OF THE CIRCULATION.
Vessels connected with the heart: chambers of the
heart—Position of the heart—Pulmonic circle:
systemic circle—Structure of the heart, artery, and
vein—Consequences of the discovery of the circulation to the
discoverer—Action of the heart: sounds occasioned by its
different movements—Contraction: dilatation—Disposition
and action of the valves—Powers that move the blood—Force
of the heart—Action of the arterial tubes: the pulse:
action of the capillaries: action of the veins—Self-moving
power of the blood—Vital endowment of the capillaries:
functions—Practical applications.
252. The blood, being necessary to nourish the tissues and to
stimulate the organs, must be in motion in order to be borne to them.
An apparatus is provided partly for the purpose of originating an
impelling force to put the blood in motion, and partly for the purpose
of conveying the blood when in motion to the different parts of the
body.
253. The heart is the impelling organ; the great vessels in immediate
connexion with it are the transmitting organs (fig. CXIV. 1, 2). The
heart is divided into two sets of chambers (fig. CXIV. 3, 4, 10, 11),
one for the reception of the blood from the different parts of the body
(fig. CXIV. 3, 10); the other for the communication of the impulse
which keeps the blood in motion (fig. CXIV. 4, 11). The chamber which
receives the blood is termed an auricle (fig. CXIV. 3, 10), and is
connected with a vessel termed a vein (fig. CXIV. 1, 2, 9); that which
communicates impulse to the blood is termed a ventricle (fig. CXIV. 4,
11), and is connected with a vessel termed an artery (fig. CXIV. 7,
12). The vein carries blood to the auricle; the auricle transmits it to
the ventricle; the ventricle propels it into the artery; the artery,
carrying it out from the ventricle, ultimately sends it again into the
vein, the vein returns it to the auricle, the auricle to the ventricle,
the ventricle to the artery, and thus the blood is constantly moving in
a circle; hence the name of the process, the circulation of the blood.
[Illustration: Fig. CXIV.
View of the heart with its several chambers exposed, and the great
vessels in connection with them. 1. The superior vena cava; 2. the
inferior vena cava; 3. the chamber called the right auricle; 4. the
chamber called the right ventricle; 5. the line marking the passage
between the two chambers, and the points of attachment of one margin of
the valve; 6. the septum between the two ventricles; 7. the pulmonary
artery arising from the right ventricle, and dividing at 8, into right
and left for the corresponding lungs; 9. the four pulmonary veins
bringing the blood from the lungs into 10, the left auricle; 11. the
left ventricle; 12. the aorta arising from the left ventricle, and
passing down behind the heart to distribute blood, by its divisions and
subdivisions, to every part of the body.]
254. In nourishing the tissues and stimulating the organs, the blood
parts with its nutritive and stimulating constituents, and receives in
return some ingredients which can no longer be usefully employed in the
economy, and others which are positively injurious. An apparatus is
established for its renovation and depuration; this organ is termed the
lung (fig. LIX. 5), and to this organ the blood must in like manner be
conveyed. Thus the blood moves in a double circle, one from the heart
to the body and from the body back to the heart, termed the systemic
circle; the other from the heart to the lung and from the lung back
to the heart, termed the pulmonic circle. Hence in the human body the
heart is double, consisting of two corresponding parts precisely the
same in name, in nature, and in office; the one appropriated to the
greater, or the systemic, and the other to the lesser, or the pulmonic
circulation (fig. CXIV.).
255. There is a complete separation between these two portions of the
heart (fig. CXIV. 6), formed by a strong muscular partition which
prevents any communication between them except through the medium of
vessels.
256. The heart is situated between the two lungs (fig. LIX. 2, 5),
in the lower and fore part of the chest, nearly in the centre, but
inclining a little to the left side. Its position is oblique (fig.
LIX. 2, 5). Its basis is directed upwards, backwards, and towards the
right (fig. LIX. 2); its apex is directed downwards, forwards, and
towards the left, opposite to the interval between the cartilages
of the fifth and sixth ribs (fig. LIX. 2). It is inclosed in a bag
termed the pericardium (fig. CXV.), which consists of serous membrane.
The pericardium is considerably larger than the heart, allowing
abundant space for the action of the organ (fig. CXV.). One part of
the pericardium forms a bag around the heart (fig. CXV.); the other
part is reflected upon the heart so as to form its external covering
(fig. CXV.), and is continued for a considerable distance upon the
great vessels that go to and from the heart in such a manner that this
bag, like all the serous membranes, constitutes a shut sac. Both that
portion of the pericardium which is reflected upon the heart, and that
which forms the internal surface of the bag around it, is moistened
during life by a serous fluid, which, after death, is condensed into a
small quantity of transparent water. That portion of the pericardium
which rests on the diaphragm (fig. LXX. 1) is so firmly attached to it
that it cannot be separated without laceration, and by this attachment,
together with the great vessels at its base, the heart is firmly held
in its situation, although in the varied movements of the body it is
capable of deviating to a slight extent from the exact position here
described.
[Illustration: Fig. CXV.
View of the heart enveloped in its pericardium, the fore part of the
latter being cut open and reflected back.]
257. When the interior of the heart is laid open there are brought
into view four chambers (fig. CXIV. 3, 4, 10, 11), two for each circle.
Those belonging to the pulmonic circle are on the right (fig. CXIV. 3,
4), those to the systemic on the left side of the body (fig. CXIV. 10,
11); hence the terms right and left are applied to these respective
parts of the heart.
258. The veins which carry the blood to the right or the pulmonic
chambers are two, one of which brings it from the upper, and the other
from the lower parts of the body: the first is called the superior and
the second the inferior vena cava (fig. CXIV. 1, 2). Both pour their
blood into the first chamber, termed the right auricle (fig. CXIV.
3); from the right auricle the blood passes into the second chamber,
denominated the right ventricle (fig. CXIV. 4): from which springs
the artery which carries the blood from the heart to the lung, the
pulmonary artery (fig. CXIV. 7). This is the pulmonic circle. From
the lung the blood is returned to the heart by four veins, termed the
pulmonary veins (fig. CXIV. 9), which pour the blood into the third
chamber of the heart, the left auricle (fig. CXIV. 10). From the left
auricle it passes into the fourth chamber, the left ventricle (fig.
CXIV. 11), from which springs the artery which carries out the blood
to the system, termed the aorta (figs. CXIV. 12, and CXVII. 11). This
is the systemic circle. In the system the minute branches of the aorta
unite with the minute branches that form the venæ cavæ, which return
the blood to the right auricle of the heart, and thus the double circle
is completed.
259. The two chambers called the auricles occupy the basis of the
heart (fig. CXIV. 3, 10). The right auricle is situated at the basis
of the right ventricle (figs. CXIV. 3, and CXVI. 4). It is partly
membranous and partly muscular. At its upper and back part is the
opening of the vena cava superior (fig. CXVI. 1), which returns the
blood to the heart from the head, neck, and all the upper parts of the
body. At its lower part is the opening of the vena cava inferior (fig.
CXVI. 2), which returns the blood from all the lower parts of the body.
[Illustration: Fig. CXVI.
