[Fig. 22]
Fig. 22--*Surface of capillary* highly magnified, showing its coat of thin
cells placed edge to edge.
*Functions of the Capillaries.*--On account of the thinness of their walls,
the capillaries are able to serve a twofold purpose in the body:
1. They admit materials into the blood vessels.
2. They allow materials to pass from the blood vessels to the surrounding
tissues.
When it is remembered that the blood, as blood, does not escape from the
blood vessels under normal conditions, the importance of the work of the
capillaries is apparent. To serve its purpose as a carrier, there must be
places where the blood can load up with the materials which it is to
carry, and places also where these can be unloaded. Such places are
supplied by the capillaries.
The capillaries also serve the purpose of spreading the blood out and of
bringing it very near the individual cells in all parts of the body (Fig.
21).
*Functions of Arteries and Veins.*--While the capillaries provide the means
whereby materials may both enter and leave the blood, the arteries and
veins serve the general purpose of passing the blood from one set of
capillaries to another. Since pressure is necessary for moving the blood,
these tubes must connect with the source of the pressure, which is the
heart. In the arteries and veins the blood neither receives nor gives up
material, but having received or given up material at one set of
capillaries, it is then pushed through these tubes to where it can serve a
similar purpose in another set of capillaries (Fig. 23).
*Divisions of the Circulation.*--Man, in common with all warm-blooded
animals, has a double circulation, a fact which explains the double
structure of his heart. The two divisions are known as the _pulmonary_ and
the _systemic_ circulations. By the former the blood passes from the right
ventricle through the lungs, and is then returned to the left auricle; by
the latter it passes from the left ventricle through all parts of the
body, returning to the right auricle.
The general plan of the circulation is indicated in Fig. 23. All the blood
flows continuously through both circulations and passes the various parts
in the following order: right auricle, tricuspid valve, right ventricle,
right semilunar valve, pulmonary artery and its branches, capillaries of
the lungs, pulmonary veins, left auricle, mitral valve, left ventricle,
left semilunar valve, aorta and its branches, systemic capillaries, the
smaller veins, superior and inferior venae cavae, and then again into the
right auricle.
In the pulmonary capillaries the blood gives up carbon dioxide and
receives oxygen, changing from a dark red to a bright red color. In the
systemic capillaries it gives up oxygen, receives carbon dioxide and other
impurities, and changes back to a dark red color.
In addition to the two main divisions of the circulation, special circuits
are found in various places. Such a circuit in the liver is called the
_portal_ circulation, and another in the kidneys is termed the _renal_
circulation. To some extent the blood supply to the walls of the heart is
also outside of the general movement; it is called the _coronary_
circulation.
[Fig. 23]
Fig. 23--*General scheme of the circulation*, showing places where the
blood takes on and gives off materials. 1. Body in general. 2. Lungs. 3.
Kidneys. 4. Liver. 5. Organs of digestion. 6. Lymph ducts. 7. Pulmonary
artery. 8. Aorta.
*Blood Pressure and Velocity.*--The blood, in obedience to physical laws,
passes continuously through the blood vessels, moving always from a place
of greater to one of less pressure. Through the contraction of the
ventricles, a relatively high pressure is maintained in the arteries
nearest the heart.(21) This pressure diminishes rapidly in the small
arteries, becomes comparatively slight in the capillaries, and falls
practically to nothing in the veins. Near the heart in the superior and
inferior venae cavae, the pressure at intervals is said to be _negative_.
This means that the blood from these veins is actually drawn into the
right auricle by the expansion of the chest walls in breathing.(22)
The velocity of the blood is greatest in the arteries, less in the veins,
and _much_ less in the capillaries than in either the arteries or the
veins. The slower flow of the blood through the capillaries is accounted
for by the fact that their united area is many times greater than that of
the arteries which supply, or the veins which relieve, them. This allows
the same quantity of blood, flowing through them in a given time, a wider
channel and causes it to move more slowly. The time required for a
complete circulation is less than one minute.
