THE HEART AND CIRCULATION.
=The Heart.=--Shielded within the chest are, as has been said, the
heart and lungs. The heart lies on the left side behind the sternum
and the cartilages of the fourth to seventh ribs in a closed, conical,
membranous sac, the _pericardium_, which is attached by its base to the
central tendon of the diaphragm, and whose point extends up between
the pleuræ of the lungs. This sac has an external fibrous layer and an
internal serous layer that is reflected back over the heart itself,
forming a closed sac, within which a thin fluid is secreted that serves
to reduce friction during the movements of the heart, the two inner
surfaces sliding over each other with every beat.
[Illustration: FIG. 39.--The heart. (Stoney.)]
[Illustration: FIG. 40.--Left auricle and ventricle, opened and part of
their walls removed to show their cavities: 1, Right pulmonary vein cut
short; 1´, cavity of left auricle; 3, 3´, thick wall of left ventricle;
4, portion of same with papillary muscle attached; 5, the other
papillary muscles; 6, 6´, the segments of the mitral valve; 7, in aorta
is placed over the semilunar valves; 8, pulmonary artery; 10, aorta and
its branches. (Allen Thomson.)]
The heart itself is a hollow conical organ composed of cardiac muscle,
a combination of smooth and striated fibers found nowhere else in the
body. It lies obliquely, base up, between the lungs, suspended by the
great blood-vessels and with the apex directed downward, forward, and
to the left, the apex beat being normally felt in the fifth intercostal
space, one inch inside and two inches below the left nipple. In size
it varies in different people and is generally smaller in women than
in men. On the average it is five inches long, three and a half inches
broad, and two inches thick. A man’s heart usually weighs about eleven
ounces and that of a woman nine ounces. It never leaks except from
disease and such leakage is fatal.
_The Cavities._--The heart contains four cavities, two _auricles_ above
and two _ventricles_ below, with a longitudinal septum between the
auricle and ventricle on the right and those on the left. The posterior
surface is largely made up of the left ventricle and the anterior of
the right ventricle. The right auricle, which receives the blood from
the general circulation, has a capacity of about two fluid ounces and
is larger than the left, which receives the blood returning from the
lungs, though its walls are thinner. Of the ventricles the left is the
larger and its walls are about three times as thick as those of the
right, for it has to send the blood all over the body. All the cavities
are lined with smooth, transparent, serous membrane, the _endocardium_,
which is continuous with the intima of the great vessels.
[Illustration: FIG. 41.--Orifices of the heart, seen from above, both
the auricles and the great vessels being removed: _PA_, Pulmonary
artery and its semilunar valves; _Ao_, aorta and its valves; _RAV_,
tricuspid, and _LAV_, bicuspid valves; _mv_, segments of mitral valve;
_lv_, segment of tricuspid valve. (Huxley.)]
_The Valves._--The opening from the auricle into the ventricle on
either side is guarded on the ventral side by a valve formed of folds
of endocardium. The valve on the right side has three flaps or cusps
and is called the _tricuspid valve_, while that on the left has two
flaps, larger and thicker than those of the tricuspid, and is known as
the _bicuspid_ or _mitral valve_. The flaps of either valve are kept
from being forced into the auricle in closing by fine tendinous cords,
the _chordæ tendineæ_, which are attached to the _columnæ carneæ_,
muscular bands or columns projecting from the walls of the ventricle,
which contract and hold the chordæ tendineæ taut. The opening into the
pulmonary artery is from the posterior part of the right ventricle
and is guarded by the _semilunar_ or _pulmonary valve_, while the
aortic opening from the left ventricle is guarded by a similar valve,
the _aortic valve_, the most important valve in the body. All these
valves are planned primarily to prevent regurgitation of the blood
during contraction of the heart muscle. Pressure in the ventricle must
exceed that in the arteries before the semilunar valves will open and
the blood can be driven out, just as the auriculo-ventricular valves
remain closed until the pressure in the auricles exceeds that in the
ventricles.