View of the heart with the great vessels in connection with it, on the
right side, its different chambers being laid open and its structure
shown. 1. The vena cava superior; 2. the vena cava inferior; 3. cut
edge of the right auricle turned aside to show, 4. the cavity of the
right auricle into which the two venæ cavæ pour the blood returned
from all parts of the body; 5. hook suspending the reflected portion
of the wall of the auricle; 6. the right ventricle; 7. cut edge of the
wall of the ventricle, a portion of which has been removed to show 8.
the cavity of the ventricle; 9. situation of the opening between the
auricle and ventricle, called the auricular orifice of the ventricle;
10. valve placed between the auricle and ventricle, one margin being
firmly attached to the auriculo-ventricular opening in its entire
extent, the other lying loose in the cavity of the ventricle; 11.
probe passed from the auricle into the ventricle underneath the valve,
showing the course of the blood from the former chamber to the latter;
12. the columnæ carneæ attached by one extremity to the walls of the
ventricle, the other extremity ending in tendinous threads attached to
the loose margin of the valve; 13. passage to the pulmonary artery;
14. the three semilunar valves placed at the commencement of 15. the
pulmonary artery; 16. the two great branches into which the trunk of
the pulmonary artery divides, one branch going to each lung.]
260. The auricle communicates with its corresponding ventricle by a
large opening, termed the auricular orifice of the ventricle (figs.
CXIV. 5, and CXVI. 9). All around the opening is placed a thin but
strong membrane (fig. CXVI. 10), one margin of which is firmly attached
to the wall of the ventricle (figs. CXIV. 5, and CXVI. 9), while the
other is free (fig. CXVI. 10). This membrane receives the name, and, as
will be seen immediately, performs the office of a valve.
261. The ventricle is much thicker and proportionally stronger than the
auricle (fig. CXVI. 3, 6). It is composed almost entirely of muscular
fibre. Over nearly the whole extent of its internal surface are placed
irregular masses of muscular fibres, many of which stand out from the
wall of the ventricle like columns or pillars (fig. CXVI. 12); hence
they are called fleshy columns (columnæ carneæ). Some of these fleshy
columns are adherent by one extremity to the wall of the ventricle,
while the other extremity terminates in tendinous threads which are
attached to the membrane that forms the valve (fig. CXVI. 12).
262. From the upper and right side of this chamber springs the
pulmonary artery (fig. CXVI. 15); at the entrance of which are placed
three membranes of a crescent or semilunar shape, termed the semilunar
valves (fig. CXVI. 14).
263. The structure of the left side of the heart is perfectly analogous
to that of the right. Its auricle, like that on the left side, is
placed at the base of the ventricle (figs. CXIV. 10, and CXVII. 2), and
like it also is thin, being composed chiefly of membrane. At its upper
and back part (figs. CXIV. 9, and CXVII. 1) are the openings of the
four pulmonary veins, two from the right, and two from the left lung.
264. At the passage of communication between the left auricle and
ventricle is placed a valve analogous to that on the right side (fig.
CXVII. 7).
[Illustration: Fig. CXVII.
View of the heart with the great vessels in connection with it, on the
left side, its chambers being laid open as in the preceding figure.
1. The four pulmonary veins opening into, 2. the cavity of the left
auricle; 3. the cut edge of the wall of the auricle; 4. the appendix
of the auricle; 5. the cavity of the left ventricle; 6. the cut edge
of the wall of the ventricle, the greater portion of the wall having
been removed to show the interior of the chamber; 7. valve placed
between the auricle and ventricle; 8. columnæ carneæ terminating in
tendinous threads attached to the loose margin of the valve; 9. probe
passed underneath the valve and its tendinous threads, raising them
from the wall of the ventricle similar to a refluent current of blood;
10. passage to 11. the aorta; 12. two of the semilunar valves placed
at the mouth of the aorta, the third having been cut away; 13. arch of
the aorta; 14. the three semilunar valves at the commencement of the
pulmonary artery seen in action, completely closing the mouth of the
vessel.]
265. The walls of the left ventricle are nearly as thick again as those
of the right, and its fleshy columns are much larger and stronger. From
the upper and back part of this fourth chamber (fig. CXVII. 11) springs
the great systemic artery, the aorta, around the mouth of which are
placed three semilunar valves (fig. CXVII. 12), similar to those at the
mouth of the pulmonary artery.
266. The partition which divides the two sets of chambers from each
other (fig. CXIV. 6) is wholly composed of muscular fibres, and is
called the septum of the heart.
267. The external surface of the heart is covered by a thin but
strong membrane continued over it from the pericardium. Between this
membranous covering and its fleshy substance is lodged, even when the
body is reduced to the greatest degree of thinness, a quantity of
fat. Immediately beneath this fat are the fleshy fibres that compose
the main bulk of the organ. These fibres are arranged in a peculiar
manner. The arrangement is not perceptible when the heart is examined
in its natural state, but after it has been subjected to long-continued
boiling, which, besides separating extraneous matters from the fibres,
hardens and loosens without displacing them, the manner in which they
are disposed is manifest. Just at the point where the muscular fibres
that constitute the septum of the auricles are set upon those which
form the septum of the ventricles, and parallel with the origin of the
aorta, the heart is not muscular but tendinous. The substance called
tendon, it has been shown, is often employed in the body to afford
origin or insertion to muscular fibres, performing, in fact, the
ordinary office of bone, and substituted for it in situations where
bone would be inconvenient. From the tendinous matter just indicated
most of the fibres that constitute the muscular walls of the heart
take their origin. From this point the fibres proceed in different
directions: those which go to form the wall of the auricles ascend;
those which form the wall of the ventricles pursue an oblique course
downwards, and the arrangement of the whole is such, that a general
contraction of the fibres must necessarily bring all the parts of the
heart towards this central tendinous point. The object and the result
of this arrangement will be manifest immediately.
268. The internal surface of the chambers of the heart, in its whole
extent, is lined by a fine transparent serous membrane, which renders
it smooth and moist; and, like all other organs which have important
functions to perform, it is plentifully supplied with blood-vessels and
nerves.
269. Such is the structure of the organ that moves the blood. The
artery, the tube that carries it out from the heart, is a vessel
composed of three distinct layers of membrane superimposed one upon
another, and intimately united by delicate cellular tissue. These
layers are termed tunics or coats. The external coat (fig. CXVIII. 3),
which is also called the cellular, consists of minute whitish fibres,
which are dense and tough, and closely interlaced together in every
direction. They form a membrane of great strength, the elasticity of
which, especially in the longitudinal direction, is such that, in
addition to its other names, it has received that of the elastic coat.
[Illustration: Fig. CXVIII.
Portion of an artery, showing the several coats of which it is composed
separated from each other. 1. The internal or serous coat; 2. the
middle or fibrous coat; 3. the external or cellular coat.]
270. The middle or the fibrous tunic is composed of yellowish
flattened fibres which pass in an oblique direction around the calibre
of the vessel, forming segments of circles, which, uniting, produce
complete rings (fig. CXVIII. 2). This tunic is thick, consisting of
several layers of fibres which it is easy to peel off in succession.
They form a firm, solid, elastic, but, at the same time, brittle
membrane.