*Summary of Causes of Circulation.*--The chief factor in the circulation of
the blood is, of course, the heart. The ventricles keep a pressure on the
blood which is sufficient to force it through all the blood tubes and back
to the auricles. The heart is aided in its work by the elasticity of the
arteries, which keeps the blood under pressure while the ventricles are in
a state of relaxation. It is also aided by the muscles and elastic tissue
in all of the blood vessels. These, by keeping the blood vessels in a
state of "tone," or so contracted that their capacity just equals the
volume of the blood, enable pressure from the heart to be transmitted to
all parts of the blood stream. A further aid to the circulation is found
in the valves in the veins, which enable muscular contraction within the
body, and variable pressure upon its surface, to drive the blood toward
the heart. The heart is also aided to some extent by the movements of the
chest walls in breathing. The organs Of circulation are under the control
of the nervous system (Chapter XVIII).
HYGIENE OF THE CIRCULATION
*Care of the Heart.*--The heart, consisting largely of muscle, is subject
to the laws of muscular exercise. It may be injured by over-exertion, but
is strengthened by a moderate increase in its usual work.(23) It may even
be subjected to great exertion without danger, if it be trained by
gradually increasing its work. Such training, by giving the heart time to
gain in size and strength, prepares it for tasks that could not at first
be accomplished.
In taking up a new exercise requiring considerable exertion, precautions
should be observed to prevent an overstrain of the heart. The heart of the
amateur athlete, bicyclist, or mountain climber is frequently injured by
attempting more than the previous training warrants. The new work should
be taken up gradually, and feats requiring a large outlay of physical
energy should be attempted only after long periods of training.
Since the heart is controlled by the nervous system, it frequently becomes
irregular in its action through conditions that exhaust the nervous
energy. Palpitations of the heart, the missing of beats, and pains in the
heart region frequently arise from this cause. It is through their effect
upon the nervous system that worry, overstudy, undue excitement, and
dissipation cause disturbances of the heart. In all such cases the remedy
lies in the removal of the cause. The nervous system should also be "toned
up" through rest, plenty of sleep, and moderate exercise in the open air.
*Effect of Drugs.*--A number of substances classed as drugs, mainly by
their action on the nervous system, produce undesirable effects upon the
organs of circulation. Unfortunately some of these are extensively used,
alcohol being one of them. If taken in any but small quantities, alcohol
is a disturbing factor in the circulation. It increases the rate of the
heart beat and dilates the capillaries. Its effect upon the capillaries is
shown by the "bloodshot" eye and the "red nose" of the hard drinker.
Another bad effect from the use of much alcohol is the weakening of the
heart through the accumulation of fat around this organ and within the
heart muscle. The use of alcohol also leads in many cases to a hardening
of the walls of the arteries, such as occurs in old age. This effect makes
the use of alcohol especially dangerous for those in advanced years.
Tobacco contains a drug, called nicotine, which has a bad effect upon the
heart in at least two ways: 1. When the use of tobacco is begun in early
life, it interferes with the growth of the heart, leading to its weakness
in the adult. 2. When used in considerable quantity, by young or old, it
causes a nervous condition both distressing and dangerous, known as
"tobacco heart."
Tea and coffee contain a drug, called caffeine, which acts upon the
nervous system and which may, on this account, interfere with the proper
control of the heart. In some individuals the taking of a very small
amount of either tea or coffee is sufficient to cause irregularities in
the action of the heart. Tea is considered the milder of the two liquids
and the one less liable to injure.
*Effect of Rheumatism.*--The disease which affects the heart more
frequently than any other is rheumatism. This attacks the lining membrane,
or endocardium, and causes, not infrequently, a shrinkage of the heart
valves. The heart is thus rendered defective and, to perform its function
in the body, must work harder than if it were in a normal condition.
Rheumatic attacks of the heart do most harm when they occur in early
life--the period when the valves are the most easily affected. Any tendency
toward rheumatism in children has, therefore, a serious significance and
should receive the attention of the physician. Any one having a defective
heart should avoid all forms of exercise that demand great exertion.
*Strengthening of the Blood Vessels.*--Disturbances of the circulation,
causing too much blood to be sent to certain parts of the body and an
insufficient amount to others, when resulting from slight causes, are
usually due to weakness of the walls of the blood vessels, particularly of
the muscular coat. Such weakness is frequently indicated by extreme
sensitiveness to heat or cold and by a tendency to "catch cold." From a
health standpoint the preservation of the normal muscular "tone" of the
blood vessels is a problem of great importance. Though the muscles of the
blood vessels cannot be exercised in the same manner as the voluntary
muscles, they may be called actively into play through all the conditions
that induce changes in the blood supply to different parts of the body.