The _heart beat_ is caused by the twisting of the heart upon its axis
during contraction of the muscle. Normally it beats rhythmically and
regularly, whatever a person does, at a rate of about seventy-two
contractions to the minute in the adult. To the regular _cardiac
cycle_, as it is called, there are two periods, the _systole_ and the
_diastole_, the former representing the period of contraction of the
ventricles, when the blood is sent to the lungs and over the body,
and the latter representing the period of rest following the emptying
of the ventricles, during which they are refilled. Contraction of
the heart occupies one-fifth of the time of one beat, dilatation
two-fifths, and the pause two-fifths. There are really two systoles,
one of the auricles and one of the ventricles, but they come so close
together that they are practically simultaneous so far as sound is
concerned, though they can be distinguished by sight. During systole
the tricuspid and mitral valves close sharply to prevent regurgitation
into the auricles, while the semilunar valves open to let the blood
out. The cardiac cycle is, therefore, as follows:
[Illustration: FIG. 42.--Diagram of the circulation. (After Kirke.)]
=Circulation.=--The blood, after it has given off its oxygen and
collected carbon dioxide, returns to the heart through two main
channels, the _superior_ and _inferior venæ cavæ_, the former bringing
the blood from the upper part of the body, including the head, neck,
and arms, and the latter from the lower part below the diaphragm. The
two vessels empty along with the _coronary sinus_, which is guarded
by the _coronary valve_, into the right auricle. At the same time
that they empty into this auricle the four _pulmonary veins_, the
only veins that carry arterial or oxygenated blood, are emptying the
fresh blood from the lungs into the left auricle. When both auricles
are full, they contract and send the blood into the ventricles, the
_auricular systole_. As the blood comes through into the ventricles it
probably comes around by the walls and closes the auriculo-ventricular
valves, though just how the valves close is not certain. When the two
ventricles are full they in turn contract, the _ventricular systole_,
and the blood is forced out, that in the right ventricle passing to the
lungs for its new supply of oxygen through the _pulmonary artery_, the
only artery to carry venous blood, and that from the left ventricle
entering the aorta for general distribution through the body. Following
the systole is a pause, the _diastole_, while the heart fills again.
[Illustration: FIG. 43.--The fetal circulation.]
_Circulation in Fetus._--In the fetus there is direct communication
between the two auricles through the _foramen ovale_, which normally
closes at birth, though occasionally it remains open. There is also
communication between the pulmonary artery and the arch of the aorta
through the _ductus arteriosus_. The freshly oxidized blood comes
to the fetus through the placenta, from which it is brought along
the umbilical cord in the umbilical vein to the liver and thence to
the inferior vena cava, where it mixes with the blood from the lower
extremities. By the inferior vena cava it is carried to the right
auricle, where the _Eustachian valve_--a valve between the inferior
vena cava and the auriculo-ventricular opening, larger in the fetus
than in later life where it serves no special purpose--guides it across
the auricle and through the foramen ovale to the left auricle. From
this auricle, together with a small amount of blood from the lungs,
it goes to the left ventricle and is distributed by the aorta almost
entirely to the head and upper extremities. Hence their large size and
perfect development at birth. Returned from the upper extremities by
the superior vena cava, the blood enters the right auricle again and,
passing over the Eustachian valve this time, descends to the right
ventricle, from which the greater part passes by the pulmonary artery
and the ductus arteriosus to the descending aorta, though a small
amount keeps on through the pulmonary artery to the lungs. In the aorta
it mixes with the blood from the left ventricle and part goes to supply
the lower extremities, though the greater part is carried back to the
placenta through the two umbilical arteries. The fact that the greater
part of the blood traverses the liver accounts for its large size at
birth, while the lower extremities, which receive for the most part
blood that has already circulated through the upper extremities, are of
small size and imperfectly developed.
=Arteries.=--After birth the arterial blood for the general circulation
leaves the heart by the aorta, the main distributing artery of the
body. Through this and its branches it is carried throughout the body
in what, with the return of the venous blood by the venæ cavæ and other
smaller veins, is known as the _systemic circulation_. The _aorta_
ascends from the left ventricle and arches backward to the left over
the root of the left lung to descend along the spinal column at the
left to the fourth lumbar vertebra, about opposite the umbilicus,
where, considerably diminished in size by the branches it has given
off, it divides into the two _common iliacs_. For convenience its
different parts are named, according to their position, the _ascending
aorta_, the _arch of the aorta_, and the _descending aorta_, the last
being subdivided into the _thoracic_ and the _abdominal aorta_.