271. The inner tunic, thin, colourless, nearly transparent, and
perfectly smooth, is moistened by a serous fluid, and is thence called
the serous coat (fig. CXVIII. 1). To the naked eye it presents no
appearance of fibres, yet notwithstanding its extreme delicacy, it is
so strong that, after the other coats of the artery have been entirely
removed in a living animal, it is capable of resisting the impetus of
the circulation, and of preventing the dilatation of the artery. The
arteries themselves are supplied with arteries, vessels that nourish
their tissues, and which are sent to them from neighbouring branches,
seldom or never from the vessel itself to which they are distributed.
Each individual part of an artery is supplied by its own appropriate
vessels, which form but few communications above and below, so that
if care be not taken in surgical operations to disturb these nutrient
arteries very little, the vessel will perish for want of sustenance.
272. The vein, the tube that carries back the blood to the heart,
is composed of the same number of tunics as the artery, which, with
the exception of the middle, are essentially the same in structure,
but they are all much thinner. The external tunic consists of a less
dense and strong cellular membrane; the middle tunic, instead of being
formed of elastic rings, is composed of soft and yielding fibres,
disposed in a longitudinal direction; while the inner coat, which is
still more delicate than that of the artery, is arranged in a peculiar
manner. The inner coat of most veins, at slight intervals, is formed
into folds (fig. CXX. 5), one margin of which is firmly adherent to
the circumference of the vessel, while the other margin is free and
turned in the direction of the heart. These membranous folds are termed
valves. In all veins the diameter of which is less than a line the
valves are single; in most veins of greater magnitude they are placed
in pairs, while in some of the larger trunks they are triple, and in
a few instances quadruple, and even quintuple. The veins, like the
arteries, are supplied with nutrient vessels and nerves.
273. All the arteries of the body proceed from the two trunks already
described; that connected with the pulmonic circle, the pulmonary
artery, and that connected with the systemic circle, the aorta. These
vessels, as they go out from the heart and proceed to their ultimate
termination, are arborescent, that is, they successively increase in
number and diminish in size, like the branches of a tree going off from
the trunk (fig. CXIX. 1, 2, 3). Each trunk usually ends by dividing
into two or more branches (fig. CXIX. 1, 2), the combined area of which
is always greater than that of the trunk from which they spring, in
the proportion of about one and a half to one. As the branch proceeds
to its ultimate termination it divides and subdivides, until at length
the vessel becomes so minute, that it can no longer be distinguished
by the eye. These ultimate branches are called capillary vessels,
from their hair-like smallness (fig. CXIX. 4); but this term does not
adequately express their minuteness. It has been stated (234) that the
red particle of the blood, at the medium calculation, is not more than
the three-thousandth part of an inch in diameter; yet vast numbers of
the capillary vessels are so small that they are incapable of admitting
one of these particles, and receive only the colourless portion of the
blood.
[Illustration: Fig. CXIX.
View of the manner in which an artery divides and subdivides into its
ultimate branches. 1. Trunk of the artery; 2. large branches into which
it subdivides; 3. small branches, successively becoming smaller and
smaller until they terminate in 4. the capillary branches.]
274. Every portion of an artery, by reason of the elasticity of its
coats, preserves nearly a cylindrical form, and as the area of the
branches is greater than that of the trunks, the blood, in proceeding
from the heart to the capillaries, though passing through a series of
descending cylinders, is really flowing through an enlarging space.
275. The disposition of the veins, like that of the arteries, is
arborescent, but in an inverse order; for the course of the veins is
from capillary vessels to visible branches, and from visible branches
to large trunks (fig. CXX. 1, 2, 3). In every part of the body where
the capillary arteries terminate the capillary veins begin, and
the branches uniting to form trunks, and the small to form large
trunks, and the trunks always advancing towards the heart, and always
increasing in magnitude as they approach it, form at length the two
veins which it has been stated (258) return all the blood of the body
to the right auricle of the heart.
[Illustration: Fig. CXX.
View of the manner in which the minute branches of the vein unite to
form the larger branches and the trunks. 1. Capillary venous branches;
2. small branches formed by the union of the capillary; 3. larger
branches formed by the union of the smaller and gradually increasing in
size, to form the great trunk, 4. a portion of which is laid open to
show its inner surface and the arrangement of 5. the valves formed by
its inner coat.]
276. The veins are very much more numerous than the arteries, for
they often consist of double sets, and they are at the same time
more capacious and more extensible. Reckoning the whole of the blood
at one-fifth of the weight of the body, it is estimated that, of
this quantity, about one-fourth is in the arterial and the remaining
three-fourths in the venous system. The combined area of the branches
of the veins is much greater than that of the two trunks in which they
terminate (fig. CXX. 1, 2, 3, 4): the blood, therefore, in returning to
the heart, is always flowing from a large into a smaller space.
277. The divisions and subdivisions of the artery freely communicate
in all parts of the body by means of what are called anastomosing
branches, and this communication of branch with branch and trunk with
trunk is termed anastomosis. The same intercommunication, but with
still greater freedom and frequency, takes place among the branches
of veins. In both orders of vessels the communication is frequent in
proportion to the minuteness of the branch and its distance from the
heart. It is also more frequent in proportion as a part is exposed
to pressure; hence the minute arteries and veins about a joint are
distinguished for the multitude of their anastomosing branches; and
above all, it is frequent in proportion to the importance of the organ;
hence the most remarkable anastomosis in the body is in the brain. By
this provision care is taken that no part be deprived of its supply of
blood; for if one channel be blocked up, a hundred more are open to the
current, and the transmission of it to any particular region or organ
by two or more channels, instead of through one trunk, is a part of the
same provision. Thus the fore-arm possesses four principal arteries
with corresponding veins, and the brain receives its blood through four
totally independent canals[6].
278. That the blood is really a flowing stream, and that it pursues the
course described (258), is indubitable. For,
(1.) With the microscope, in the transparent parts of animals, the
blood can be seen in motion (fig. CXXI.); and if its course be
attentively observed, its route may be clearly traced.
[Illustration: Fig. CXXI.
View of the circulation of the blood as seen under the microscope in
the web of the frog's foot.]
(2.) The membranes termed valves are so placed as to allow of the
freest passage to the blood in the circle described, while they either
altogether prevent or exceedingly impede its movement in any other
direction.
(3.) The effect of a ligature placed around a vein and an artery, and
of a puncture made above the ligature in the one vessel and below it in
the other, demonstrate both the motion of the blood and the course of
it. When a ligature is placed around a vein, that part of the vessel
which is most distant from the heart becomes full and turgid on account
of the accumulation of blood in it; while the part of the vessel which
is between the ligature and the heart becomes empty and flaccid,
because it has carried on its contents to the heart, and it can receive
no fresh supply from the body. When, on the contrary, a ligature is
placed around an artery, that portion of the vessel which lies between
the ligature and the heart becomes full and turgid, and the other
portion empty and flaccid. This can only be because the contents of
the two vessels move in opposite directions,—from the heart to the
artery, from the artery to the vein, and from the vein to the heart.
At the same time, if the vein be punctured above the ligature, there
will be little or no loss of blood; while if it be punctured below
the ligature, the blood will continue to flow until the loss of it
occasions death, which could not be unless the blood were in motion,
nor unless the direction of its course were from the artery to the vein
and from the vein to the heart.