The usual forms of physical exercise necessitate such changes and
indirectly exercise the muscular coat. The exposure of the body to cold
for short intervals, because of the changes in the circulation which this
induces, also serves the same purpose. A cold bath taken with proper
precautions is beneficial to the circulation of many and so also is a
brisk walk on a frosty morning. Both indirectly exercise and strengthen
the muscular coat of the blood vessels. On the other hand, too much time
spent indoors, especially in overheated rooms, leads to a weakening of the
muscular coat and should be avoided.
*Checking of Flow of Blood from Wounds.*--The loss of any considerable
quantity of blood is such a serious matter that every one should know the
simpler methods of checking its flow from wounds. In small wounds the flow
is easily checked by binding cotton or linen fiber over the place. The
absorbent cotton, sold in small packages at drug stores, is excellent for
this purpose and should be kept in every home. A simple method of checking
"nosebleed" is that of drawing air through the bleeding nostril, while the
other nostril is compressed with the finger.(24) Another method is to
"press with the finger (or insert a small roll of paper) under the lip
against the base of the nose." (25) Where the bleeding is persistent, the
nostril should be plugged with a small roll of clean cotton or paper. When
this is done, the plug should not be removed too soon because of the
likelihood of starting the flow afresh.
In dealing with large wounds the services of a physician are
indispensable. But in waiting for the physician to arrive temporary aid
must be rendered. The one who gives such aid should first decide whether
an artery or a vein has been injured. This is easily determined by the
nature of the blood stream, which is in jets, or spurts, from an artery,
but flows steadily from a vein. If an artery is injured, the limb should
be tightly bandaged on the side of the wound nearest the heart; if a vein,
on the side farthest from the heart. In addition to this, the edges of the
wound should be closed and covered with cotton fiber and the limb should
be placed on a support above the level of the rest of the body. A large
handkerchief makes a convenient bandage if properly applied. This should
be folded diagonally and a knot tied in the middle. Opposite ends are then
tied, making a loose-fitting loop around the limb. The knot is placed
directly over the blood vessel to be compressed and a short stick inserted
in the loop. The necessary pressure is then applied by twisting the
handkerchief with the stick. Time must not be lost, however, in the
preparation of a suitable bandage. The blood vessel should be compressed
with the fingers while the bandage is being prepared.
*Summary.*--The blood, to serve as a transporting agent, must be kept
continually moving through all parts of the body. The blood vessels hold
the blood, supply the channels and force necessary for its circulation,
and provide conditions which enable materials both to enter and to leave
the blood stream. The heart is the chief factor in propelling the blood,
although the muscles and the elastic tissue in the walls of the arteries
and the valves in the veins are necessary aids in the process. In the
capillaries the blood takes on and gives off materials, while the arteries
and veins serve chiefly as tubes for conveying the blood from one system
of capillaries to another.
*Exercises.*--1. Of what special value in the study of the body was the
discovery of the circulation of the blood?
2. State the necessity for a circulating liquid in the body.
3. Show by a drawing the general plan of the heart, locating and naming
the essential parts. Show also the connection of the large blood vessels
with the cavities of the heart.
4. Compare the purpose served by the chordae tendineae to that served by
doorstops (the strips against which the door strikes in closing).
5. Explain how the heart propels the blood. To what class of pumps does it
belong? What special work is performed by each of its divisions?
6. Define a valve. Of what use are the valves in the heart? In the veins?
7. By what means is pressure from contracting muscles in different parts
of the body made to assist in the circulation?
8. Of what advantage is the elasticity of the arteries?
9. How is blood forced from the capillaries back to the heart?
10. Why should there be a difference in structure between the two sides of
the heart?
11. Following Fig. 23, trace the blood through a complete circulation,
naming all the divisions of the system in the order of the flow of the
blood.
12. If the period of rest following the period of contraction of the heart
be as long as the period of contraction, how many hours is the heart able
to rest out of every twenty-four?
13. State the functions of the capillaries. Show how their structure
adapts them to their work.
14. What kind of physical exercise tends to strengthen the heart? What
forms of exercise tend to injure it? State the effects of alcohol and
tobacco on the heart.