[Illustration: FIG. 44.--The aortæ and their branches. (Leidy.)]
From the ascending aorta come off the _coronary arteries_ which supply
the heart muscle itself, as the coronary sinuses carry off the venous
blood from the heart. From the arch are given off the _left common
carotid_ and _left subclavian_ and the _innominate_, which divides into
the _right common carotid_ and _right subclavian_.
The _common carotids_ pass up the neck behind the sterno-cleido-mastoid
muscles in a line from the sterno-clavicular joint to a point midway
between the mastoid process and the angle of the lower jaw and divide
opposite the upper border of the thyroid cartilage into the _internal_
and _external carotids_, of which the former with its branches supplies
the anterior part of the brain, the eye and forehead, and the latter
the neck and face.
The _subclavian_ is the artery of the upper extremity but its vertebral
branch goes to the brain, where with its fellow it forms the _basilar
artery_, whose branches together with the branches of the internal
carotid form the _circle of Willis_ at the base of the brain. Other
branches of the subclavian are the _thyroid axis_, with branches to
the neck and shoulders; the _internal mammary_, with branches to the
chest walls, mediastinum, and diaphragm, such as the musculo-phrenic
and superior epigastric; and the _superior intercostal_. At the lower
border of the first rib, over which it passes, the name _axillary_ is
substituted for _subclavian_, while at the lower border of the axilla,
where it starts down the arm, it is called the _brachial artery_. At
the elbow the brachial divides into the _radial_ and _ulnar arteries_.
The _axillary artery_ sends branches to the chest and shoulder and is
more frequently injured than any other artery except the popliteal.
_Aneurism_ may occur in it and is very likely to occur in the thoracic
aorta.
From the _thoracic aorta_ branches go to various of the chest contents,
while the _abdominal aorta_ supplies the abdominal viscera. Among the
branches of the abdominal aorta are: the _celiac axis_, which has a
gastric, an hepatic, and a splenic branch; the _superior_ and _inferior
mesenteric_ to the intestines; the _renal_; the _suprarenal_; the
_spermatic_ or _ovarian_; the _inferior phrenic_; and the _lumbar_.
The _common iliacs_ divide at the upper edge of the sacrum into the
_external_ and _internal iliacs_, of which the latter with its branches
supplies the walls and viscera of the pelvis and the inner part of the
thigh. The external iliac and its branches go to the thigh, leg, and
foot.
=Veins.=--Of the veins few need be mentioned by name. The deep veins
have the same names as the arteries they accompany, though there are
two _innominate veins_ where there is only one innominate artery,
the _subclavian_ and _internal jugular_ veins on either side joining
to form an innominate vein and the two innominates in turn forming
the _superior vena cava_. Of the superficial veins the _external_
and _internal jugular_ correspond to the common carotid arteries and
return the blood from the head and face. The external jugular vein is
important because it is the largest superficial vein in the neck and
is often cut in suicide. The _median vein_ is found at the bend of
the elbow and is used in letting blood and in giving salt solution,
while the _basilic_ is on the inner side and the _median cephalic_
on the outer side of the upper arm. _Varicosity_ often occurs in the
_internal_ or _long saphenous_ and the _external_ or _short saphenous_
in the leg. The _inferior vena cava_ is formed by the juncture of the
two _common iliac veins_.
=Portal Circulation.=--The portal system of veins includes four large
trunks which collect the blood from the viscera of digestion, the
_superior_ and _inferior mesenteric veins_ from the intestines, the
_splenic vein_ from the spleen, and the _gastric_ from the stomach.
These join together to form the _portal vein_, the only vein that
breaks up into capillaries. This divides and ramifies through the
liver, whence it emerges as the _hepatic veins_. The whole is known as
the portal circulation.
=Pulmonary Circulation.=--Of the pulmonary circulation and its vessels
a few words might also be said. The _pulmonary artery_, which carries
the blood from the right ventricle to the lungs, is only about two
inches long and divides into a right and a left pulmonary artery, which
pierce the pericardium and go to their respective lungs. The right one
is the larger and longer, for it has farther to go and gives off a
branch to supply the third lobe of the right lung. The vessels finally
divide and subdivide, terminating in the _pulmonary capillaries_.
