artery, called the aorta, which, in its passage, sends off branches
to the heart, arms, legs, head, and every other part of the body.
These branches, in the course of their progress, are divided and
subdivided into innumerable minute ramifications, the last of which
are invisible. These small ramifications unite again into branches
continually larger and larger, till they form two great tubes, called
the venae cavae; which large veins are inserted into the right
auricle of the heart; where a vein, termed the coronary vein of the
heart, which returns the blood from the heart itself, also
terminates.
From what has been said, it will be evident, that strictly speaking,
there are only two arteries and seven veins in the body; one
pulmonary artery, which carries the blood from the right ventricle of
the heart to the lungs, and four pulmonary veins, which bring it back
again; then the aorta or large artery, which carries the blood from
the left ventricle of the heart to all parts of the body; the two
venae cavae, and the coronary vein of the heart, which bring it back
again.
At the beginning of both arteries, where they leave the heart, are
placed valves, which allow the blood to flow freely from the heart
into the arteries, but which prevent its return to the heart. There
are likewise valves between the auricles and ventricles, which permit
the blood to flow from the former into the latter, but prevent its
return into the auricles. The veins are likewise furnished with
valves, which allow the blood to flow from their minute branches
along the larger toward the heart, but prevent its returning to these
minute branches.
The blood being brought back from all parts of the body into the
right auricle of the heart, distends this cavity, and thus causes it
to contract; this auricle, by contracting, forces the blood into the
right ventricle; this muscular cavity being distended and irritated
by the blood, contracts, and propels the blood through the pulmonary
artery into the lungs: from hence it is brought back by the pulmonary
veins, to the left auricle of the heart, by whose contraction it is
forced into the left ventricle. The contraction of this ventricle
propels the blood, with great force, into the aorta, through the
innumerable ramifications of which, it is carried to every part of
the body, and brought back by veins, which accompany these arterial
ramifications, and form the venae cavae, which conduct the blood into
the right auricle of the heart, from whence it is again sent into the
right ventricle, which sends it through the pulmonary artery, to the
lungs; the pulmonary veins bring it back again to the heart, from
whence it is propelled through the aorta, to all parts of the body:
thus running a perpetual round, called the circulation of the blood.
Thus then we see, that the circulation consists of two circles or
stages, one through the lungs, which may be called the pulmonary, or
lesser circle, and the other through all parts of the body, which may
be termed the aortal, or greater circle.
That the blood circulates in this manner, is evident, from the valves
placed at the origin of the arteries, and in the large branches of
the veins, which prevent the return of the blood to the heart, in any
other manner than that I have described. This is likewise evident, in
the common operation of blood letting: when the arm is tied, the vein
swells below the ligature, instead of above, and we do not make the
opening above the ligature, or on the side next the heart. If the
vein were opened above the ligature, it would not bleed. For it only
swells next the hand, which shows that the blood does not flow into
the vein downwards from the heart, but upwards from the hand.
If the ligature be too tight, the blood will not flow through the
opening in the vein. The reason of this, is, that the artery is
compressed, in this case, as well as the vein; and as the veins
derive their blood from the arteries, it follows that if the blood's
motion be obstructed in the latter, none can flow from them into the
former: when we wish to open an artery, the orifice must be made
above the ligature.
Another proof of the circulation being performed in this manner, is
derived from microscopic observations, on the transparent parts of
animals, in which the blood can be seen to move towards the
extremities, along the arteries, and return by the veins.
The blood, however, does not flow out of the heart into the arteries
in a continued stream, but by jets, or pulses; when the ventricles
are filled with blood from the auricles, this blood stimulates them,
and thereby causes them to contract; by such contraction, they force
the blood, which they contain, into the arteries; this contraction is
called the systole of the heart. As soon as they have finished their
contraction, they relax, till they are again filled with blood from
the auricles, and this state of relaxation of the heart, is called
the diastole.
This causes the pulsation or beating of the heart. The arteries must,
of course, have a similar pulsation, the blood being driven into them
only by starts; and accordingly we find it in the artery of the
wrist; this beating we call the pulse; the like may also be observed
in the arteries of the temples, and other parts of the body. The
veins, however, have no pulsation, for the blood flowing on, in an
uninterrupted course, from smaller tubes to wider, its pulse becomes
entirely destroyed.
The different cavities of the heart do not contract at the same time;
but the two auricles contract together, the ventricles being at that
time in a state of relaxation; these ventricles then contract
together, while the auricles become relaxed.
Both the arteries and veins may be compared to a tree, whose trunk is
divided into large branches; these are subdivided into smaller, the
smaller again into others still smaller; and we may observe,
likewise, that the sum of the capacities of the branches, which arise
from any trunk, is always greater than the capacity of the trunk.
