THE CIRCULATION
The blood circulates in a closed system of tubes, continuous from the
heart back to the heart. The walls of these vessels separate the blood
from the tissues. Nowhere, except in the spleen, does it come into
contact with any cells other than the lining cells of the vessels in
which it flows, and the exception made by the spleen is more apparent
than real. The spleen (p. 79) is a kind of sponge invested with a firm
capsule. Small arteries discharge their blood into its spaces; small
veins collect it. But the organ is essentially a part of the vascular
system. Its spaces take the place of the capillary vessels which
connect arteries with veins in other situations.
[Illustration: FIG. 10.—THE HEART CUT IN THE PLANE OF ITS LONG AXIS,
AND THE VESSELS WHICH OPEN INTO AND OUT OF IT.
Chordæ tendineæ attach the margins of the auriculo-ventricular valves
to musculi papillares which project from the inner aspect of each
ventricle.]
The blood makes a double circuit. From the right heart it passes
through the vessels of the lungs. Returning to the left heart, it is
driven through the body. Although the heart consists of two separate
pumps, it makes but a single organ. Its division into right auricle and
ventricle and left auricle and ventricle is but slightly indicated on
the surface. In most invertebrate animals the two pumps are distinct.
In some the lung-heart and the body-heart are on opposite aspects of
the body. But one must not, when thinking of the morphology of the
vertebrate heart, picture it as formed by the juxtaposition of two,
originally separate, pumps. Truly, in its very earliest stage of
growth, it is represented by two tubes which lie, in the embryo, far
apart. But these, before we can speak of the existence of a heart, fuse
into a single tube, with four contractile bulbs in series. As the heart
develops, the dilatation at its hinder or venous end and the dilatation
at its anterior or arterial end disappear. A partition is formed which
divides the two middle bulbs into right and left auricle and right and
left ventricle respectively. Immediately after birth the lungs are,
for the first time, distended with air. Up to that particular minute
they have had no functional use. Nothing would be gained by compelling
all the blood of the body to traverse the vessels of the embryo’s
lungs. Until birth, therefore, the inter-auricular septum is perforate.
The blood takes a short-cut, through the foramen ovale, from right
auricle to left. But by birth-time a curtain has grown down on the
left side of the foramen. When the lungs are expanded by the forcible
enlargement of the chest-cavity which contains them, their bloodvessels
are distended by the same extensile force. Blood is sucked into them
from the right side of the heart. A difference in pressure on the two
sides is established. A condition is set up which is favourable to what
may almost be termed the adherence of the flap which hangs down on the
left side of the foramen ovale. The growth of its margin very rapidly
obliterates the hole. Occasionally the closure of the foramen is not
complete. A child grows up with a perforate inter-auricular septum. If
the aperture be very small it causes little inconvenience. Shortness
of breath and blueness of lips indicate its existence if it be large
enough to lead to deficient aeration of the blood.
The two sides of the heart being quite separate, it is clear that all
blood ejected by the right ventricle into the lungs must return to the
left auricle, to be driven by it round the body. Yet it does not follow
that the heart must at each stroke drive exactly the same quantity of
blood into the pulmonary artery and into the aorta. On the average,
each of the two sides ejects the same amount—about 3 ounces. Nor does
it follow that as much blood is lodged in the lungs as in the whole of
the rest of the body. The amount varies, but on the average the lungs
contain not more than one-fifteenth of the whole blood. The heart may
be likened to two cogwheels; the blood-stream to a chain, folded into
a figure eight, against which the cogwheels work. Synchronously each
cogwheel lifts a link, the right one of the smaller, the left one of
the larger loop. Any given link returns to its starting-place in half
a minute. Such an illustration gives an idea of the arrangement of
the circulation as a whole, although the motion of a fluid is widely
different from the motion of a chain.
If, the jugular vein of the neck being cut, a colouring matter—such,
for example, as ferrocyanide of sodium or methylene-blue—is injected
into its central end towards the heart, it begins to appear in the
blood which issues from its distal end in half a minute. In this
short space of time it has passed through the right heart, through
the lungs, through the left heart, and through the vessels, arteries,
capillaries, and veins, of the head. Half a minute is therefore the
“circulation-time.” Not that all the blood-corpuscles of the body
make the circuit as rapidly as this. The time taken depends upon the
particular route they follow in the greater or systemic circulation.
Some traverse the vessels which supply the walls of the heart itself—a
short journey; others go down to the foot and up again. But the
average circulation time does not exceed a minute or a minute and a
half. It is particularly in the veins of the liver and other abdominal
viscera that blood tends to linger. Usually half the blood of the
body, or even more, is lodged in these capacious reservoirs. It is
thanks to their capacity for storing blood that a supply is provided
adequate to meet any special demand. If a man runs a few hundred
yards, two-thirds of the whole blood of the body is transferred to his
limbs. It is quickly withdrawn from the abdominal vessels when it is
wanted elsewhere; but, failing an efficient cause for removing it, its
accumulation induces lethargy. Even tight-lacing has been defended by
an eminent physiologist on the ground that it prevents accumulation
of blood in the abdomen. But tight-lacing diminishes the capacity of
the chest, hampers the action of the heart, checks the circulation,
distorts the abdominal viscera, and generally deforms and jams the
domestic machinery, even though the professor be right in his view that
visceral compression may favour alertness of mind. More by token, it
interferes with this admirable adjustment by which blood is distributed
to the various parts of the body in proportion to their needs. The
brain is the only organ which has any difficulty in securing all it
wants, and its claim to so much blood might be disputed. Nature has
not provided for long-continued passivity of the body associated with
strained activity of mind. When the stimulus to mental activity is not
unreasonable, most “nervous” people are apt to discover that their
brains are better supplied with blood than is good for their health.
The effect upon the distribution of blood throughout the body of
squeezing the viscera is experienced after taking a deep breath and
contracting the muscles of the abdomen. The contents of the abdomen are
compressed between the depressed diaphragm and its muscular wall.
Certain other forces co-operate with the beat of the heart in causing
blood to circulate. Two such factors are especially deserving of
attention. In the first place, the movement of blood in veins is
largely dependent upon external pressure. The veins are valved at
frequent intervals, the folds in their interior being of course
directed towards the heart. Any external force which empties a section
of a vein drives blood forward. A “good stretch” brings the lateral
pressure of contracting muscles to bear upon the walls of the veins
which lie between them, or beneath. More blood is delivered to the
heart. The exercise of standing erect in the attitude of attention,
and then slowly raising the arms until the thumbs meet above the head,
and slowly lowering them again, has a remarkable effect in quickening
the circulation—increasing the blood-supply of the brain. Changes
of posture, by relieving pressure on subcutaneous veins, removes an
impediment to the flow of blood.
The second of the factors to which we have referred as adjuvant
to the heart’s action is the negative pressure of inspiration. In
explaining the effect of this force upon the circulation, the relation
of the lungs to the thorax must be taken into account. The box in
which the lungs are enclosed is too big for them; nevertheless,
being extensible and elastic, they always fill it. They follow its
movements when in inspiration the muscles between the ribs enlarge
it, and when in expiration it diminishes again. No air or fluid, save
the moisture which lubricates the surface of the pleura, reducing
friction, occupies the (potential) space between the lungs and the
chest. But the moment the chest is punctured the lungs collapse.