(4.) If fluids be injected into the veins or arteries, whether of the
dead or of the living body, they readily make their way and fill the
vessels, if thrown in the direction stated to be the natural course
of the circulation; but they are strongly resisted if forced in the
opposite direction.
279. Such is the description, and with the exception of the first
proof, such the evidence of the circulation of the blood in the
human body, pretty much as it was given by the discoverer of it, the
illustrious Harvey. Before the time of Harvey, a vague and indistinct
conception that the blood was not without motion in the body had been
formed by several anatomists. It is analogous to the ordinary mode
in which the human mind arrives at discovery (chap. iii., p. 103),
that many minds should have an imperfect perception of an unknown
truth, before some one mind sees it in its completeness and fully
discloses it. Having, about the year 1620, succeeded in completely
tracing the circle in which the blood moves, and having at that
time collected all the evidence of the fact, with a rare degree of
philosophical forbearance, Harvey still spent no less than eight years
in re-examining the subject, and in maturing the proof of every point,
before he ventured to speak of it in public. The brief tract which at
length he published was written with extreme simplicity, clearness,
and perspicuity, and has been justly characterised as one of the most
admirable examples of a series of arguments deduced from observation
and experiment that ever appeared on any subject.
280. Cotemporaries are seldom grateful to discoverers. More than one
instance is on record in which a man has injured his fortune and lost
his happiness through the elucidation and establishment of a truth
which has given him immortality. It may be that there are physical
truths yet to be brought to light, to say nothing of new applications
of old truths, which, if they could be announced and demonstrated
to-day, would be the ruin of the discoverer. It is certain that there
are moral truths to be discovered, expounded, and enforced, which, if
any man had now penetration enough to see them, and courage enough to
express them, would cause him to be regarded by the present generation
with horror and detestation. Perhaps, during those eight years of
re-examination, the discoverer of the circulation sometimes endeavoured
in imagination to trace the effect which the stupendous fact at the
knowledge of which he had arrived would have on the progress of his
favourite science; and, it may be, the hope and the expectation
occasionally arose that the inestimable benefit he was about to confer
on his fellow men would secure to him some portion of their esteem
and confidence. What must have been his disappointment when he found,
after the publication of his tract, that the little practice he had
had as a physician, by degrees fell off. He was too speculative,
too theoretical, not practical. Such was the view taken even by his
friends. His enemies saw in his tract nothing but indications of a
presumptuous mind that dared to call in question the revered authority
of the ancients; and some of them saw, moreover, indications of a
malignant mind, that conceived and defended doctrines which, if not
checked, would undermine the very foundations of morality and religion.
When the evidence of the truth became irresistible, then these persons
suddenly turned round and said, that it was all known before, and
that the sole merit of this vaunted discoverer consisted in having
circulated the circulation. The pun was not fatal to the future fame of
this truly great man, nor even to the gradual though slow return of the
public confidence even during his own time; for he lived to attain the
summit of reputation.
281. It is then indubitably established that the whole blood of the
body in successive streams is collected and concentrated at the heart.
The object of the accumulation of a certain mass of it at this organ
is to subject it to the action of a strong muscle, and thereby to
determine its transmission with adequate force and precision through
the different sets of capillary vessels.
282. In the accomplishment of this object the heart performs a twofold
action; that of contraction and that of dilatation. The auricles
contract and thereby diminish their cavities, then dilate and thereby
expand them, and the one action alternates with the other. There is the
like alternate contraction and dilatation of the ventricles. The first
action is termed systole, the second diastole, and both are performed
with force.
283. When the heart is laid open to view in a living animal, and its
movements are carefully observed, it is apparent that the two auricles
contract together; that the two ventricles contract together; that
these motions alternate with each other, and that they proceed in
regular succession. The interval between these alternate movements is,
however, exceedingly short, and can scarcely be perceived when the
heart is acting with full vigour; but it is evident when its action is
somewhat languid.
284. When the ventricles contract, the apex of the heart is drawn
upwards, and at the same time raised or tilted forwards. It is during
this systole of the ventricles, and in consequence of this result of
their action, that the apex of the heart gives that impulse against the
walls of the chest which is felt in the natural state between the fifth
and sixth ribs, and which just perceptibly precedes the pulse at the
wrist.
285. When the ear is applied to the human chest, over the situation of
the heart, a dull and somewhat prolonged sound is heard, which precedes
and accompanies the impulse of the heart against the chest. This dull
sound is immediately succeeded by a shorter and sharper sound: after
this there is a short pause; and then the dull sound and impulse are
again renewed. The duller sound and stronger impulse are ascribed to
the contraction of the ventricles, and the sharper sound and feebler
impulse to that of the auricles.
286. The movement of the heart is effected by the contraction of its
muscular fibres. Those fibres rest, as upon a firm support, on the
tendinous matter to which they are attached, from which they diverge,
and towards which their contraction must necessarily bring all the
parts of the heart (267). The result of their contraction is the
powerful compression of all the chambers of the heart, and thereby the
forcible ejection of their contents through the natural openings.
287. But the chambers, alternately with forcible contraction, perform
the action of forcible dilatation. This movement of dilatation is
effected by the reaction of the elasticity of the tendinous matter on
which the muscular fibres are supported (267). This highly elastic
substance, by the contraction of the fibres, is brought into a state of
extreme tension. The contraction of the fibres ceasing, that moment the
tense tendon recoils with a force exactly proportionate to the degree
of tension into which it had been brought. Thus the very agent that is
employed forcibly to close the chamber is made the main instrument of
securing its instantaneous re-opening. A vital energy is appointed to
accomplish what is indispensable, and what nothing else can effect,
the origination of a motive power; a physical agent is conjoined to
perform the easier task to which it is competent; and the two powers,
the vital and the physical, work in harmony, each acting alternately,
and each, with undeviating regularity and unfailing energy, fulfilling
its appropriate office.
288. When the chambers of the heart which open into each other, and
which as freely communicate with the great vessels that enter and
proceed from them, are forcibly closed, and the blood they contain is
projected from them, how is one uniform forward direction given to the
current? Why, when the right ventricle contracts, is the blood not sent
back into the right auricle, as well as forward into the pulmonary
artery? There is but one mode of preventing such an event, which is to
place a flood-gate between the two chambers; and there a flood-gate
is placed, and that flood-gate is the valve. As long as the blood
proceeds onwards in the direct course of the circulation, it presses
this membrane close to the side of the heart, and thereby prevents it
from occasioning any impediment to the current. When, on the contrary,
the blood is forced backwards, and attempts to re-enter the auricle,
being of course driven in all directions, some of it passes between the
wall of the ventricle and the valve. The moment it is in this situation
it raises up the valve, carries it over the mouth of the passage, and
shuts up the channel. There cannot be a more perfect flood-gate.
289. This is beautiful mechanism; but there is another arrangement
which surpasses mere mechanism, however beautiful. It has been shown
(260) that one edge of the membrane that forms the valve is firmly
adherent to the wall of the ventricle, while the other edge, when not
in action, appears to lie loosely in the ventricle (fig. CXVI. 10).