The minutest branches of the arteries, being reflected, become veins,
or else they enter veins that are already formed, by anastomosis, as
it is called; the small veins continually receiving others, become,
like a river, gradually larger, till they form the venae cavae, which
conduct the blood to the heart.
Anatomical injections prove, that the last branches of the arteries
terminate in the beginning of veins; but it is the opinion of many
celebrated physiologists, that the arteries carry the blood to the
different parts of the body to nourish them, and that the veins
commence by open mouths, which absorb or suck up what is superfluous,
and return it back to the heart.
From what has been said, it must be evident that there is a
considerable resemblance between the circulation of the blood in the
animal body, and the circulation of the aqueous fluid on the surface
of the globe. In the latter case the water is raised from the ocean,
by the heat of the sun, and poured down upon the dry land, in minute
drops, for the nourishment and economy of its different parts. What
is superfluous is collected into little rills; these meeting with
others, form brooks; the union of which produce rivers, that conduct
the water to its original source, from which it is again circulated.
In the same manner, the blood is sent by the heart to different parts
of the body, for the nourishment and economy of its different parts;
what is superfluous is brought back by veins, which, continually
uniting, form those large trunks, which convey the vital fluid to the
heart.
The blood does not circulate, however, in the manner which I have
mentioned, in all parts of the body; for that which is carried by
arteries to the viscera, serving for digestion, such as the stomach,
bowels, mesentery, omentum, and spleen, is collected by small veins
which unite into a large trunk called the vena portarum; this vein
enters the liver, and is subdivided in it like an artery,
distributing through the liver a great quantity of blood, from which
the bile is secreted: and, having served this purpose, the blood is
collected by small veins; these unite and form the hepatic vein,
which pours the blood into the vena cava, to be conducted to the
heart.
The reason of this deviation, is probably, to diminish the velocity
of the blood in the liver, for the secretion of the bile; which could
not have been effected by means of an artery.
The force which impels the blood, is, first, the contraction of the
heart, which propels the blood into the arteries with great velocity;
but this is not the only force concerned in keeping up the
circulation; this is evident, from the diminished heat, and weakened
pulse, in a paralytic limb, which ought not to take place, if the
blood were propelled merely by the action of the heart.
The arteries are possessed of an elastic and muscular power, by means
of which they contract when they are distended or stimulated. It is
however by the muscular power alone, that they assist in propelling
the blood; for the elasticity of their coats can serve no other
purpose than preserving the mean diameter of the vessel. If we
suppose the arteries to be dilated by the blood, poured into them by
the heart, they will, by their contraction, as elastic tubes,
undoubtedly propel the blood: but supposing them to be perfectly
elastic, the force of the heart will be just as much diminished in
dilating them as the force of the blood is increased by their
contraction. We are not however acquainted with any substance
perfectly elastic, or which restores itself with a force equal to
that with which it was distended: hence the elastic power of the
arteries will subtract from, instead of adding to, the power of the
heart. It is evident, therefore, that it must be by the muscular
power of the arteries, which causes them to contract like the heart,
that they propel the blood.
That such is the case, appears from the muscular structure of the
arteries observed by anatomists; as also from the effects of
mechanical irritation of their coats, which causes them to contract;
this is likewise evident from the inflammation produced by the
application of stimulating substances to particular parts; for
instance, cantharides and mustard. It appears likewise, from the
secretion in some parts being preternaturally increased, while the
motion of the general mass of the blood continues unaltered.
The contraction of the arteries always propels the blood towards the
extreme parts of the body: this must necessarily happen, because the
valves at the origin of the arteries prevent its return to the heart,
it must therefore move in the direction in which it finds least
resistance.
If it were not for this muscular power of the arteries, the force of
the heart would not alone be able to propel the blood to the extreme
parts of the body, and overcome the different kinds of resistance it
has to encounter. Among the causes that lessen the velocity of the
blood, may be mentioned the increasing area of the artery; for it was
before observed, that the sum of the cavities of the branches from
any trunk exceeded the cavity of the trunk: and from the principles
of hydrostatics, the velocities of fluids, propelled by the same
force, in tubes of different diameters, are inversely as the squares
of the diameters, so that in a tube of double the diameter, the
velocity will only be one fourth; in one of the triple, only one
ninth: and since the arteries may be looked upon as conical, it is
evident that the velocity of the blood must be diminished from this
cause.
The curvilinear course of the arteries likewise gives considerable
resistance; for at every bending the blood loses part of its momentum
against the sides; and this loss is evidently proportioned to the
magnitude of the angle, at which the branch goes off. Convolutions
are frequently made, in order to diminish the force of the blood in
particular organs; this is especially the case with the carotid
artery before it enters the brain.