Air is sucked into the pleural cavity. The lungs fill the chest
only so long as there is neither air nor fluid between it and them.
Lung-tissue is extremely delicate. Each air-cell is a cup of thin
membrane holding together a basket-work of capillary vessels. So long
as the chest-wall is stationary the negative pressure in the pleural
cavity has no effect upon these slender tubes. But when the chest
expands, the capillaries are between two minus pressures, the pull of
the chest-wall and the resistance offered to the entrance of air into
the lungs by the passages through which it has to pass. The calibre of
the lung-capillaries is increased, just as it would be increased were
they hanging in an air-pump while the piston was drawn out. More blood
passes to the left heart through the wider capillaries. Ejected into
the aorta, it raises the pressure in the arterial system. A record of
the pressure in an artery shows a rhythmic rise for each heart-beat. It
shows also a rise with inspiration and a fall with expiration. These
larger undulations correspond with the movements of the chest, although
they are necessarily somewhat late on respiration, for the first effect
of the dilatation of the capillaries is to cause them to hold more
blood and to deliver less. The first effect of expiration, on the other
hand, is to urge on the blood which the dilated vessels contain. In
any case a single beat is needed to throw into the aorta the blood
which has been received by the right auricle.
The expansion of the chest influences the flow of blood in yet another
way. The heart and the great vessels which join and leave it are
themselves enclosed within the chest, subject to the negative pressure
produced within that cavity by the elasticity of the lungs. The lungs
pull upon the pericardium, the membranous covering of the heart. When
this pull is increased owing to the forcible expansion of the chest,
blood is sucked into the great veins, just as air is sucked into the
windpipe. The thick-walled aorta, containing blood at high pressure,
does not feel the effect of slight variations in the pressure round
it. The soft-walled veins are expanded during inspiration to a not
inconsiderable degree. What relief a deep yawn gives by hastening
a languid circulation! Leaning over an account-book late in the
afternoon, every condition is unfavourable to the flow of blood. It
accumulates in the legs and in the abdomen. The head is thrown back
and the mouth opened wide, while the chest expands in a long deep
inspiration. Down on the liver, stomach, and intestines presses the
flattened diaphragm, squeezing their blood towards the heart. The
negative pressure within the chest sucks this up, and draws down the
blood contained in the great veins of the neck. The capillaries of the
lungs are widened, allowing blood to pass more quickly from the right
side to the left side of the heart. The heart responds to the call upon
it, throwing all that it receives into the aorta. Only a great effort
of the will had kept the pale brain at work; in the attic it suffers
more than organs on the lower storeys from insufficient pressure. For
a short time after the yawn it finds itself nourished with an adequate
supply of blood.
The negative pressure in the thorax is considerable at all times. If
a manometer—a =U=-shaped tube with mercury in its loop—be connected
with a cannula passed through the wall of the chest, the difference
of level of the mercury in the two limbs of the =U= is a measure of
the force with which the lungs are endeavouring to shrink away from
the chest-wall. Even at the end of expiration the mercury in the limb
next the chest stands about 6 millimetres higher than the mercury in
the outer limb. During a deep inspiration the pressure in the chest
falls 30 millimetres below the atmospheric pressure. Hence a problem
is presented of which no completely satisfactory solution has yet been
given. How comes it that lymph is not sucked into the pleural cavity?
In health there is no more pleural fluid than just suffices to keep
the membrane moist. The endothelial cells which cover the surface of
the pleura resist further exudation. Valves in the lymphatic vessels
prevent backward flow. Yet in disease, when the pleura is inflamed,
lymph pours out quickly, often to be reabsorbed with equal rapidity
when the pleurisy subsides. This flow uphill, from a lower to a higher
pressure, can be explained only as a phenomenon due to the “secretory”
capacity of endothelium. As an answer to the hydrostatic problem this
is hardly satisfactory.
The circulation of the blood is the result of the difference between
the pressure in the vessels through which it leaves the heart, and that
in the vessels through which it is returned. The pressure in the aorta
amounts to about 200 millimetres of mercury. In the venæ cavæ it is
nil, or, owing to the aspiration of the thorax, less than nil.
=The Heart.=—Inspection of the liver, the spleen, or the kidney helps
but little to the comprehension of the mechanism of these organs.
It is quite otherwise in the case of the heart. Its mechanics being
comparatively simple, physiology is concerned with measurements, with
the conditions under which it can and cannot work, and with the action
upon it of the nervous system and of drugs. The heart of any mammal
will suffice for anatomical study. A sheep’s heart is about the same
size as that of a man, and exactly similar, save in minute particulars,
which do not appreciably affect its mode of working.
The heart is a hollow muscle, composed of minute contractile cells.
Each cell is a cylinder, about twice as long as it is broad, with an
oval nucleus in its centre. There is no impropriety in speaking of the
heart as a single muscle. Muscles which we can move at will, “voluntary
muscles,” consist of fibres, each from 1 inch to 2 inches long, and
of about the thickness of a piece of thread (Fig. 16). Every fibre is
surrounded by a membranous sheath, its sarcolemma, which completely
isolates it from the others. Each has its separate nerve-supply. A
voluntary muscle-fibre is a cell-complex. The single embryonic cell
which grew into the fibre underwent nuclear division until hundreds
of nuclei were formed, but its cell-substance was not divided into
territories appertaining to the several nuclei. In heart-muscle, on
the other hand, nuclear division has been followed by cell division;
but minute protoplasmic bridges are left between the cells. The whole
of the heart-substance is thus in structural continuity. The cells are
not invested with sarcolemma. As the result of this arrangement, an
impulse started in one part of the heart spreads over the whole, with
certain limitations as to the directions in which it is able to travel,
whereas in voluntary muscle a separate impulse must be delivered to
each fibre. The wave of contraction commences in the great veins, the
venæ cavæ and pulmonary veins, near their junction with the heart,
spreads from cell to cell throughout the auricles, and onwards down
the ventricles to the apex of the heart. The substance of the heart
has not, however, a homogeneous appearance. Its cells are collected
into fascicles, which lie in various planes and cross the axis of
the heart at various angles. In a boiled sheep’s heart it is easy to
separate one fascicle from another, and to distinguish the sheets into
which the fascicles are collected. The four valves of the heart lie in
almost the same plane. They are supported by a fibrous plate divided
into four rings (Fig. 11). Most of the fascicles are attached to this
plate, though some which encircle the auricles are independent of it.
With one or with both ends attached to the plate, fascicles loop over
the auricles. They run down the ventricles with a twist from right to
left. Those on the surface turn in at the apex of the heart, and run
up the inner surface of the ventricles. Some of them go to form the
free columns which are found on the inner surface of the ventricles,
pointing towards the valves—musculi papillares. The fibrous plate
which supports the valves cuts off almost all of the muscle which makes
the walls of the auricles from that which constitutes the ventricular
walls; but a thin sheet is continued from the inner surface of the
auricles down the interventricular septum. To a considerable extent the
walls of the two auricles and of the two ventricles are respectively
continuous, insuring synchronous contraction.