The venous capillaries then gather together to form a main vein in
each lobule, these veins uniting into two trunks for each lung, the
_pulmonary veins_, which empty into the left auricle.
=Nerves of Heart.=--The muscular fibers of the heart have the power
of rhythmical contraction. Independent nerve centers or ganglia are
also found in the muscular walls and influence the mechanism of the
heart, especially the acceleratory mechanism. Thus, in some of the
lower animals the heart can be removed from the body, and if placed in
normal salt solution will go on beating for some time. The heart is
controlled, however, by two nerves, the _vagus_ or _pneumogastric_ and
the _sympathetic_. Of these the vagus is the inhibitory mechanism. It
acts as a check and makes the heart’s action regular and rhythmic. If
it is cut, the action of the heart becomes very rapid and irregular.
The sympathetic is the acceleratory mechanism. When the vagus alone
is stimulated, it first slows, then stops the heart, for it weakens
the systole and prolongs diastole. Acceleration follows stimulation
of the sympathetic, both the rapidity and the force of the beat
being increased. When a person faints from a blow in the abdomen, it
is because the pneumogastric is affected and inhibits the action of
the heart. The work of the heart is very dependent upon its nervous
condition and functional diseases of the heart are practically wholly
due to nervous derangement.
=Heart Sounds.=--Through the stethoscope two heart sounds may be heard.
They are known as the first and second sounds. The first is a soft,
rushing sound, stronger and louder than the other, and is caused in
part by the contraction of the muscle itself when the blood is forced
out and in part by the closure of the auriculo-ventricular valves.
The second sound is shorter and sharper, a snap, and is caused by the
closure of the semilunar valves when the contraction of the ventricles
ceases and they begin to refill. In certain diseased conditions, where
the edges of the valves are roughened, they do not snap properly and
the sound varies from the normal.
=The Heart Beat.=--The rate of the heart beat is proportionate to the
size of the person and increases in rapidity as the size diminishes.
If the ear is placed over the abdomen of a pregnant woman, the heart
of the fetus can be heard beating very rapidly. In prolonged labor it
may become more rapid or very faint and warn the doctor that something
should be done. The usual rate of the pulse in the fetus is 140 to 150
times a minute, though it varies with size and sex. At birth it drops
to 140 to 130; for the first year it is 130 to 115; for the second
year 115 to 105; for the third year 105 to 95; from the seventh to the
fourteenth years 80 to 90; from the fourteenth to the twenty-first
years 75 to 80; from twenty-one to sixty 60 to 75. In old age it rises
a little and is 75 to 80. The rate is higher in the average woman
than in the average man and increases with exercise, with increase of
temperature, and in high altitudes, where the atmospheric pressure is
less.
At each beat of the heart from four to six ounces of blood are
expelled into the pulmonary artery and the aorta, and in 22 or 23
beats all the blood in the body passes through the heart. The power
exerted by the heart every minute in thus driving the blood upon its
course has been estimated as sufficient to raise its own weight,
three-quarters of a pound, the height of the Washington monument or 150
meters; for the ventricles have to force the blood into vessels already
full.
=Factors Affecting Circulation.=--There are three main factors in the
circulation: 1. the systole, which gives the blood its first impulse;
2. the peripheral resistance in the capillaries, which serves to hold
it in check, slowing the circulation and doing away with its rhythmic
character, and 3. the elasticity of the walls of the arteries.
If a ligature is tied about an artery, there is a swelling on the side
toward the heart, while in the case of a vein, the swelling is on the
side away from the heart, that is, the swelling is in either case on
the side from which the blood comes. When an artery is cut, however,
the blood comes out rhythmically in spurts, though from a cut vein it
oozes slowly and regularly. For the blood is pumped out by the heart
rhythmically and its rhythmic beating against the walls of the artery
is felt in the _pulse_, which follows slightly after the beat of the
heart itself. The pulse is due to the fact that the vessels into which
the blood is forced are already full. This causes a local dilation at
the beginning of the artery which passes with diminishing force along
its entire length, the distention being due to the fact that more force
is needed to drive the blood through the small arteries and capillaries
than to stretch the elastic walls of the aorta and the large arteries.