The angles which the ramifications of the arteries make, are greater
or more obtuse nearer the heart, and more acute as the distance
increases; by which means the velocity of the blood is rendered more
equal in different parts.
The anastomosing or union of different branches of arteries, likewise
retards the velocity of the blood, the particles of which, from
different vessels, impinging, disturb each other's motion, and
produce a compound force, in which there is always a loss of
velocity: and it is evident, from the composition of forces, that
this loss must be proportioned to the obliquity of the angle at which
the vessels unite.
The adhesion of the blood to the sides of the vessels, likewise
causes a loss of velocity in the minuter branches, which may be owing
to a chemical affinity: the viscidity or imperfect fluidity of the
blood is another retarding cause. All these causes united, would
render it impossible for the heart to propel the blood with the
velocity with which it moves in the very minute branches of the
arteries, if these arteries were not endowed with a living muscular
power like the heart, by which they contract and propel their
contents.
In the veins, the motion of the blood is occasioned partly by the vis
a tergo, and partly by the contraction of the neighbouring muscles,
which press upon the veins; and these veins being furnished with
valves, the return of the blood towards the arteries is prevented; it
must therefore move towards the heart.
That the contraction of the muscles of the body tends very much to
promote the circulation of the blood, is evident, from the increase
of the circulation from exercise, and likewise from the languid
motion of the blood in sedentary persons, and those given to
indolence. Hence we may account for the different diseases to which
such persons are subject, and know how to apply the proper remedies.
Hence likewise, we see the reason why rest is so absolutely necessary
in acute and inflammatory diseases, where the momentum of the blood
is already too great.
It has been doubted by anatomists, whether the veins were possessed
with muscular power; but this seems now to be confirmed. Haller found
the vena cava near the heart to contract on the application of
stimulants, though he could see no muscular fibres; these, however,
have been discovered by succeeding anatomists.
The magnitude of the veins is always greater than that of the
corresponding arteries; hence the velocity of the blood must be less
in the veins; and hence likewise we may account for their want of
pulsation; for the action of the heart upon the arteries is at first
very great; but as we recede from the heart, this effect becomes less
perceptible; the arterial tube increases both in size and
muscularity, in proportion to its distance from the source of
circulation. The powers of the heart are spent in overcoming the
different resistances which I have noticed, before the blood enters
the veins; hence the blood will flow uniformly in these last.
The blood is subject in the veins to retarding causes, similar to
those which operate in the arteries, but perhaps not in an equal
degree; for the flexures are less frequent in the veins than in the
arteries. As the capacity of the arterial tube increases with its
distance from the heart, the velocity, from this cause, as has
already been observed, is continually diminished; but a contrary
effect takes place in the veins; for the different branches uniting,
form trunks, whose capacities are smaller than the sums of the
capacities of the branches, hence the velocity of the blood in the
veins will increase as it approaches the heart.
Another retarding cause may be mentioned, namely, gravity, which acts
more on the venous than the arterial system. The effects of gravity
on the veins may be exemplified, by a ring being pulled off the
finger with ease when the hand is elevated; also by the swellings of
the feet that occur in relaxed habits, which swellings increase
towards night, and subside in the morning, after the body has been in
a horizontal posture for some hours.
In weak persons, the frequency of the pulse is increased by an erect
posture, which may probably depend on gravity; as we know, from the
observations of Macdonald and others, that an erect posture will make
a difference of 15 or 20 beats in a minute. The experiments alluded
to, were made by gently raising a person fastened to a board, where
there being no muscular exertion, respiration would not be increased;
so that the whole effect was probably owing to gravity accelerating
the column of arterial blood.
The inverted posture produces a still more remarkable effect in
accelerating the pulse, than the erect, for it sometimes causes it to
beat 10 or 12 times more in the former case than in the latter.
While we are on this subject, it may not be improper to take notice
of the effects of swinging on the circulation, which have been found
by Dr. Carmichael Smyth, and others, to diminish the strength and
velocity to such a degree, as to bring on fainting. These effects
have never been satisfactorily accounted for; but they would seem to
admit of an easy explanation on mechanical principles: they are
undoubtedly owing, at least in a great measure, to the centrifugal
force acquired by the blood.
By a centrifugal force, I mean, the tendency which revolving bodies
have to fly off from the centre, which arises from their tendency to
move in a straight line, agreeably to the laws of motion. Hence a
tumbler of water may be whirled in a circle vertically without
spilling it; the centrifugal force pushing the water against the
bottom of the tumbler. In the same manner when the human body is made
to revolve vertically in the arch of a circle, this centrifugal force
will propel the blood from the head and heart towards he extremities;
hence the circulation of the blood will be weakened, and the energy of
the brain diminished. The contrary, however, will take place on a
horizontal swing, as I have frequently observed, both on myself and
others; for the centrifugal force in this case will propel the blood
from the extremities towards the head.