The arrangement of the fascicles accounts for the changes in form
which the heart undergoes when it contracts. Systole commences in the
cardiac ends of the venæ cavæ and pulmonary veins. They empty the last
of their blood into the auricles, and close to prevent regurgitation,
their mouths not being valved. Then the auricles quickly shrink in
all dimensions, and as soon as their contraction is at its height
the ventricles contract, while the auricles relax. The ventricular
wave runs from base to apex too rapidly to be followed with the eye,
and ends, owing to the involution of the fascicles, in the musculi
papillares. As soon as ventricular systole has commenced, the auricles
relax. After emptying their contents into the aorta and pulmonary
artery, the ventricles relax, their contraction giving way first at
the apex, and being longest held at the base. Then follows a pause
(diastole), during which both auricles and ventricles are flaccid. If
the pericardium is open, the heart is seen to become round instead of
oval in transverse outline during systole. It shortens. Its apex twists
a little to the right, and projects forward. But if it is within its
pericardium the shortening is not accompanied with any displacement of
the apex. Instead of the apex mounting, the base descends. The front
of the right ventricle, at some little distance from the apex, presses
the chest-wall forwards in the fifth intercostal space, about an inch
to the inner side of a line falling vertically through the nipple. This
pressing forwards is felt as the “impulse of the heart.”
[Illustration: FIG. 11.—A SECTION APPROXIMATELY AT RIGHT ANGLES TO THE
LONG AXIS OF THE HEART, EXPOSING THE FOUR VALVES WHICH LIE VERY NEARLY
IN THE SAME PLANE.
The semilunar valve which guards the aperture of the pulmonary artery
is the nearest to the breast-bone.]
The contraction of the heart is not a see-saw of auricles and
ventricles. During diastole blood is falling from the veins through
the auricles into the ventricles. In a sense, the auricles are not
necessary parts of the double pump. They collect blood while the
ventricle is contracting, thus preventing it from heading up in the
veins. They save time. Their contraction completes the filling of the
ventricle, so that the instant the ventricular contraction begins blood
enters the aorta and pulmonary artery.
=The Valves.=—If ever expressions of admiration were appropriate in
a treatise on the animal body, such preface might be permitted to a
description of the cardiac valves. Which means no more than this:
Men make pumps. Therefore they are in a position to appreciate the
mechanism of the heart. We cannot admire what we do not understand.
If we made secreting organs or self-contracting springs, glands and
muscles would evoke our commendation. We should recognize that Nature’s
apparatus is even better adapted to its work than any that men can
make. This is the admission which is forced from us when we study the
heart.
The apertures connecting auricles and ventricles are extremely wide,
allowing the contents of the former to be emptied into the latter
almost instantaneously. If we attempted to make a pump fulfilling this
condition, we should find that it failed in several respects. In the
first place, the rush of fluid from the one chamber into the other
would press the flaps of the valves back against the wall of the second
chamber. They would cling to the wall, and would not float up quickly
into place when the second chamber was squeezed. Let us call the two
chambers A and V for brevity’s sake. When V contracted, some of the
fluid would be thrown back into A, because, the resistance in that
direction being lower than the resistance offered by the column of
fluid above the pump (the resistance in the aorta is very high), the
contents of V would rush past the margins of the A-V valve. This would
happen even though its flaps were not pressed back against the wall.
Further, at the height of contraction the membranous valve would bulge
backwards into A, making a cup towards V which V could not empty. In
the heart these difficulties have been overcome.
The tricuspid valve, which separates the right auricle from the right
ventricle, has three flaps. The mitral valve, on the left side of the
heart, has but two. The flaps are composed of tough membrane, but are
comparatively thin. The following direction for deciding at an autopsy
whether or not they were healthy at the time of death was given many
years ago by a surgeon of repute: “You ought to be able to see the
dirt under your thumbnail when you place it beneath one of the flaps.”
Surgery has improved in cleanliness as well as in other ways; indeed,
the possibility of advance has been due to the recognition of the need
for transcendental cleanliness. But this is a digression. The margins
of the flaps are crenulated. Threads—chordæ tendineæ—are attached to
them like the stay-ropes of a tent. At their other end these tendons
are attached to the musculi papillares already mentioned. The bunch
of tendons from each papillary muscle spreads, to be inserted into
the contiguous margins of two flaps. We have mentioned some of the
difficulties which have been overcome in the construction of the pump.
(1) The flaps do not flatten back against the wall of the ventricle
during systole of the auricle. It must be remembered that during
diastole of both chambers blood is flowing through the auricle into
the ventricle. The latter being partly filled before systole of the
auricle commences, the flaps are floated up. This is greatly favoured
by the form of the inner wall of the ventricle. It is not flat, but
raised in pillars—columnæ carneæ. The spaces between these pillars
cause backwash currents, which lift the flaps and help to bring them
into apposition as soon as systole of the ventricle commences. (2) No
blood which has entered the ventricle is thrown back into the auricle.
The valve “balloons” over the blood in the ventricle before the
contraction of the auricle has ceased. The thin margins of its flaps
come together with great rapidity. The tendinous cords holding their
edges on the ventricular side, they meet, not edge to edge, but folded
flap to folded flap. (3) The valve does not bulge into the auricle. On
the contrary, at the height of systole it is pulled into the ventricle
by the contracting musculi papillares. As the ring to which the valve
is attached is diminished in size, by the contraction of the base of
the heart, which continues, it will be remembered, until after the apex
has begun to relax, the edges of the flaps are folded farther and still
farther over by the pull of the musculi papillares, and the blood is
squeezed out from between the wall of the ventricle and the indrawn
valve.
The “semilunar valves,” which close the apertures into the aorta and
pulmonary artery, have each three flaps. The aortic semilunar valve,
which has the higher pressure to bear, shows its characteristic
features in a rather more marked degree than the other. Each of its
three flaps is a half-cup. At the centre of the margin of the half-cup
is a small fibrous nodule. The edge of the cup on either side of this
is very thin. Fine elastic fibres radiate from the nodule to all parts
of the flap. The wall of the aorta shows three bays, or “sinuses,”
one behind each flap. Hence, when the valve is forced by the rise of
pressure in the ventricle, the flap is not flattened back against the
wall of the aorta. There is always a certain amount of backwash in the
pocket behind it. The instant the pressure in the ventricle begins
to fall, the three flaps come together with a click, so smart as to
be plainly audible over most of the front of the chest. The click is
the “second sound” of the heart. The auriculo-ventricular valves also
make a sound when they close; but this “first sound of the heart”
has a different character. It is prolonged, soft, low-pitched. It is
customary to represent the sounds by the syllables “lūbb dŭp—lūbb
dŭp,” the pause during diastole being of about the same length as
the sounds when the heart is beating with its normal rhythm. The
duration of systole is little affected by variations in the rate of
beat. It is diastole that is shortened or prolonged. The second sound
is due entirely to the closure of the semilunar valves. It is heard
most clearly when the stethoscope is placed over the region where
the aorta comes nearest to the wall of the chest—at the second rib
cartilage on the right side of the breast-bone. The first sound is
loudest near the apex of the heart. It is generally agreed that it is
not wholly due to the closure of the auriculo-ventricular valves, but
possesses a second constituent. Some persons assert that they can with
the ear distinguish the clearer valvular sound at the commencement
from the general rumble which overtakes it. The main part of the
sound, if it have two constituents, or the whole sound, if there be
no distinguishable valvular constituent—observers differ—is just
the noise of a distant cab (_bruit du cab_) or the waves on a far-off
beach; it is the sound which the ear picks up from any irregular
mixture of tones which it cannot analyse. It is the resonance-tone of
the ear. That the membranous valves play the leading part in producing
the first sound cannot be doubted, whether by their first closure or
by their subsequent vibration. We should be inclined to attribute to
them the whole performance, were it not that the first sound, or at
any rate a sound, is heard during the beating of a bloodless heart.