It is this elastic character of the arteries that makes the blood flow
constant, for otherwise the blood would come intermittently in jets, as
it is pumped from the heart. The elastic walls of the vessels, however,
offer a certain resistance to the pumping of the fluid through them and
at the same time, by relaxing between whiles, allow a certain amount of
fluid to be retained in them, so that they continue full and the flow
is more or less constant. The insufficient outlet also helps to make
the flow constant.
By the time the blood reaches the veins its rhythmic character has
been done away with, but though there are no elastic walls in the
veins, it still has force enough after the slowing in the capillaries
to return to the heart. In this it is aided to a certain extent by
the valves and by the action of the skeletal muscles as they contract
and expand, especially in the arms and legs, where the blood runs
perpendicularly and there is a high column to be supported. There are
also more veins than arteries, each large artery having two large
veins, the venæ comites, to help get the blood back to the heart, and
the veins anastomose freely. Thus, if the blood cannot get back by one
channel it does by another. In parts like the brain, where it is very
important that there should be no compression, since any disturbance
of circulation would lead to serious results, the vessels are enclosed
in thick walls, and in the liver, through which all the blood passes
and where compression is sure to cause trouble, the veins are simply
caverns carved out in the organ and have no walls. They lie open when
the organ is opened. _Varicose veins_ are the result of valves giving
way through inherited weakness or disease so that others have an unduly
large weight to support.
=The Pulse.=--The _pulse wave_ is characterized by a quick rise and
a slow fall, though this cannot ordinarily be distinguished by the
finger. In some slow fevers, however, the fall is very long and
distinct ripples can be felt. This is known as the _dicrotic pulse_.
With age the arterial walls grow stiffer and more rigid and less
adapted to their work. In certain cases of heart disease the heart does
not transmit all the beats to the pulse and to get the true rate the
heart must be listened to.
The rate at which the pulse wave travels varies with the size of the
artery and the force of the heart beat but is about 15 to 20 feet
a second. The flow is most rapid in the arteries because they are
nearest the heart, where the pressure is greatest, and slowest in the
capillaries, where the area is greatest, the sectional area of the
capillaries, known as the _peripheral area_ because it is farthest from
the heart, being larger than that of the large arteries. Thus rapidity
of flow varies with pressure and with area.
=Blood Pressure.=--Liquids, moreover, are incompressible and exert
pressure on the walls of the tubes through which they pass. The amount
of pressure depends upon the inflow and outflow, increasing directly
with the inflow and inversely with the outflow, that is, the smaller
the outlet the greater the pressure, and _vice versa_. The pressure
is also greatest nearest to the inflow and gradually decreases with
distance until at the point of outflow there is practically no
pressure. So, in the arteries the blood pressure is greatest in the
large vessels nearer the heart and gradually decreases as they branch
into smaller and smaller vessels. In passing through the capillaries,
owing to their small size and resultant increased friction, the
blood meets with more resistance, the peripheral resistance, and
this resistance usually regulates the pressure in the arteries. The
greater the peripheral resistance, as a rule, the greater the arterial
pressure. The pressure in the capillaries is very slight and in the
veins there is practically no pressure. In fact, in the large veins
near the heart the pressure is negative and the blood is almost sucked
into the heart.
Pressure, then, is greatest in the arteries and least in the veins,
while the rate of flow is fastest in the arteries--300 to 500
millimeters a second--and slowest in the capillaries--75 millimeters
a second--being a little faster again in the veins--200 millimeters a
second.
Blood pressure is gauged by opening a vessel and inserting a
_manometer_, the pressure being determined by the height to which the
mercury is raised. In man the pressure in the arteries is 120 to 160
millimeters. It is considerably heightened during inspiration by the
increased pressure of the lungs on the heart and great vessels. In
pericarditis the opposite is true.
When the blood pressure is high, the _pulse_ is small and travels fast,
because the wall of the artery is already highly stretched. Such a
pulse is hard and incompressible. A large pulse occurs where the heart
is strong and the pressure is low, owing to peripheral dilatation. A
low-pressure pulse is soft and compressible if the heart beat is weak.
A slow pulse is generally stronger than a rapid one.
The _nerve supply of the blood-vessels_ comes from the spinal cord
through the vasomotor nerves, which are connected with the sympathetic
system and are distributed to the smooth muscle fibers of the vessels.