Were this edge really loose the refluent current would carry it back
completely into the auricle, and so counteract its action as a valve;
but it is attached to the tendinous threads proceeding from the fleshy
columns that stand along the wall of the ventricle (fig. CXVI. 12). By
these tendinous threads, as by so many strings, the membrane is firmly
held in its proper position (fig. CXVI. 10, 12); and the refluent
current cannot carry it into the auricle. Thus far the arrangement is
mechanical. But each of these fleshy columns is a muscle, exerting
a proper muscular action. Among the stimulants which excite the
contractility of the muscular fibre, one of the most powerful is
distension. The refluent current distends the membrane; the distension
of the membrane stretches the tendinous threads attached to it; the
stretching of its tendinous threads stretches the fleshy column; by
this distension of the column it is excited to contraction; by the
contraction of the column its thread is shortened; by the shortening
of the thread the valve is tightened, and that in the exact degree
in which the thread is shortened. So, the greater the impetus of the
refluent blood, the greater the distension of the membrane; and the
greater the distension of the membrane, the greater the excitement of
the fleshy column; the greater the energy with which it is stimulated
to act, the greater, therefore, the security that the valve will be
held just in the position that is required, with exactly the force that
is needed. Here, then, is a flood-gate not only well constructed as far
as regards the mechanical arrangement, but so endowed as to be able to
act with additional force whenever additional force is requisite; to
put forth on every occasion, as the occasion arises, just the degree of
strength required, and no more.
290. The contraction of the heart is the power that moves the blood;
and this contraction generates a force which is adequate to impel it
through the circle. From experiments performed by Dr. Hales it appears
that if the artery of a large animal, such as the horse, be made to
communicate with an upright tube, the blood will ascend in the tube to
the height of about ten feet above the level of the heart, and will
afterwards continue there rising and falling a few inches with each
pulsation of the heart. In this animal, then, the heart acts with a
force capable of maintaining a column of ten feet. Now a column of ten
feet indicates a pressure of about four pounds and a half in a square
inch of surface. Suppose the human heart to be capable of supporting
a column of blood eight feet high, this will indicate a pressure of
four pounds to the square inch; but the left ventricle of the heart,
while it injects its column of blood into the aorta, has to overcome
the inertia of the quantity of blood projected; of the mass already
in the artery, and of the elasticity of the vessel yielding to a
momentary increase of pressure: it is probable, therefore, that the
heart acts with a force of six pounds on the inch. The left ventricle,
when distended, has about ten square inches of internal surface;
consequently the whole force exerted by it may be about sixty pounds.
According to the calculation of Hales, it is fifty-one and a half.
Now, it is proved by numerous experiments, that, after death, a slight
impulse with the syringe, certainly much less than that which is acting
upon the blood in the same artery during life, is sufficient to propel
a solution of indigo, or fresh drawn blood, from a large artery into
the extreme capillary. If, therefore, after death, a slight force will
fill the capillaries, a force during life equal to sixty pounds must be
adequate to do so.
291. The heart, with a force equal to the pressure of sixty pounds,
propels into the artery two ounces of blood at every contraction. It
contracts four thousand times in an hour. There passes through the
heart, therefore, every hour, eight thousand ounces or seven hundred
pounds of blood. It has been stated (216) that the whole mass of blood
in an adult is about twenty-eight pounds: on an average the entire
circulation is completed in two minutes and a half; consequently a
quantity of blood equal to the whole mass passes through the heart
from twenty to twenty-four times in an hour. But though the average
space of time requisite to accomplish a complete circulation may be
two minutes and a half, yet when a stream of blood leaves the heart,
different portions of it must finish their circle at very different
periods, depending in part upon the length of the course which they
have to go, and in part upon the degree of resistance that obstructs
their passage. A part of the stream, it is obvious, finishes its course
in circulating through the heart itself; another portion takes a longer
circuit through the chest; another extends the circle round the head;
and another visits the part placed at the remotest distance from the
central moving power. Such is the velocity with which the current
sometimes goes, that, in the horse, a fluid injected into the great
vein of the neck, on one side, has been detected in the vein on the
opposite side, and even in the vein of the foot, within half a minute.
292. It has been shown (282) that the different chambers of the
heart have a tendency to perform their movements in a uniform manner,
and in a successive order; that they contract and dilate in regular
alternation, and at equal intervals; but, moreover, they continue
these movements equally without rest and without fatigue. On go the
motions, night and day, for eighty years together, at the rate of
a hundred thousand strokes every twenty-four hours, alike without
disorder, cessation, or weariness. The muscles of the arm tire after
an hour's exertion, are exhausted after a day's labour, and can by no
effort be made to work beyond a certain period. There is no appreciable
difference between the muscular substance of the heart and that of the
arm. It is true that the heart is placed under one condition which
is peculiar. Muscles contract on the application of stimuli; and
different muscles are obedient to different stimuli,—the voluntary
muscles to the stimulus of volition, and the heart to that of the
blood. The exertion of volition is not constant, but occasional; the
muscle acts only when it is excited by the application of its stimulus:
hence the voluntary muscle has considerable intervals of rest. The
blood, on the contrary, is conveyed to the heart without ceasing, in a
determinate manner, in a successive order; and this is the reason why
through life its action is uniform: it uniformly receives a due supply
of its appropriate stimulus. But why it is unwearied, why it never
requires rest, we do not know. We know the necessities of the system
which render it indispensable that it should be capable of untiring
action, for we know that the first hour of its repose would be the
last of life; but of the mode in which this wonderful endowment is
communicated, or of the relations upon which it is dependent, we are
wholly ignorant.
293. The force exerted by the heart is vital. It is distinguished from
mechanical force in being produced by the very engine that exerts it.
In the best-constructed machinery there is no real generation of power.
There is merely concentration and direction of it. In the recoil of the
spring, in the reaction of condensed steam, the energy of the expansive
impulse is never greater than the force employed to compress or
condense, and the moment this power is expended all capacity of motion
is at an end. But the heart produces a force equal to the pressure of
sixty pounds by the gentlest application of a bland fluid. Here no
force is communicated to be again given out, as in every mechanical
moving power; but it is new power, power really and properly generated;
and this power is the result of vital action, and is never in any case
the result of action that is not vital.
294. The heart projects the blood with a given force into the arterial
tubes. The arteries in the living body are always filled to distension,
and somewhat beyond it, by the quantity of blood that is in them. It
has been shown that the elasticity of their coats is such as to give
to them, even after death, the form of open hollow cylinders (274).
During life they are kept in a state of distension by the quantity of
blood they contain. By virtue of their elasticity they react upon
their contents with a force exactly proportioned to the degree of their
distension, that is, with a force at least adequate to keep them always
open and rigid.
295. These open and rigid tubes, already filled to distension, and
somewhat beyond it, receive at every contraction of the heart a
forcible injection of a new wave of blood. The first effect of the
injection of this new wave into a tube previously full to distension,
is to cause the current to proceed by jerks or jets, each jerk or
jet corresponding to the contraction of the heart. And, accordingly,
by this jet-like motion, the flow of the blood in the artery is
distinguished from that in the vein, in which latter vessel the current
is an equal and tranquil stream.