If an animal be killed and the heart removed from its thorax with the
utmost despatch, it will beat for about a minute while lying in the
palm of one’s hand. When a stethoscope is applied to the ventricle,
a “first sound” is heard. This was described as a muscular sound,
owing to a misconception. It is similar to the sound which is heard
when a stethoscope rests upon a contracting biceps. Until recently
the voluntary contraction of a muscle was believed to be vibratory—a
tetanus. The sound corresponds to a rate of about thirty-six vibrations
to the second. There being reasons for thinking that muscle contracting
naturally does not vibrate as fast as this, the sound was interpreted
as the first overtone of the muscle-note. Muscle was said to vibrate
eighteen times a second. The similarity of the first sound of the
heart and the ordinary muscle-sound led physiologists to infer that
the contraction of the heart also was a tetanus. But this was a
mistake. Neither voluntary muscular action nor the contraction of
the heart is an interrupted contraction in this sense. In the case
of the musculature of the heart especially, contraction is a steady
shrinking, followed by a steady relaxation. The sound produced by the
bloodless heart is due to the various displacements which occur when it
contracts. Its interior is very irregular, with its columns, papillary
muscles, tendinous cords, valves. The displacement of these various
structures is responsible for the noise.
The sounds of the heart afford to the physician a means of ascertaining
with the utmost nicety the condition of the valves. If the sounds
are altered from the normal in the least degree, the valves are not
healthy. Alteration of the structure of a valve is in ordinary parlance
heart-disease. It is usually indicated by an addition to the normal
sound. Such addition is termed a “murmur”; in French, _un bruit de
souffle_. Either term is somewhat misleading to the tyro. We remember
a fellow-student to whom our chief had in vain expounded the nature
of a murmur. “Surely, Mr. S., you can hear the murmur in this case.”
We others could hear it as we stood around the bed. After listening
for a minute, S. replied: “I think I could hear it, sir, if the heart
wasn’t making such a thundering noise.” The thundering noise was the
murmur. It is the business of the physician to recognize that there is
a departure from the normal, to analyse its character, to determine
the time at which it is heard in relation to the cardiac cycle, and to
locate the place on the chest where it is heard most loudly. He is then
in a position to state which of the valves is affected and what is the
nature of its lesion. Is it a lesion obstructing an orifice, or is it
causing regurgitation of blood? Or is one of the valves, as is commonly
the case in heart-disease, imperfect in both respects?
A murmur, in the strictest sense, is a sound added to a heart-sound.
It is due in all cases to vibration of a fluid column (“fluid vein” is
the term in physics). When fluid passing under pressure along a tube
of a certain calibre enters a tube of smaller calibre, no vibration
occurs. When it passes from a tube of smaller calibre into a larger
tube or space, it is thrown into vibration. Under normal conditions no
vibration occurs in the heart. The auriculo-ventricular orifices are so
large that auricle and ventricle form a single cavity when the valve is
open. The ventricles drive the blood into tubes of smaller dimensions
than themselves. These are not the conditions which set up vibration
in a fluid column. But if one of the orifices is constricted, owing to
thickening or partial adhesion of its valve, the fluid column vibrates
on entering the space beyond it. The sound is propagated forwards,
beyond the constriction, not behind it, and transmitted to the wall
of the ventricle, aorta, or pulmonary artery, as the case may be.
When either of the auriculo-ventricular orifices is constricted, the
vibration of the fluid column can be felt as well as heard. The finger
placed against the chest-wall at the spot where the impulse of the
heart occurs is sensible of a thrill. The vibration may occur whilst
blood is flowing _through_ an auricle into a ventricle, before the
auricle contracts. In time, it is presystolic. The murmur produced by
regurgitation into an auricle is synchronous with systole. The murmur
due to regurgitation into a ventricle past an incompetent semilunar
valve is postsystolic.
We have said that the heart is so formed that no vibrating fluid
vein is produced when it is functioning normally. Murmurs are due to
alterations in the valves which are visible after death. This statement
needs modification. Not infrequently functional murmurs are heard,
which disappear again after a time—in a few weeks, or even days,
perhaps. The explanation of murmurs of this class is very difficult.
They are heard most frequently in anæmic persons, and appear in these
cases to be due to the heart having shrunk, owing to the blood in
circulation being deficient in quantity, until the cavities of the
ventricles have a smaller diameter than that of the great arteries into
which they expel their contents.
Such is the explanation of the physical cause of murmurs given by
Chauveau and Marey, the physiologists who have paid most attention to
this subject. But it must be remembered that the valves which, when
diseased, are the sources of the murmurs are membranous structures. It
may be that fluid veins would be produced by them if they were rigid
ledges which jutted into the blood-stream; but, being membranous, they
are capable of vibration. Certain physicists are of opinion that a
murmur is caused, not by the vibration of a fluid vein, as such, but by
the vibration of the membranous structure which impedes the passage of
the fluid. The physics of the problem is of little consequence to the
physician. The murmur is produced at the spot where a diseased valve is
situated, and is propagated forwards. It enables him to ascertain with
accuracy what is amiss with the heart.
=Bloodvessels.=—The greater circulation occurs through a closed
system of vessels which unite the left ventricle with the right
auricle. The aorta gives off lateral branches. Its branches branch.
Subdivision continues until the vessels are just wide enough to allow
blood-corpuscles to pass in single file, or but little wider. When
a bough of a tree divides, the united cross-sections of its twigs,
their soft bark being stripped off, may be a little larger than the
cross-section of the bough; but the disparity is usually small.
The united cross-sections of the smaller arteries is considerably
greater than that of the trunks which give origin to them. By the
time the capillaries are reached, their total bed—their united
cross-section—is about 640 times as great as that of the aorta. This
estimate is based upon the diminution in the rate at which blood
flows through the vessels. The velocity with which a stream flows
through a channel varies as the cross-section of the channel. In a
capillary vessel the blood flows at the rate of from 0·5 millimetre
to 1 millimetre per second. In the aorta the velocity is about 320
millimetres per second. In the re-formation of the venous system a
converse process of reduction occurs, but not with anything like the
same rapidity. The united calibre of the two venæ cavæ, in which the
reduction is complete, is about twice that of the aorta. From this it
follows that the veins hold much more blood than the arteries; and
since veins are more easily distended, the amount that they can hold
varies within wide limits. They constitute to some extent a reservoir
for blood.