It has been already observed, that the pulsations of the artery which
we feel at the wrist, are occasioned by its alternate dilatations and
contractions, which vary according to the strength and regularity of
the circulation, which is liable to be affected by the smallest
changes in the state of health. Hence physicians make use of the
pulse as a criterion whereby to judge of the health of the body. And we
may observe that there are few more certain characteristics of the
state of the body than the pulse; yet the conclusions that have been
drawn from it have often been erroneous; and this has arisen from
trusting to observation without the aid of reason.
That we may better understand the phenomena of the pulse, I shall lay
down the following postulata. 1st. It is now generally believed, that
every part of the arterial system is endowed with irritability, or a
power of contracting on the application of a stimulus, and that the
blood acting on this contractibility, if the term may be allowed,
causes contraction; and that the alternate relaxation and contraction
gives the phenomenon pulsation. 2d. The greater the action of the
stimulus of the blood, the greater will be the contraction, that is,
the nearer will the sides of the artery approach towards the axis.
3d. That the velocity with which a muscular fibre, in a state of
debility, contracts, is at least equal to that with which a fibre in
a state of strength contracts, is a fact generally allowed by
physiologists.
We shall afterwards see, that a deficient action of stimulus on the
vessels may arise, either directly from diminishing the quantity of
blood contained in them, or indirectly, from the application of too
great a stimulant power, which has diminished the capability of
contracting inherent in the vessels.
From these postulata, it will be evident, that the greater the action
of the arteries, that is, the more powerful their contraction, the
longer will be the intervals between the pulsations.
For the velocity being at least equal in debility and in strength,
the times between the pulsations will be proportioned to the approach
of the sides of the artery towards its axis: but the approach of the
sides towards the axis is greater when the arteries are in a state of
vigour than when debilitated; consequently the intervals between the
pulsations will be greater when the arteries are in a state of vigour
than when debilitated.
Hence it is evident, that a frequency of pulse must generally
indicate a diminished action or debility; while a moderate slowness
indicates a vigorous or just action.
Hence likewise the opinion of increased action, which has been
supposed to take place in fevers, because a frequent pulse was
observed, must be false, because the frequency arises from a directly
opposite state, and indicates a diminished action of the vascular
system.
In a sound and adult man the frequency of the pulse is about seventy
beats in a minute; and in an infant, within the first five or six
months, the pulse is seldom less than one hundred and twenty, and
diminishes in frequency as the child grows older. But though seventy
beats in the minute may be taken as a general standard; yet in
persons of irritable constitutions the frequency is greater than
this, and many, who are in the prime of life, have the pulse only
between fifty and sixty.
It is generally observed, that the pulse is slower in the morning,
that it increases in frequency till noon, after dinner it again
becomes slow, and in the evening its frequency returns, which
increases till midnight.
These phenomena may be rationally explained on the principles just
laid down. When we rise in the morning, the contractibility being
abundant, the stimulus of the blood produces a greater effect, the
pulse becomes slow, and the contractions strong; it becomes more
frequent, however, till dinner time, from a diminished
contractibility; after dinner, from the addition of the stimulus of
food and chyle, it again decreases in frequency, and becomes slow
till the evening, when its frequency returns, because the
contractibility becomes exhausted: and this frequency continues till
the vital power have been recruited by sleep.
By the same principles it is easy to explain the quickness of the
pulse in infancy, its gradual decrease till maturity, its slowness
and strength during the meridian of life, and the return of its
frequency during the decline.
Having now described the phenomena of the circulation, it will be
proper to examine the changes produced by this function on the blood;
and, in the first place, it may be observed, that the blood which
returns by the vena cava to the heart, is of a dark colour inclining
to purple; while that which passes from the left ventricle into the
arteries, is of a bright vermilion hue. The blood which is found in
the pulmonary artery has the same dark purple colour with that in the
vena cava, while that in the pulmonary vein resembles the aortal
blood in its brightness. Hence it would appear, that the blood,
during its passage through the lungs, has its colour changed from a
dark purple to a bright vermilion, in which state it is brought by
the pulmonary vein to the left auricle of the heart; this auricle,
contracting, expels the blood into the corresponding ventricle, by
whose action, and that of the arteries, it is distributed to all
parts of the body. When it returns, however, by the veins, it is
found to have lost its fine bright colour. It would appear,
therefore, that the blood obtains its red colour during its passage
through the lungs, and becomes deprived of it during its circulation
through the rest of the body.