They are of two classes, the _vasoconstrictors_, which diminish the
lumen of the vessels, and the _vasodilators_, which increase the size
of the vessels. By these nerves the general tone of the arteries is
kept up. They are distributed chiefly to vessels in the skin and
in the abdominal organs and the constrictors are probably the more
important. When the constrictors are stimulated, three phenomena occur:
1. diminished flow through the vessel, due to its diminished size; 2.
increased general arterial pressure, and 3. increased flow through
the other arteries. When the dilators are stimulated the opposite
effect is produced: 1. the flow through the vessel is increased; 2.
there is decreased arterial pressure, and 3. there is decreased flow
through the other arteries. The palor of fright is due to the action
of the vasoconstrictor nerves of the face and blushing to the action
of the vasodilators. Heat stimulates the vasodilators so that more
blood goes to the skin, perspiration begins, and the body is cooled by
evaporation. Cold stimulates the vasoconstrictors and the blood is kept
within the body, where it cannot cool. If a part has too much blood,
an impulse passes by the vasoconstrictors to lessen the supply, while
if more blood is needed a message goes to the central nervous system
and an impulse passes by the vasodilators to flush the organ. The more
active a part is in functioning the greater the number of capillaries,
except in the brain, which has only large vessels. The vessels of the
intestines contain much blood and are capable of containing all the
blood in the body.
=The Blood.=--The blood itself, which thus circulates through the body,
carrying nutrition to the tissues and removing waste, is a complex
fluid of a bright red color. Its amount has been calculated to be
about one-thirteenth of the body weight. One-fourth of it is generally
in the heart, lungs, and large arteries and veins, one-fourth in the
liver, one-fourth in the skeletal muscles, and one-fourth variously
distributed through the other organs. If there is too little blood, the
vital processes cannot go on as they should, while too great a supply
causes weakness rather than strength. So the tendency is to keep the
amount constant and any blood added is disposed of and any blood lost
is replaced. In starvation it is the last tissue to be used up, for on
it the life of the other tissues depends.
_Composition._--In composition the blood is practically the same in
all arteries and fundamentally the same everywhere, but in passing
through certain organs certain substances are added to or taken from
it, so that its character changes more or less. Thus it varies somewhat
in composition in different parts of the body, as in the liver and
kidneys. It has five main functions: 1. the conveying of fuel from the
digestive tract to the tissues, or force production; 2. the carrying of
oxygen to the tissues; 3. the carrying of tissue-building materials,
or tissue building; 4. the distribution of heat; and 5. the removal of
waste products.
The blood is slightly alkaline in reaction, of a saltish taste, and has
a specific gravity of 1055. Its temperature is about 100° Fahrenheit or
37.8° Centigrade. It is made up of two parts, the _plasma_ or fluid
portion and the _corpuscles_ or solid portion. The plasma, again, which
is transparent and almost colorless, consists of two materials, the
_blood serum_ and _fibrin_. Fibrin does not exist as such in the body
nor in freshly shed blood, but there is a substance named _fibrinogen_
which is worked on by another substance, the _fibrin ferment_, to form
fibrin. Both fibrin ferment and fibrinogen can be isolated from the
blood.
_Coagulability._--In the body the blood is perfectly fluid and under
normal conditions does not coagulate. But, though fluid when first
shed, upon standing it gradually becomes viscid, that is, in two or
three minutes, then jelly-like, in five to ten minutes, and grows
firmer and firmer until there finally appears around this jelly-like
mass or clot a yellowish fluid, the _serum_. The _clot_ is made up of
the corpuscles and fibrin. If some blood is drawn and set on ice until
the corpuscles settle, the plasma can then be drawn off, and after it
has stood a while in a warm place coagulation will take place, a mass
of fibrin forming in the middle. It takes from one to two hours for
clotting to be complete. In very slow clotting at a low temperature the
white corpuscles appear in a layer on top of the clot, the buffy coat.
Of _fibrin_ little is known, but its formation is the most important
step in clotting, as its presence is absolutely essential. If it
is removed by whipping, the blood will not clot. It is a delicate,
stringy material, elastic and contractile, and contains certain salts
of lime and magnesium, upon whose presence its power of coagulation
depends. The coagulability of blood differs in different people and is
occasionally so little as to make operation dangerous.