296. The second effect of this new wave is to occasion some further
distension of the already distended artery, and accordingly, when
the vessel is exposed in a living animal, and its action carefully
observed, a slight augmentation of its diameter is distinguishable at
every contraction of the heart. This new wave while it distends must at
the same time slightly elongate the vessel; cause its straight portions
to bend a little, and its curved portions to bend still more; and,
consequently, in some situations, to lift it a little from its place,
giving it a slight degree of locomotion;—and these two causes combined
produce the pulse. When the finger is pressed gently on an artery, at
the instant of the contraction of the heart, the vessel is felt to
bound against the finger with a certain degree of force: this, as just
stated, is owing to a slight distension of the vessel by the new wave
of blood, together with a slight elongation of it, and a gentle rising
from its situation.
297. The blood, in flowing through the arterial trunks and branches
to the capillaries, through the arterial to the venous capillaries, and
through the venous branches and trunks back to the heart, is exposed
to numerous and powerful causes of retardation: such, for example,
as the friction between the blood and the sides of the vessels, the
numerous curves and angles formed by the branches in springing from the
trunks, the tortuous course of the vessels in many parts of the body,
and the increasing area of the arterial branches as they multiply and
subdivide. Yet the extraordinary fact has been recently discovered,
that the blood moves with the same momentum or force in every part of
the arterial system, in the aorta, in the artery in the neck which
carries the blood to the head (the carotid artery), in the artery of
the arm (the humeral artery), in the artery of the lower extremity
(the femoral artery); in a word, in the minute and remote capillary,
and in the large trunk near the heart. Having contrived an instrument
by which the force of the blood as it flows in its vessel could be
accurately indicated by the rise of mercury in a tube, M. Poiseuille
found that the elevation of the mercury is uniformly the same in the
different arteries of the same animal, whatever the size of the artery
and its distance from the heart. This tube was inserted, for example,
into the common carotid artery of a horse: the diameter of the vessel
was 34/100ths of an inch; its distance from the heart was thirty-nine
inches; the height to which the mercury rose in the graduated tube was
accurately marked. The tube was then inserted into a muscular branch of
the artery in the thigh: the diameter of this vessel was 7/100ths of an
inch, and its distance from the heart 67½ inches. According to the mean
of nine observations, the mercury rose in both tubes to precisely the
same elevation. Here is another instance of the beautiful adjustments
everywhere established in the living economy. The blood is sent by a
living engine, moving under laws peculiar to the state of life, into
living vessels, which in their turn acting under laws peculiar to the
state of life, so accommodate themselves to the current as absolutely
to offer no resistance to its progress; so accommodate themselves to
the moving power, as completely and everywhere to obviate the physical
impediments to motion inseparable from inorganic matter.
298. That the arterial tubes do possess and exert a truly vital power,
modifying the current of the blood they contain, is indubitably
established.
1. If in a living animal the trunk of an artery be laid bare, the mere
exposure of it to the atmospheric air causes it to contract to such a
degree, that its size becomes obviously and strikingly diminished. This
can result only from the exertion of a vital property, for no dead tube
is capable in such a manner of diminishing its diameter.
2. If during life an artery be opened and the animal be largely bled,
the arteries become progressively smaller and smaller as the quantity
of blood in the body diminishes. If the bleeding be continued until the
animal dies, and the arteries of the system be immediately examined,
they are found to be reduced to a very small size; if again examined
some time after death, they are found to have become larger, and they
go on growing successively larger and larger until they regain nearly
their original magnitude, which they retain until they are decomposed
by putrefaction.
3. M. Poiseuille distended with water the artery of an animal just
killed. This water was urged by the pressure of a given column of
mercury. The force of the reaction of the artery was now measured by
the height of a column of mercury which the water expelled from the
artery could support. It was found that the artery reacted with a
force greater than that employed to distend it, and greater than the
same artery could exert some time after death; but since mechanical
reaction can never be greater than the force previously exerted upon
it (293), it follows that the excess of the reaction indicated in this
case was vital.
4. If an artery be exposed and a mechanical or chemical stimulus be
applied to it, its diameter is altered, sometimes becoming larger and
sometimes smaller, according to the kind of agent employed.
299. Any one of these facts, taken by itself, affords a demonstration
that the arterial trunks and branches are capable of enlarging and
diminishing their diameter by virtue of a vital endowment. There is
complete evidence that the exertion of this vital power on the part
of the arterial trunk is not to communicate to the blood the smallest
impulsive force; the engine constructed for the express purpose of
working the current generates all the force that is required; but the
labour of the engine is economized by imparting to the tubes that
receive the stream a vital property, by which they wholly remove the
physical obstructions to its motion.
300. Driven by the heart through the arterial branches into the
capillaries, the blood courses along these minute vessels urged by the
same power. The most careful observers, from Haller and Spalanzani
down to the present time, concur in stating that the pulsatory
movement communicated by the heart to the blood in the great arteries
is distinctly visible under the microscope in the capillaries. "I
have often observed in frogs and tadpoles, and once in the bat," says
Wedemeyer, "that when the circulation was becoming feeble, the blood
in the finest capillaries advanced by jerks, corresponding with the
contractions of the heart. I remarked the same appearance in the fine
veins several times in the toad and tadpole, and once in the frog." If
an experimenter so dispose the circulation of the limb of an animal
that the flow of blood be confined to the branches of a single artery,
and a corresponding vein, it is found that the blood stagnates in the
vein whenever the current in the artery is stopped by a ligature, but
no sooner is the ligature removed from the artery, than the blood
begins again to flow freely along the vein, the capillaries of the
artery which have to send on the current to those of the vein being
now again within the influence of the heart. And if the impulse of
the heart be removed from the capillary system, by placing a ligature
around the aorta, the capillary circulation is uniformly and completely
stopped.
301. It was found by Dr. Hales, that, under ordinary circumstances,
the blood rises in a tube connected with a vein to the height only of
six inches, while it has been shown (290) that in the artery it ascends
as high as ten feet. This prodigious difference between the venous and
the arterial tension led to the conclusion that the impulsive force of
the heart was all but exhausted before the blood reached the veins, and
set physiologists on the search for other powers to carry on the venous
circulation. It was overlooked that the blood has an open and ready
escape from the great trunks of the veins through the right chambers of
the heart, and that in consequence of this free escape of their fluid,
these vessels indicate no greater tension than is just sufficient to
lift the blood to the heart, and to overcome friction[7]. M. Magendie
having laid bare the chief artery and vein of a living limb, and having
raised the vessels in such a manner that he could place a ligature
around the former, without including the latter, found that the flow
of blood from a puncture made below a ligature on the vein, was rapid
or slow, according as the heart was allowed to produce a greater or
less degree of tension in the artery, which tension was regulated by
compressing the artery between the fingers. After a similar preparation
of a limb, a ligature was placed around the vein; a tube was then
inserted into it; it was found that the blood ascended in the tube from
the obstructed vein just as high as from the artery.
302. Thus we are able to trace the action of the heart from the
beginning to the end of the circle. Of this circle it is the sole
moving power; but it is a living engine acting in combination with
living vessels. The force it exerts is a vital force, economized by the
agency of a vital property communicated to the vessels, by virtue of
which they spontaneously and completely remove all physical obstruction
to the progress of the stream through its channels.