The capillary vessels are the tubes of the circulatory system in which
blood comes into use. On the average they are about 0·5 millimetre
long. Through them the blood flows slowly. Through their walls alone
is there any exchange worth mentioning between the blood within the
vascular system and the lymph by which it is surrounded. Interest
therefore centres in these vessels. Their walls are formed of
endothelial tiles. In the centre of each thin transparent tile is a
boss, where its lens-shaped nucleus is situate. The outline of the tile
is sinuous. Its margin dovetails with the margins of those adjacent to
it. Oxygen and carbonic acid, nutrient substances and waste products,
pass rapidly through the endothelial cells. Leucocytes have the power
of pushing the cells aside, in order that they may make their way out
of the blood into the lymph which fills the tissue-spaces. With the
exception of the lens and cornea of the eye, cartilage, and the various
epidermal structures, all tissues are traversed by capillary vessels.
It is not difficult to calculate the number of such vessels in the
body exclusive of the liver and the lungs. The diameter of the aorta
is 28 millimetres, that of a capillary about 0·008 millimetre. The
cross-section of all the capillaries added together is 640 times that
of the aorta, as already stated.
Many schemata have been devised to illustrate the vascular system;
but all are misleading, inasmuch as they fail to give any idea of the
extent to which the subdivision of its vessels is carried. If the
water-pipes supplying a town branched until the original conduit was
represented by five to six thousand million little pipes, the friction
which the pumping-station would have to overcome would be very great.
But little force would remain in the water when it reached the smallest
pipe. Still greater is the resistance to the flow of blood, which
is slightly viscous, and contains solid corpuscles, which increase
friction. Two thousand miles of capillary tubing in the body of a man,
without reckoning the vessels of his liver and lungs!
[Illustration: FIG. 12.—A PORTION OF THE WALL OF A SMALL ARTERY CUT
TRANSVERSELY AND HIGHLY MAGNIFIED.
Its inner coat consists of a lining sheet of
epithelial scales supported by connective tissue
and a strong elastic membrane. This membrane is
perforated with holes which place the lymph-spaces
on its two sides in continuity. The middle coat
is composed of plain muscle-fibres and patches of
elastic membrane; the outer coat of elastic fibres,
mostly longitudinal, and connective tissue.]
Water is supplied to houses in rigid tubes. Arteries are elastic,
and their elasticity is self-regulating. The cause of this will be
apparent if a section of an artery is examined. It contains much
elastic tissue. It also contains plain muscle-fibres. The smaller the
artery, the greater is the amount of muscle relatively to the other
constituents of its wall. The wall of a vein contains very little
muscle, and not much elastic tissue. The muscle of all arterial walls
is in a chronic state of tone. To some extent the degree of tone is
varied automatically. Pressure within an artery acts as a stimulus
to the muscle-fibres of its wall. Any increase leads the fibres to
contract more strongly. Any diminution induces them to relax. The
arteries resist distension; they do not narrow to any great extent
when pressure falls. But more important than this automatic mechanism
for maintaining a uniform pressure in the capillaries in general are
the changes of pressure in particular localities, brought about by the
mediation of vaso-constrictor and vaso-dilator nerves. In almost all
organs and parts of the body the automatic tone of arteries is enhanced
by impulses which flow continuously down vaso-constrictor nerves. These
impulses start from, or, to speak more accurately, pass through, the
vaso-motor centre in the medulla oblongata. From every part of the body
impulses ascend to this centre, urging it to keep up the blood-pressure
by universal constriction. Yet no separate organ would be interested
in sending such a message if it were not open to it to ask at the
same time that the constriction of its own vessels might be relaxed.
Hence it may be said that every individual in the community is crying
out for universal economy, with more generous treatment of himself.
The response made by the State to the latter part of his demand is in
proportion to the vehemence with which it is presented.
If the spinal cord of an animal be cut across near the medulla
oblongata, respiration being maintained by pumping air into and out of
the lungs, the heart continues to beat with undiminished force, but
the pressure in the large arteries falls to one-third of its normal
height. Constricting impulses no longer pass down the spinal cord from
the vaso-motor centre. This experiment also illustrates the truth of
the statement that models of the vascular system—arrangements of pumps
and indiarubber tubes—are more likely to mislead than to inform. In an
artificial schema the relaxation of the constriction of the small tubes
on the proximal side of the capillary vessels would reduce friction.
Fluid would reach the capillaries in larger quantity, and pass through
them more quickly. The pressure in the tubes which represented veins
would consequently approach more nearly to that on the arterial side.
But when the spinal cord is divided the pressure falls in the veins,
as well as in the arteries. This is due to another factor, and one of
very great importance in the regulation of the circulation. The blood
from the digestive organs is collected by the “portal system” of veins.
These do not join the inferior vena cava; they go to the liver, where
they again break up into capillaries. It is not until after this second
distribution through minute vessels that the blood is re-collected by
the hepatic veins and forwarded to the heart. As in the case of the
arteries, the portal system of vessels is controlled by the nervous
system. When the spinal cord is divided they also dilate. The whole
vascular system becoming more capacious, blood-pressure falls in veins
as well as in arteries.
When the digestive organs are active, other parts of the body are kept
short of blood. It chanced to the writer, in his student days, to
spend the early summer in Paris, with a big healthy Yorkshireman as
companion. We dined together each night at one of the restaurants of
the Palais Royal _à prix fixe_. After dinner, with British regularity,
my friend called for the _Times_. Then followed a short period of
placid reading, interrupted by the remark: “How cold it is!” Half an
hour later, giving himself a shake: “Suppose we go and dine somewhere
else?” His well-ordered digestive organs had made short work of the
two-franc dinner. They had been ably supported by the vaso-motor system
of nerves which provided them with the bulk of the blood, while limbs
and skin ran short.
Vaso-constrictor nerves leave the spinal cord by the roots (called
“rami communicantes”) of sympathetic ganglia. Beyond the ganglia they
apply themselves to the large arteries whose course they follow. The
constrictor nerves for the face and neck leave the spinal cord within
the chest by the roots of the first four thoracic nerves. They do not
at once apply themselves to the great artery of the head. Until the
upper part of the neck is reached, they traverse the ganglionated
sympathetic cord, which lies behind the carotid artery and internal
jugular vein. If in a rabbit this cord be cut, the vessels of its
ear dilate, as evidenced by the rosy blush which is observed when a
light is held behind it. If the upper part of the sympathetic cord be
stimulated, the ear grows pale. The redness of the ear remains for
many days after section of the nerve; but gradually the engorgement
diminishes, and the vessels acquire the power of automatically
regulating the flow.
The classical experiment with the rabbit’s ear suffices to show the
relation of bloodvessels and nerves which holds good for all areas of
the skin. The condition of the skin is the chief factor in regulating
the temperature of the body. In a cold atmosphere its vessels are
severely constricted to limit loss of heat. When one passes into a
warm room the constriction is relaxed. The skin is flushed; heat is
thrown off by radiation. The sweat-glands secrete water, which is
evaporated by the heat of the skin. Constriction and remission of
constriction are the processes which diminish or increase loss of heat.
This mechanism is different in the case of glands and some other
structures which, when active, require an abundant supply of
blood. Such organs are provided with vaso-dilator in addition to
vaso-constrictor nerves. The most conspicuous example of this is to be
seen in the case of the submaxillary gland. The nerve to this gland
runs for some distance as an isolated thread—the chorda tympani.