15. How may rheumatism injure the heart?
16. Give directions for checking the flow of blood from small and from
large blood vessels.
PRACTICAL WORK
In showing the relations of the different parts of the heart, a large
dissectible model is of great service (Fig. 24). Indeed, where the time of
the class is limited, the practical work may be confined to the study of
the heart model, diagrams of the heart and the circulation, and a few
simple experiments. However, where the course is more extended, the
dissection of the heart of some animal as described below is strongly
advised.
*Observations on the Heart.*--Procure, by the assistance of a butcher, the
heart of a sheep, calf, or hog. To insure the specimen against mutilation,
the lungs and the diaphragm must be left attached to the heart. In
studying the different parts, good results will be obtained by observing
the following order:
1. Observe the connection of the heart to the lungs, diaphragm, and large
blood vessels. Inflate the lungs and observe the position of the heart
with reference to them.
2. Examine the sac surrounding the heart, called the _pericardium_. Pierce
its lower portion and collect the pericardial fluid. Increase the opening
thus made until it is large enough to slip the heart out through it. Then
slide back the pericardium until its connection with the large blood
vessels above the heart is found. Observe that a thin layer of it
continues down from this attachment, forming the outer covering of the
heart.
3. Trace out for a short distance and study the veins and arteries
connected with the heart. The arteries are to be distinguished by their
thick walls. The heart may now be severed from the lungs by cutting the
large blood vessels, care being taken to leave a considerable length of
each one attached to the heart.
[Fig. 24]
Fig. 24--Model for demonstrating the heart.
4. Observe the outside of the heart. The thick, lower portion contains the
cavities called _ventricles_; the thin, upper, ear-shaped portions are the
_auricles_. The thicker and denser side lies toward the left of the
animal's body and is called the _left_ side of the heart; the other is the
_right_ side. Locate the right auricle and the right ventricle; the left
auricle and the left ventricle.
5. Lay the heart on the table with the front side up and the apex pointing
from the operator. This places the left side of the heart to his left and
the right side to his right. Notice the groove between the ventricles,
called the inter-ventricular groove. Make an incision half an inch to the
right of this groove and cut toward the base of the heart until the
pulmonary artery is laid open. Then, following within half an inch of the
groove, cut down and around the right side of the heart. The wall of the
right ventricle may now be raised and the cavity exposed. Observe the
extent of the cavity, its shape, its lining, its columns of muscles, its
half columns of muscles, its tendons (chordae tendineae), the tricuspid
valve from the under side, etc. Also notice the valve at the beginning of
the pulmonary artery (the right semilunar) and the sinuses, or
depressions, in the artery immediately behind its divisions.
6. Now cut through the middle of the loosened ventricular wall from the
apex to the middle of the right auricle, laying it open for observation.
Observe the openings into the auricle, there being one each for the vena
cava superior, the vena cava inferior, and the coronary vein. Compare the
walls, lining, shape, size, etc., with the ventricle below.
7. Cut off the end of the left ventricle about an inch above the apex.
This will show the extension of the cavity to the apex; it will also show
the thickness of the walls and the shape of the cavity. Split up the
ventricular wall far enough to examine the mitral valve and the chordae
tendineae from the lower side.
8. Make an incision in the left auricle. Examine its inner surface and
find the places of entrance of the pulmonary veins. Examine the mitral
valve from above. Compare the two sides of the heart, part for part.
9. Separate the aorta from the other blood vessels and cut it entirely
free from the heart, care being taken to leave enough of the heart
attached to the artery to insure the semilunar valve's being left in good
condition. After tying or plugging up the holes in the sides of the
artery, pour water into the small end and observe the closing of the
semilunar valve. Repeat the experiment until the action of the valve is
understood. Sketch the artery, showing the valve in a closed condition.
*To illustrate the Action of a Ventricle.*--Procure a syringe bulb with an
opening at each end. Connect a rubber tube with each opening, letting the
tubes reach into two tumblers containing water. By alternately compressing
and releasing the bulb, water is pumped from one vessel into the other.
The bulb may be taken to represent one of the ventricles. What action of
the ventricle is represented by compressing the bulb? By releasing the
pressure? Show by a sectional drawing the arrangement of the valves in the
syringe bulb.
[Fig. 25]
Fig. 25--Illustrating elasticity of arteries.