303. Some German physiologists of great eminence, after a careful and
patient observation of the blood, have satisfied themselves that in
addition to the contraction of the heart, it is necessary to admit a
second original and independent motive force, namely, a self-moving
power inherent in the particles of the blood itself. The blood we
know is a living substance. No reason can be assigned why the power
of originating motion should not be communicated to such a substance
as well as to the muscular fibre, of which, indeed, one constituent
of the blood affords the basis. Such a power, if found to be inherent
in the particles of the blood, would explain some phenomena connected
with the circulation not yet clearly elucidated; but the proof of the
self-moving power of the blood does not yet seem to be complete. It
is, however, impossible to explain the phenomena of the circulation,
or to obtain a satisfactory view of some of the other functions of the
economy, without supposing the particles of the blood to be endowed
with a vital power of repulsion, in consequence of which they are
prevented from uniting when in contact, and the fluidity of the mass is
maintained.
In this account of the powers that move the blood, no notice has been
taken of the physical agents supposed to act as auxiliaries to the
heart, in carrying on the circulation, such as the suction power of the
thorax, and of the auricles of the heart, and the capillary attraction
of the vessels; because, without questioning the existence of such
agents, or denying that advantage may be taken of them, it seems pretty
clear that their influence is but trivial, and they assumed importance
only when the vital endowments of the tissues were not well understood.
304. The ultimate end for which the apparatus of the circulation is
constructed, and for which all its action is exerted, is to convey
arterial blood to the capillary arteries. These vessels are totally
distinct in structure and in office from the larger arterial tubes. All
the tunics of these minute vessels diminish in thickness and strength
as the tubes lessen in size, but more especially the middle or the
fibrous coat; which, according to Wedemeyer, may still be distinguished
by its colour in the transverse section of any vessel whose internal
diameter is not less than the tenth of a line; but that it entirely
disappears in vessels too small and too remote to receive the wave of
blood in a manifest jet. But while the membranous tunics diminish,
the nervous filaments distributed to them increase: the smaller and
thinner the capillary, the greater the proportionate quantity of its
nervous matter; and this is most manifest in organs of the greatest
irritability. The coats of the capillaries successively becoming
thinner and thinner, at length disappear altogether, and the vessels
ultimately terminate in membraneless canals formed in the substance of
the tissues. "The blood in the finest capillaries," says Wedemeyer,
"no longer flows within actual vessels; it is not contained in tubes
whose parietes are formed by a membranous substance distinguishable by
its texture and compactness from the adjoining cellular tissue: it is
contained in the different tissues in channels which it forms in them
for itself; and, under the microscope, the stream is seen easily and
rapidly to work out for itself a new passage in the tissues which it
penetrates."
305. Some of these fine capillaries, before they entirely lose their
membranous tunics, communicate directly with veins. Of the capillaries
which terminate by direct communication with veins, some are large
enough to admit of three or four of the red particles of the blood
abreast; the diameter of others is sufficient to admit only of one;
while others are so small that they can transmit nothing but the serum
of the blood. As long as the capillary is of sufficient magnitude
to receive three or four of the particles abreast, it is evident
that it possesses regular parietes; but by far the greater number,
before they communicate with veins, lose altogether their membranous
coats. There are no visible openings or pores in the sides or ends
of the capillaries by means of which the blood can be extravasated,
preparatory to its being imbibed by the veins. There is nowhere
apparent a sudden passage of the arterial into the venous stream; no
abrupt boundary between the division of the two systems. The arterial
streamlet winds through long routes, and describes numerous turns
before it assumes the nature and takes the direction of a venous
streamlet. The ultimate capillary rarely passes from a large arterial
into a large venous branch.
306. The vital power which it has been shown (298) is possessed by
the arterial trunks and branches, is still more intense in the minute
capillaries. If alcohol, strong acetic acid, naphtha, and other
stimulating fluids, be injected into the arteries of a living animal,
it is found that they are not transmitted through the capillaries
at all, or, at all events, that they make their way through them
with extreme difficulty; whereas mild, unirritating fluids pass with
rapidity and ease. Wedemeyer exposed and divided the main artery in
the fore-leg of a horse, together with the corresponding vein in the
shoulder. Several syringes-full of tepid water were now injected into
the lower end of the artery. The gentlest pressure was sufficient to
force the fluid through the capillaries. At each injection the water
issued in a full stream from the aperture of the vein, the flow of the
fluid ceasing as soon as the injection was stopped. Next, instead of
water, four syringes-full of pure cold brandy were injected. To propel
this fluid through the capillaries, so as to render its smell and taste
perceptible at the aperture of the vein, required a great degree of
pressure; and when at last the fluid issued from the vein, it merely
trickled in a feeble stream.
The experiment being repeated on another horse with vinegar, six
syringes-full of which being injected in rapid succession, at first
this fluid passed as easily as water, afterwards it flowed with greater
difficulty and in a small stream; before long the force required to
propel it was extreme, and at last the obstruction to its passage
became complete, so that no fluid whatever issued from the vein.
These experiments, whenever repeated, afforded the same result, and
they demonstrate that the capillaries are capable of being stimulated
to contract upon their contents, and that they can contract with such
force as to stop the current. It is manifest that the power by which
they do this is vital, because after death all fluids, the mildest and
the most acrid, pass through them with equal facility.
307. Drs. Thompson, Philip, Hastings, and others in this country,
have applied stimulants of various kinds to the capillary arteries,
in order to observe with the microscope the changes which the vessels
undergo. The results of these experiments, performed independently,
agree with each other; and all the observers concur in stating that
those results are so obvious and decisive as to admit of no question.
Wedemeyer, fully aware of all that had been done on this subject by
the English physiologists, repeated their experiments with his usual
patience and care, vigilantly watching the effects with his microscope.
His observations completely coincide with those of our countrymen. The
circulation being observed in the mesentery of the frog and in the web
of its foot, it was apparent that no change whatever took place in
the diameter of the small arteries, nor in that of the capillaries,
as long as the circulation was allowed to go on in its natural state;
but as soon as stimulants were applied to them, an alteration of their
diameter was visible. Alcohol, without much apparent contraction of the
vessels, stopped the flow of the blood. Muriate of soda, in the course
of three or four minutes, caused the vessels to contract one-fifth of
their calibre, which contraction was followed by dilatation and gradual
retardation and stoppage of the blood. Ammonia caused immediate and
direct dilatation, and the effect of galvanism was still more striking.
In a space of time varying from ten to thirty seconds, nay, sometimes
immediately after the completion of the galvanic circle, the vessels
contracted, some a fourth, others half, and others three-fourths, of
their calibre. The flow of the blood through the contracted vessels
was accelerated. The contraction sometimes lasted a considerable time,
occasionally several hours; in other instances the contraction ceased
in ten minutes, and the vessels resumed their natural diameter. A
second application of galvanism to the same capillaries seldom caused
any material contraction.
308. The evidence, then, is abundant that stimulants are capable of
modifying to a great extent the action of the capillary arteries,
sometimes causing them to contract, at other times to dilate; sometimes
quickening the flow of blood through them, at other times retarding it,
and frequently altogether arresting its motion. This contractile power
of the capillaries must be a vital endowment, for no such property is
possessed by any substance destitute of life, and there is satisfactory
evidence that it is communicated, regulated, and controlled by the
organic nerves, which, as has been shown, increase as the size of the
vessels and the thickness of their membranous tunics diminish. The
powerful influence of these nerves upon the capillary vessels is placed
beyond doubt or controversy by the obvious local changes produced in
the capillary circulation by sudden, and even by mental, impressions,
by the flush of the cheek and the sparkle of the eye, at a thought
conceived or a sound heard; changes which can be effected, as far as we
have any knowledge, by no medium excepting that of the nerves. The part
performed by electricity, the physical agent by which it is conceived
the nerves operate, will be considered hereafter.