Stimulation of the chorda tympani has the double effect of dilating
the arteries of the gland and of causing it to secrete. But the
administration of atropin prevents secretion. Vaso-dilation is then the
only visible effect. Stimulation may increase sixfold the outflow of
blood from the veins of the gland. It rushes through with such rapidity
that it retains its bright arterial hue. The gland also receives a
twig from the sympathetic cord in the neck, which, as already stated,
controls the vessels of the face. By stimulating the one nerve or the
other the physiologist can at will increase or diminish the amount of
blood flowing through the submaxillary gland. Stimulating any sensory
nerve causes in a reflex manner an increased outflow of constrictor
impulses from the centre in the medulla oblongata to all parts of
the body, with the exception of the part to which the sensory nerve
appertains. Its own constituency receives an increased supply of blood.
It is not difficult to appreciate the importance of this double action.
A part is injured. The restrictions placed upon its supply of blood are
suspended. Lest its increased consumption should lead to a general fall
in pressure, all other parts have their supply curtailed. The effect
is even more pronounced than this. The whole blood-pressure is raised
above its ordinary level. The flow of blood to the injured part is
therefore greater than it would be were relaxation of its arteries the
only change.
The most important of all constrictor nerves are the splanchnics which
control the supply to the stomach and intestines. When these nerves
are cut, the digestive organs become engorged to such an extent that
a pronounced fall of the general blood-pressure is the result. Their
stimulation renders the digestive organs anæmic. We have already
shown that the relaxation of vaso-constriction occurs in a reflex
manner. The reflex relaxation of the splanchnic area is a matter of
great importance, because it can be brought about by stimulation of
one of the sensory nerves of the heart. The higher the blood-pressure,
the harder the heart would work if left to itself. It is an impetuous
organ, always trying to quicken its pace and to increase the force
of its beat. Excessive zeal would get it into trouble if severe
precautions were not taken to hold it in check. True, it is encouraged
by certain “accelerator nerves”—sympathetic filaments which leave the
spinal cord by the anterior roots of the second and third thoracic
nerves; but the influence which the accelerators exert under normal
conditions is not, it would seem, very pronounced. The nerves which
restrain the heart are much more in evidence than those which urge
it on. The arrangements for diminishing the work of the heart are of
two kinds. In the first place, branches derived from the vagus act
as a continuous check. From a certain spot in the medulla oblongata,
the cardio-inhibitory centre, impulses are always descending to slow
the heart. They are of reflex origin, but a high blood-pressure in
the centre increases the facility with which they are transmitted.
Some of these stimuli originate in the heart itself, ascending and
descending the vagus nerve. The remainder come from various sources.
A severe injury to any part of the body slows the heart. Injury
to the intestines, such as occurs in peritonitis, is particularly
effective in increasing vagus inhibition. Slowing of the heart lowers
blood-pressure. When both vagi are cut, the heart begins to gallop
whatever may be the pressure against which it has to work.
A sensory nerve of the heart, termed the “depressor,” is the chief
agent in lowering blood-pressure. Its course is not the same in all
animals, but it runs more or less in conjunction with the vagus.
Usually it joins its superior laryngeal branch. Impulses which ascend
this nerve inhibit the constriction of the splanchnic vessels. They
open a floodgate which brings down the general pressure. The severe
pain and extreme distress of angina pectoris are the cry of the heart
when blood-pressure is too high—when it feels unable to work against
it. This was recognized by physiologists long before a remedy was
known. A systematic search was instituted for a drug which could be
used with safety to lower blood-pressure. The discovery that the
inhalation of amyl nitrite answers this purpose and fulfils this
condition was the result.
[Illustration: FIG. 13.—MANOMETER FOR MEASURING BLOOD-PRESSURE.
A =U=-tube contains mercury, on which floats
a rod supporting a scratching point, which makes
a “tracing” on blackened paper wrapped round a
revolving drum. Between the manometer and the
cannula which is introduced into the central end
of a cut artery is a three-way cock, which leads
to a pressure-bottle containing a half saturated
solution of sodic sulphate. This solution prevents
blood from clotting. Before it is connected with
the artery the apparatus is filled from the
pressure-bottle. The cock is then turned into
the second position, and the bottle raised until
the mercury in the manometer stands at a level
somewhat higher than that which it may be expected
to attain under the influence of blood-pressure.
The cannula being then inserted into an artery,
the cock is turned into the third position, which
places the manometer in connection with the
blood, and excludes the pressure-bottle. As the
mercury is a little higher than blood-pressure,
some of the sodic sulphate solution enters the
artery, but no blood enters the cannula. The
scratching point, rising and falling with every
variation in blood-pressure, makes a record on the
soot-blackened paper, which is subsequently removed
from the drum, and varnished.]
When we consider the hydrostatics of the circulation, it becomes
evident that changes in the force with which the heart beats,
and changes in the calibre of the bloodvessels, work together
in determining blood-pressure. Both vessels and heart contract
automatically—the former continuously, the latter rhythmically. The
heart of a frog, if it is enclosed in a moist chamber, beats for a
long time after its removal from the animal. Even when cut in pieces,
in certain ways, the separate pieces beat. A strip from the ventricle
of a tortoise’s heart, kept gently stretched by the weight of a light
lever attached to one of its ends, continued to contract rhythmically
for forty-eight hours. When the heart has come to a pause, it cannot
be started again by stimulating any nerve. It has in the most marked
degree its own views as to the rapidity and force with which it ought
to beat. But within certain limits it is under nervous control. The
accelerators hasten it, to its own detriment. They belong to the
division of katabolic nerves—a name given them to indicate that they
waste the tissues, impoverishing their condition. The vagus nerve slows
the heart. It protects it from itself. Its action is anabolic. The
condition of the heart is improved under its influence. If it has been
kept in check for a time by stimulation of the vagus, the heart beats
more strongly when this nerve ceases to act than it did before it was
induced to rest.
The arteries also are under the influence of two antagonistic sets
of nerves. Those which increase their tonic contraction are almost
universal in their distribution. It may be that those which actively
check it are equally widespread, but the evidence is not altogether
free from ambiguity. On certain organs—such as the salivary glands,
already instanced—which require great variations in the amount of
blood supplied to them, the influence of dilator nerves is very marked.
The simplest hypothesis as to the mode of action of vaso-constrictor
and vaso-dilator nerves leaves the initiative with the muscle-ibres
of the vessel-wall. The distending internal pressure of blood is the
stimulus which induces the muscle to contract. In some invertebrate
animals—the snail, for example—if blood be prevented from entering
the heart, so that there is no distending pressure, the heart stops.