*To show the Advantage of the Elasticity of Arteries.*--Connect the syringe
bulb used in the last experiment with a rubber tube three or four feet in
length and having rather thin walls. In the opposite end of the rubber
tube insert a short glass tube which has been drawn (by heating) to a fine
point (Fig. 25). Pump water into the rubber tube, observing:
1. The swelling of the tube (pulse) as the water is forced into it. (This
is best observed by placing the fingers on the tube.)
2. The forcing of water from the pointed tubs during the interval when no
pressure is being applied from the bulb. Compare with the action of the
arteries when blood is forced into them from the ventricles.
Repeat the experiment, using a long glass tube terminating in a point
instead of the rubber tube. (In fitting the glass tube to the bulb use a
very short rubber tube.) Observe and account for the differences in the
flow of water through the inelastic tube.
*To show the Advantage of Valves in the Veins.*--Attach an open glass tube
one foot in length to each end of the rubber tube used in the preceding
experiment and fill with water (by sucking) to within about six inches of
the end. Lay on the table with the glass tubes secured in an upright
position (Fig. 26). Now compress the tube with the hand, noting that the
water rises in both tubes, being pushed in both directions. This effect is
similar to that produced on the blood when a vein having no valves is
compressed.
[Fig. 26]
Fig. 26.--*Simple apparatus* for showing advantage of valves in veins.
Now imitate the action of a valve by clamping the tube at one point, or by
closing it by pressure from the finger, and then compressing with the hand
some portion of the tube on the table. Observe in this instance that the
water is *all* pushed in the same direction. The movement of the water is
now like the effect produced on the blood in veins having valves when the
veins are compressed.
*To show the Position of the Valves in the Veins.*--Exercise the arm and
hand for a moment to increase the blood supply. Expose the forearm and
examine the veins on its surface. With a finger, stroke one of the veins
toward the heart, noting that, as the blood is pushed along on one side of
the finger the blood follows on the other side. Now stroke the vein toward
the hand. Places are found beyond which the blood does not follow the
finger. These mark the positions of valves.
*To show Effect of Exercise upon the Circulation.*--1. With a finger on the
"pulse" at the wrist or temple, count the number of heart beats during a
period of one minute under the following conditions: (_a_) when sitting;
(_b_) when standing; (_c_) after active exercise, as running. What
relation, if any, do these observations indicate between the general
activity of the body and the work of the heart?
2. Compare the size of the veins on the backs of the hands when they are
placed side by side on a table. Then exercise briskly the right hand and
arm, clenching and unclenching the fist and flexing the arm at the elbow.
Place the hands again side by side and, after waiting a minute, observe
the increase in the size of the veins in the hand exercised. How is this
accounted for?
*To Show the Effect of Gravity on the Circulation.*--Hold one hand high
above the head, at the same time letting the other hand hang loosely by
the side. Observe the difference in the color of the hands and the degree
to which the large veins are filled. Repeat the experiment, reversing the
position of the hands. What results are observed? In what parts of the
body does gravity aid in the return of the blood to the heart? In what
parts does it hinder? Where fainting is caused by lack of blood in the
brain (the usual cause), is it better to let the patient lie down flat or
to force him into a sitting posture?
*To study the Circulation in a Frog's Foot* (Optional).--A compound
microscope is needed in this study and for extended examination it is best
to destroy the frog's brain. This is done by inserting some blunt-pointed
instrument into the skull cavity from the neck and moving it about. A
small frog, on account of the thinness of its webs, gives the best
results. It should be attached to a thin board which has an opening in one
end over which the web of the foot may be stretched. Threads should extend
from two of the toes to pins driven into the board to secure the necessary
tension of the web, and the foot and lower leg should be kept moist. Using
a two-thirds-inch objective, observe the branching of the small arteries
into the capillaries and the union of the capillaries to form the small
veins. The appearance is truly wonderful, but allowance must be made for
the fact that the _motion_ of the blood is magnified, as well as the
different structures, and that it appears to move much faster than it
really does. With a still higher power, the movements of the corpuscles
through the capillaries may be studied.
NOTE.--To perform this experiment without destroying the brain, the frog is
first carefully wrapped with strips of wet cloth and securely tied to the
board. The wrapping, while preventing movements of the frog, must not
interfere with the circulation.