The most favorable temperature for clotting is that of the body,
extreme heat preventing it and cold delaying it. That the blood does
not clot in the body must be due to some relation between the blood
and the walls of the arteries and veins that prevents it, just as the
walls of the stomach are not digested by the juices secreted. Though
coagulation does not normally take place in the body, it does take
place when a blood-vessel is injured or when the blood comes in contact
with the air, a wise provision of nature, as otherwise the tendency
would be for bleeding to go on indefinitely after injury. The greater
the surface with which the blood comes in contact the more quickly it
clots. Injury to the vessel wall itself is necessary; the endothelium
must be cracked. Under extreme injury the muscular coat of the vessel
undergoes spasmodic contraction and partially closes it. Hence a wound
caused by tearing is less likely to bleed than one due to cutting.
The valves of the heart, which are covered with endothelium, are
frequently the seat of fibrin coagulation, bits of the fibrin thus
formed giving rise to conditions in various kinds of heart trouble. Or
the bits of fibrin float in the blood and perhaps lodge in the small
vessels of the brain and cause apoplexy. Pus in various parts of the
body will set up coagulation in nearby arteries. In fact, the presence
of any foreign substance in the blood causes clotting.
[Illustration: FIG. 45.--Cells of blood: _a_, Colored blood-corpuscles
seen on the flat; _b_, on edge; _c_, in rouleau; _d_, blood platelets.
(Leroy.)]
=Blood-corpuscles.=--The solid parts of the blood are the red
corpuscles, the white corpuscles, and the blood plaques or plates.
It is to the _red corpuscles_, or _erythrocytes_ which number about
5,000,000 to the cubic millimeter of blood, that the color of the
blood is due. Under the microscope they appear as small, spherical,
biconcave discs with a slightly greenish-yellow color, which have
a tendency to form in rouleaux. They are homogeneous, with no
limiting membrane, and are made up of a fine network of tissue, the
_stroma_, in which is embedded the hemoglobin or coloring matter.
This hemoglobin is a crystalline body and the most complex substance
known to chemists. The corpuscles are very flexible and can squeeze
through small apertures, as in the tiny capillaries, and regain their
shape. They are probably formed chiefly in the red bone marrow at the
ends of the bones, which under the microscope shows red corpuscles in
various stages of growth, and also in the spleen, for which no other
use is known. Their function is to carry oxygen, which forms a chemical
combination, though an extremely loose one, with the hemoglobin. As the
tissues are more greedy of oxygen than is the hemoglobin, they rob the
corpuscles of it.
[Illustration: FIG. 46.--Various forms of leucocytes: _a_, Small
lymphocyte; _b_, large lymphocyte; _c_, polymorphonuclear neutrophile;
_d_, eosinophile. (Leroy.)]
The _white corpuscles_ or _leucocytes_ are much fewer in number,
about one to from 300 to 700 of the red, the average number being
5,000 to 10,000 to the cubic millimeter. They are larger than the red
corpuscles, colorless, and spherical when at rest. Their structure is
more definite, there being a definite cell substance or protoplasm
and one or more nuclei, which vary more or less in shape and size.
The corpuscles are classed in accordance with these variations in the
nuclei. They are most numerous during digestion and are probably formed
in the lymphatic system, constantly passing from the lymphatics to the
arteries and veins. For they have the function of amœboid movement by
which they not only wander from place to place in the blood, keeping
close to the sides of the vessels, but pass through the walls of the
capillaries, probably between the cells which form their lining, into
the lymph spaces. This is known as _migration of the white corpuscles_.
In inflammation they collect in the inflamed area to assist in allaying
the inflammation by absorbing and carrying off its products. For they
carry waste products and destroy poisons, acting as scavengers and
protectors of the body. When they are unsuccessful and the inflammation
gets the better of them, they become _pus corpuscles_.
Besides the corpuscles there are seen floating in the blood small
disk-like substances with no special characteristics, the _blood
plaques_ or _plates_, whose function is unknown.
In _anemia_ the red corpuscles are diminished and the white corpuscles
and blood plaques increased in number. After excessive bleeding normal
salt solution is injected, subcutaneously or by rectum, as being nearly
equivalent to blood serum in composition, and the renewal of the
solid elements is left to time. The length of time needed for their
restoration is about a week, except in the case of the hemoglobin,
which takes longer.