In higher animals the heart has acquired a habit of contracting, which
keeps it going in the absence of its proper stimulus. The two classes
of nerves exercise opposing influences on the muscle. Vaso-constrictor
nerves increase the excitability of its fibres; vaso-dilator nerves
diminish it. Only thus can we explain their action on a common
basis. A good deal might be said as to the reasonableness of such
an explanation. Our views as to the relation of nerve-influence and
muscle-contraction are apt to go astray, owing to the fact that
generations of physiologists have observed the phenomenon of a spasm
of a muscle following on a sudden stimulus to a nerve. The two events
are evidently related. The stimulus appears to set up a new condition
in the nerve—to initiate a process which was not occurring before
the electric current was passed through it. The muscular spasm equally
appears to be an isolated event. As usual, we are misled by the analogy
of human inventions. We compare the nerve-impulse to the fall of a
hammer, the muscle-spasm to the explosion of gunpowder. We forget that
nerve and muscle are in permanent connection; that the impulse is a
sudden exaggeration of an influence which the nerve is continuously
exerting, the contraction an exaggeration of metabolic changes which
are constantly occurring in muscle. (See in this connection the
explanation of muscle-tone, p. 273.) In the case of plain muscle, nerve
stimuli do not cause contraction; they merely increase the excitability
of the muscle. It may be more difficult for us to figure to ourselves
the way in which dilator nerves diminish excitability; but the
existence of such an anabolic influence is beyond the reach of doubt.
Heart and bloodvessels are part of the same system. The heart has its
accelerator and inhibitory nerves, the bloodvessels their constrictor
and dilator nerves. For both vessel-wall and heart the stimulus to
contraction is the distending pressure of blood—although it is not
altogether necessary that this stimulus should be acting at the time.
Sympathetic and vagus nerves can to a certain extent control the
beating of a bloodless heart. The heart-tissue has acquired the habit
of beating, and the habit of listening to advice conveyed to it through
these nerves.
The self-adjustment of the blood-tubes to the pressure to which
they are exposed is exhibited in the adaptation of their degree of
contraction to the position of the body—to the weight, that is to say,
of the column of fluid which they have to support. Everyone has played
the game of “right hand or left.” When the hand is held above the head
the blood leaves it, and the hand becomes cold; but if there be need
for adjustment, and time is given for the mechanism to come into play,
it works to perfection. When we are standing erect, there is neither
too much blood in the feet nor too little in the head. But after a
fortnight in bed a convalescent finds, the first time that he stands
upright, that his legs are quickly engorged—his slippers after a few
minutes feel too tight for him—whereas the brain becomes so anæmic
that he turns giddy, or even faints.
Numberless illustrations of vaso-motor action are met with in daily
experience. It is a curious fact that the nerves which control the
calibre of the bloodvessels tend to overact their part. When an organ
demands more blood, it is supplied at the expense of the rest of the
body, and especially of the parts most nearly adjacent. This is partly
a mechanical effect. If all the houses in a terrace are supplied with
water from a common main, the bursting of a water-pipe in one of them
will reduce the supply of its neighbours more than it will reduce the
supply of houses in distant parts of the town. But vaso-motor nerves,
in their compensating adjustment, go farther than this. A thimbleful of
blood removed by a leech produces an effect upon an underlying engorged
organ altogether out of proportion to the hydrostatic requirements
of the case. “Cupping” the loins diminishes the congestion of the
kidneys. This is the explanation of the curative efficacy of various
agents which, with improvements in surgery and the introduction of
more reliable drugs, have almost disappeared from the surgeon’s
armamentarium—scarification, blisters, setons, and the like. Such
methods have been relegated to veterinary practice.
There is a marked tendency to see-saw between the skin and the mucous
membrane of the alimentary canal. During active digestion, when the
“splanchnic area” is full of blood, the skin is cold. Hot fomentations,
by dilating the vessels of the skin, diminish congestion of the
alimentary tract. An inflamed throat is relieved by a compress round
the neck. Conversely, it must be admitted that, in certain persons,
slight constriction of the vessels of the skin induces inflammation
of the mucous membrane. This is one reason for the almost universal
dread of draughts. A draught cools a limited area of the skin. Some
of us cultivate a love of draughts. They are the sensible evidence
of the entrance of fresh air. Yet we admit reluctantly that certain
fragile mortals are not altogether fanciful in supposing that a draught
may give them a catarrh or a toothache. If asked why they object to
draughts, many persons answer that they “are afraid of catching a
chill”—carrying us back to the time before the clinical thermometer
was invented; to days when the shivering fit, or “rigor,” which first
calls attention to the fact that the temperature is already two or
three degrees above the normal, was supposed to be the commencement
of the illness. The patient imagined that the “chill” caused him to
shiver, and that if he had not “caught” it he would not have been ill.
The substitution of the term “cold” for “rheum,” naming the malady
after one of its prominent symptoms, has done much to perpetuate this
superstition. “Chill” is a word we scarcely dare to mention. When
doctors could no longer attribute to witchcraft the occurrence of
disorders for which they had no other explanation, they invented the
luminous theory that inflammatory diseases—especially those of the
stomach, liver, and lungs—were produced by “a chill.” At one time all
diseases which were not evidently infectious were caused by chill. The
discovery of germs and the recognition of their maleficent activity
has stripped this cloak of ignorance off almost every case of abnormal
tissue-metabolism. It is recognized now that the germ _is_ the disease,
not the effects which the germ produces. Pneumonia is impossible in
the absence of the pneumococcus, however severe the chill to which the
patient was exposed when out in the cold and wet. Consumption is the
effect produced by the tubercle bacillus. If there are no bacilli,
there can be no consumption. Yet these two diseases illustrate the
possibility of the use of the term “chill” without impropriety. The
coccus of pneumonia may frequently be found in the mouth of a healthy
person. If everyone with whom the tubercle bacillus has at some time
come in contact were inevitably its victim, no human being would be
free from phthisis, if any still survived. There are conditions of
health, or rather of unhealth, in which the economy is less resistant
than usual to the germs. Apparently the vaso-motor disturbances of
internal organs caused by the cooling of the surface of the body, if it
occur when health is otherwise depressed, contributes to the production
of such a state.
The vaso-motor system is influenced by emotions. It is a little
difficult to express accurately the relation between emotion and
vaso-motor change. Some psychologists regard the vaso-motor change
as the emotion. “All emotions,” says a prominent exponent of this
view, “are wholly due to excitation of a particular kind of the
vaso-motor centre.” The person about to be subject to an emotion
of shame, anger, fear, disgust, recognizes a fact or circumstance,
or conjunction of circumstances, which justifies the emotion. (We
are assuming that emotions may be justified; that the intellectual
appreciation of a situation and reasoned decision regarding the
action which it demands is not sufficient.) This recognition as an
intellectual act of the higher brain is accompanied by certain forms
of enhanced activity or inhibition of activity of the vaso-motor
centre in the medulla oblongata which cause changes in the degree of
contraction of the bloodvessels of certain organs. The vascular changes
produce an alteration in the state of the organ which is reflected
in nerve-currents sent back to the brain, providing the background
of feeling which constitutes emotional tone. We are not prepared to
endorse this extreme view of the nature of an emotion. A maiden’s
blush is not an emotion of embarrassment or shame. It is its harmony.
Her mind plays the air. The sensations which originate in the flushed
skin of the face sustain it with their accompaniment. The emotional
tone keeps attention fixed on the fact or circumstance which led her
to conclude, by the exercise of her reason, that she was placed in an
awkward situation. This fixing of attention is frequently so pronounced
as to inhibit all other intellectual action. The maiden is less quick
than she would have been, had the emotion not glued her thoughts
together, in recognizing the readiest means of extricating herself
from embarrassment. All nerves found within the chest and abdomen
were in very early times termed “sympathetic.” The cord in the neck
was the “little sympathetic.” The name explains itself; but it will
be understood that it implied much more in the days when the liver,
spleen and heart were supposed to pour out emotions than it does now.