309. Exerting upon each other a vital force of repulsion, under a
vital influence derived from the organic nerves, urged by the vital
contraction of the heart, the particles of the blood reach the extreme
capillaries. Most of these capillaries terminate (304) in canals, which
they work out for themselves in the substance of the tissues. The
tissues are endowed with a vital attractive force, which they exert
upon the blood—an elective as well as an attractive force: for in
every part of the body, in the brain, the heart, the lung, the muscle,
the membrane, the bone, each tissue attracts only those constituents
of which it is itself composed. Thus the common current, rich in all
the proximate constituents of the tissues, flows out to each. As the
current approaches the tissue, the particles appropriate to the tissue
feel its attractive force, obey it, quit the stream, mingle with the
substance of the tissue, become identified with it, and are changed
into its own true and proper nature. Meantime, the particles which are
not appropriate to that particular tissue, not being attracted by it,
do not quit the current, but passing on, are borne by other capillaries
to other tissues, to which they are appropriate, and by which they
are apprehended and assimilated, When it has given to the tissues the
constituents with which it abounded, and received from them particles
no longer useful, and which would become noxious, the blood flows into
the veins to be returned by the pulmonic heart to the lung, where,
parting with the useless and noxious matter it has accumulated, and,
replenished with new proximate principles, it returns to the systemic
heart, by which it is again sent back to the tissues.
310. Particles of blood are seen to quit the current and mingle with
the tissues; particles are seen to quit the tissues and mingle with
the current. But all that we can see, with the best aid we can get,
does but bring us to the confines of the grand operations that go on,
of which we are altogether ignorant. Arterial blood is conveyed by
the arteries to the capillaries; but before it has passed from under
the influence of the capillaries it has ceased to be arterial blood.
Arterial blood is conveyed by the carotid artery to the brain; but
the cerebral capillaries do not deposit blood, but brain. Arterial
blood is conveyed by its nutrient arteries to bone, but the osseous
capillaries do not deposit blood, but bone. Arterial blood is conveyed
by the muscular arteries to muscle, but the muscular capillaries do not
deposit blood, but muscle. The blood conveyed by the capillaries of
brain, bone, and muscle is the same, all comes alike from the systemic
heart, and is alike conveyed to all tissues; yet in the one it becomes
brain, in the other bone, and in the third muscle. Out of one and the
same fluid these living chemists manufacture cuticle, and membrane, and
muscle, and brain, and bone; the tears, the wax, the fat, the saliva,
the gastric juice, the milk, the bile, all the fluids, and all the
solids of the body.
311. And they do still more; for they are architects as well as
chemists; after they have manufactured the tissue, they construct the
organ. The capillaries of the eye not only form its different membranes
and humours, but arrange them in such a manner as to constitute the
optical instrument; and the capillaries of the brain not only form
cerebral matter, but build it up into the instrument of sensation,
thought, and motion.
312. The practical applications of these phenomena are numerous and
most important; but they can be clearly and impressively stated
only when the operation of the physical agents which influence the
circulation, and which proportionally affect life and health, has been
explained.
FOOTNOTES.
[1] Computationi in alimentis faciendæ hanc formam esse Ulpianus
scribit, ut _à primâ ætate_ usque ad annum vicesimum quantitas
alimentorum triginta annorum computetur, ejusque quantitatis Falcidia
præstetur: _ab annis verò viginti_ usque ad annum vicesimumquintum
annorum viginti octo: _ab annis vigintiquinque_ usque ad annos
triginta, annorum vigintiquinque: _ab annis triginta_ usque ad annos
trigintaquinque annorum viginti duo; _ab annis trigintaquinque_
usque ad annos quadraginta annorum viginti: _ab annis quadraginta_
usque ad annos quinquaginta tot annorum computatio fit quot ætate
ejus ad annum sexagesimum deerit, remisso uno anno: _ab anno verò
quinquagesimo_ usque ad annum quinquagesimumquintum annorum novem:
_ab annis quinquagintaquinque_ usque ad annum sexagesimum annorum
septem: _ab annis sexaginta_, cujuscunque ætatis sit, annorum quinque;
eoque nos jure uti Ulpianus ait, et circa compu tationem ususfructus
faciendam. Solitum est tamen _à primâ ætate_ usque ad annum trigesimum
computationem annorum triginta fieri: _ab annis verò triginta_ tot
annorum computationem inire, quot ad annum sexagesimum deesse videntur;
nunquam ergo amplius quam triginta aunorum computatio initur. Sic
denique, et si Reipublicæ ususfructus egetur, sive simpliciter, sive
ad ludos, triginta annorum computatio fit. Si quis ex heredibus rem
propriam esse contendat, deinde hereditariam esse convincatur: quidem
putant ejus quoque Falcidiam non posse retineri; quià nihil intersit,
subtraxerit an hereditariam esse negaverit. Quod Ulpianus rectè
improbat. (Vide Justin. Pandect. lib. 35, tit. 2, ad Legem Falcidiam.)
[2] Which maximum is a little above the highest point hitherto any
where attained.
[3] Hence in the preparation of jelly as an article of diet, the parts
of young animals, as the feet of the calf, are principally employed;
whereas soups made from beef contain a large proportion of albumen,
while in those made from veal the proportion of jelly preponderates.
[4] Treatise on Ligaments, by Bransby B. Cooper, Esq.
[5] For these illustrations I am indebted to Mr. Lister, who has been
so kind as to make drawings of the objects for me.
[6] Whenever there is any interruption to the ordinary flow of the
circulating fluids, the powers of the anastomosing circulation are
capable of being increased to a surprising extent. The aorta itself
has frequently been tied in animals of considerable size without
destroying life; in the human body it has also been found obliterated
by disease in different parts of its course, in one case as high
as the termination of its curvature. In the cure for aneurism the
external iliac artery has been tied by Mr. Abernethy with success; the
subclavian artery below the clavicle by Mr. Keate; the common carotid
by Sir Astley Cooper; the subclavian artery above the clavicle by
Mr. Ramsden; the internal iliac artery by Dr. Stevens; the arteria
innominata by Dr. Mott, of New York; and lastly, the abdominal aorta
itself, by Sir A. Cooper. Mr. Grainger tied the abdominal aorta of a
dog; when the animal had recovered from that operation, the carotids
and the great trunks of the anterior extremities were tied: in this
manner the whole course of the circulation was altered. The dog, which
was of very large size, survived all these operations, and appeared to
enjoy its ordinary health. Grainger's General Anatomy, p. 251-253.
[7] See this matter very ably discussed in Dr. Arnott's excellent work
on the Elements of Physics, vol. i.
END OF VOL. I.
London: Printed by W. CLOWES and SONS, Stamford Street.
TRANSCRIBER'S NOTES.
1. (Figure LXXIV.) was incorrectly labeled as (Figure LXXVI.).
This has been corrected.
2. No Figure LXX in original book.