The vagus nerve was termed the “middle sympathetic.” Shame inhibits the
activity of the vaso-constrictor nerves of the face; dilation of the
vessels which they supply is accompanied with constriction of other
cutaneous nerves. Kipling must, we think, have embellished Nature
when he represents the very unimpressionable hero of Lungtungpen as
admitting “I niver blushed before or since; but I blushed all over
my carkiss thin.” Usually the carmine of the face contrasts with the
pallor and coldness of the hands. Still, we are not prepared to assert
that it is impossible, under circumstances as trying as those in which
Private Mulvaney and his companions were placed, for all the cutaneous
constrictor nerves to let go their grip at the same time. Terror
heightens the control of the vaso-motor centre over the vessels of the
skin; it increases vagus inhibition of the heart. Even disgust evoked
by a revolting sight or a foul smell may call the vagus so forcibly
into action as to bring the heart to a standstill.
=The Pulse.=—The arterial system is always distended. The pressure in
the largest arteries amounts to about 140 millimetres of mercury. The
source of pressure is the beat of the heart pushing the blood forward
against the resistance offered to its flow by the smallest vessels.
At every stroke another 3 ounces is added to the already overfull
vessels. In the aorta, therefore, the blood moves forward with jerks,
but by the time it reaches the capillaries the intermittent accessions
of force have been taken up by the elastic walls of the vessels and
returned to the stream in the form of constant pressure. In the very
smallest arteries the blood flows in a steady stream. If the corpuscles
in a capillary vessel are watched under the microscope, they show no
variations in rapidity synchronous with the beat of the heart. The
“pulse” in the larger arteries is the push given to the column of blood
by the sudden contraction of the left ventricle. Its propagation along
the arteries will be understood if it is remembered that the blood is
contained within elastic tubes. The first effect of the ejection into
the aorta of an additional quantity of blood is the distension of its
wall. The wave of distension travels down all the arteries of the body
with gradually decreasing force.
[Illustration: FIG. 14.—SPHYGMOGRAPH.
A, An ivory button which is pressed on the skin
over the radial artery by a metal spring. B,
A continuous screw which works against the
cogwheel C. By rotating B, the lever D is raised
to a position in which its point scratches the
travelling-plate E (covered with blackened paper).
F, A box containing clockwork which moves E. G, A
screw by means of which the pressure of the spring
is adjusted to the force of the pulse.]
[Illustration: FIG. 15.
A, A cardiogram, or tracing of the impulse of the
heart, recorded on a blackened plate borne on
the end of a vibrating tuning-fork; _a-b_, the
systole of the auricles; _b-e_, the ventricular
systole. From _c-e_ the heart is shrinking as blood
leaves it by the aorta and pulmonary artery. B,
C, D, E, F, Sphygmograms. B is shaded to show the
portion of the pulse-wave which corresponds to the
systole of the heart. C, High-tension pulse of
vigorous health. D, Low-tension pulse. E, Dicrotic
pulse of fever. F, Hog-backed pulse of hardened
(atheromatous) arteries.]
Much may be learned from the pulse with regard to the condition of
the vascular system, although it is impossible to balance the effects
of the several factors which go to the production of its various
modifications. The character of the pulse depends upon the vigour
with which the heart is beating, the efficiency or otherwise of the
cardiac valves, the quantity of blood in circulation, the suppleness
of the arterial walls, the degree to which they are contracted, the
resistance offered by the smaller vessels. Departures from the normal
may take the direction of unduly high tension or of unduly low tension.
In place of the sudden rise and more or less gradual fall, with the
slightest possible roughness due to secondary waves, which constitutes
a healthy pulse, the rise may be shorter, its subsidence prolonged.
This is a high-tension or hard pulse. The pressure in the arteries
is unduly high, or the walls of the vessels are not as elastic as
they should be. Considerable pressure is needed to obliterate such a
pulse—_i.e._, to prevent it from passing on beneath the finger. As
the converse of this condition, the difference between the beginning
of the pulse and its end may be very marked, the vessel suddenly
dilating and as suddenly collapsing. But little pressure is needed to
stop such a low-tension pulse from passing beneath the finger. Usually
it has a distinct secondary or dicrotic wave. Some tactile education
is needed by the finger that aspires to read the pulse. It was hoped
that the personal equation would be of less importance if mechanical
records were substituted for statements as to the impression produced
upon the observer. Various forms of sphygmograph (σφυγμός, pulse)
have been invented for this purpose. The form commonly used (Fig. 14)
consists of a metal spring which is adjusted so that a button beneath
its free end presses on the radial artery at the wrist. The force
with which it presses is regulated by a screw. At each pulsation its
free end is lifted by the distension and rounding of the artery. Its
movement is transmitted by means of a continuous screw, attached to
it vertically, to a cogged wheel, which in its turn raises a lever.
The end of the lever scratches blackened paper fastened on a plate
moved by clockwork. Records made in this way are useful for future
reference. They are not, however, so valuable as it was anticipated
that they would be. The form of the tracing depends to so large an
extent upon the amount of pressure exerted by the spring, and the
amount of pressure must be adapted to the vascular tone in every case.
Some of the most interesting tracings are obtained from old people
affected with atheroma of the arteries. This is a condition in which,
owing to old-standing inflammation of the subepithelial coat of the
vessels, the arteries have lost their suppleness. They are hard and
inelastic. Instead of showing the normal steep face of the pulse-wave
rising abruptly to its highest point, the tracing rises vertically for
a short distance, and then slopes upwards. The wave is flat-topped or
hog-backed.
All pulses are dicrotic, although the dicrotism may not be sufficiently
pronounced to be felt with the finger. The notch which divides the
primary from the secondary wave is produced by the closure—that is
to say, by the falling down of the aortic valve. The wave from the
commencement of its ascent to the dicrotic notch corresponds to the
period during which blood is passing from the heart into the aorta.
This part of the tracing represents systole of the ventricle after the
semilunar valve has been forced. It is the push given to the bottom of
the column by the additional 3 ounces of blood thrust into the aorta.
The effort of the ventricle then comes to an end. The pressure beneath
the semilunar valve is less than that above it. The valve closes. If
the blood were contained in an open tube, the wave would now end,
save for secondary oscillations, due to inertia of the fluid. But the
arterial system is practically closed owing to the fineness of the
tubes into which it ultimately divides. Its walls are elastic. They
distend, taking up the pressure and returning it again in the second
half of the wave. In fever, after the consumption of alcohol, and in
other conditions in which the finest bloodvessels are dilated, the
division between the two parts of the wave is very marked. Dicrotism
is plainly felt. We have used the expression “finest vessels” rather
than “capillaries,” because the ascription to the capillary vessels of
all peripheral resistance has led to misunderstanding. Resistance is
offered throughout the whole vascular system, with the exception of the
largest veins. It is greatest in the small arteries, capillaries, and
small veins. It is so adjusted as to fall to zero just before the blood
reaches the heart.