dilatation of the thorax, in the action of inspiration, is the
following. The lungs are in direct contact with the inner surface of
the thorax, and follow passively all its movements. When the volume
of the lungs is reduced to its minimum by the diminished capacity of
the thorax, in the state of expiration, they still contain a certain
bulk of air. As their volume increases with the enlarging capacity of
the thorax in the state of inspiration, this bulk of air having to
occupy a greater space expands. By this expansion of the air in the
interior of the lungs, it becomes rarer than the external air. Between
the rarified air within the lungs, and the dense external air, there
is a direct communication by the nostrils, mouth, trachea, larynx, and
bronchi. In consequence of its greater weight, the dense external air
rushes through these openings and tubes to the lungs and fills the
air vesicles, the current continuing to flow until an equilibrium is
established between the density of the air within the lungs and the
density of the external air; and thus there is established the flow of
a current of fresh air to the air vesicles.
398. The external air which, in obedience to the physical law that
regulates its motion, thus rushes to the lung in order to fill the
partial vacuum created by the dilatation of the thorax in inspiration,
produces, in passing to the air vesicles, a peculiar sound. When the
lungs are perfectly healthy, and the respiration is performed in a
natural manner, if the ear be applied to any part of the chest, a
slight noise can be distinguished both in the action of inspiration
and that of expiration. A soft murmur, somewhat resembling the sound
produced by the deep inspirations occasionally made by a person
profoundly sleeping. This sound, though appreciable even by the naked
ear, and though produced many times every minute, in every healthy
human being from the first moment of the existence of the first man,
had never been heard, or at least never attended to, until about twenty
years ago, when it was observed by accident. A physician, Dr. Laennec,
of Paris, having occasion to examine a young female labouring under,
as he supposed, some disease of the heart, and scrupling to follow
his first impulse to apply his ear to the chest, chanced to recollect
that solid bodies have the power of conducting sounds better than the
air. Thereupon he procured a quire of paper, rolled it up tightly,
tied it, and then applied one extremity to the patient’s chest and the
other to his ear. Profiting by the result, which was, that he could
hear the beating of the heart infinitely more distinctly than he
could possibly feel it by the hand, he substituted for this first rude
instrument a wooden cylinder, which he called a stethescope or chest
inspector. The attentive and practised use of this instrument is found
to be capable of revealing to the ear all that is passing in the chest
almost as clearly and certainly as it would be visible to the eye,
were the walls of the chest and the tissues of its organs transparent.
Besides the entrance of the air into the lung in inspiration, and its
exit in expiration, even the motion of the blood in the heart, and in
the great blood-vessels, are rendered by this instrument distinctly
manifest to sense; and as the ear which has once become familiar with
the natural sounds produced by these operations in the state of health,
can detect the slightest deviation occasioned by disease, the practical
application of this discovery has already effected for the pathology
of the chest, what the discovery of the circulation of the blood has
accomplished for the physiology of the body.
399. At the instant that the expanding lung admits the current of air,
it receives a stream of blood. The air rushes through the trachea to
the air vesicles impelled by its own weight; the blood flows through
the trunks of the pulmonary artery to its capillary branches, spread
out on the walls of the air vesicles, driven by the contraction of
the right ventricle of the heart. A current of air and a stream of
blood are thus brought into so close an approximation that nothing
intervenes between the two fluids, but the fine membranes of which
the air vesicles and the capillary branches of the pulmonary artery
are composed, and these membranes being pervious to the air, the air
comes into direct contact with the blood; the two fluids re-act on each
other, and in this manner is accomplished the ultimate object of the
action of inspiration.
400. On the other hand, by the action of expiration, the bulk of the
lung is diminished; the air vesicles are compressed, and a portion of
the air they contained, forced out of them by the collapse of the lung,
is received by the bronchi, transmitted to the trachea, and ultimately
conveyed out of the system by the nostrils and mouth.
401. At the same instant that a portion of air is thus expelled from
the lung and carried out of the system, a stream of blood, namely,
blood which has been acted upon by the air, arterial blood, is
propelled from the lung and is borne by the pulmonary veins to the
left side of the heart, to be transmitted to the system (fig. CXL. 10,
11, 4). In this manner, by the simultaneous expulsion from the lung of
a current of air and a stream of blood is accomplished the ultimate
object of the action of expiration.
402. That blood flows to the lung during the action of inspiration, and
is expelled from it during the action of expiration, is established by
direct experiment.
403. If the great vessel which returns the blood from the head to the
heart, called the jugular vein, be exposed to view in a living animal,
it is seen to be alternately filled and emptied according to the
different states of inspiration and expiration.
It becomes nearly empty at the moment of inspiration, because at that
moment the venous stream is hurried forward to the right chambers
of the heart, which in consequence of the general dilatation of the
chest are now expanded to receive it. This may be rendered still more
strikingly manifest to the eye. If a glass tube, blown at the middle
into a globular form, be inserted by its extremities into the jugular
vein of a living animal in such a manner that the venous stream must
pass through this globe, it is found that the globe becomes nearly
empty during inspiration, and nearly full during expiration; empty
during inspiration, because, during this action the blood flows
forwards to the right chambers of the heart; full during expiration,
because during this action the venous stream, retarded in its passage
through the lung, its motion becomes so slow in the jugular vein that
there is time for its accumulation in the glass globe. In the artery,
on the contrary, in which the course of the current is the reverse
of that in the vein, the opposite result takes place. In the carotid
artery the stream is seen to be feeble and scanty during inspiration,
but forcible and full during expiration, and if the artery be divided
the jet of blood that issues from it absolutely stops during the
action of inspiration; and the fuller and deeper the inspiration the
longer is the interval between the jets, while it is during the action
of expiration that the jet is full and strong.
404. In the course of some experiments performed by Dr. Dill and myself
with a view to ascertain with greater precision the relation between
respiration and circulation, we observed a phenomenon which places
these points in a still more clear and striking light. We happened to
divide a jugular vein. We saw that the vessel ceased to bleed during
inspiration, and that it began to bleed copiously the moment expiration
commenced; the reverse of what uniformly happens in the entire state
of the vessel. The reason is, that the division of the vein cuts off
its communication with the lung, removes it from the influence of
respiration, brings it under the influence, the sole influence of the
powers that move the arterial current, and consequently reverses its
natural condition, and so reverses the manner in which its current
flows; affording a beautiful illustration of the influence of the two
actions of respiration on the two sets of blood-vessels concerned in
the function.
405. It is then the venous system that is immediately related to
inspiration, and the arterial to expiration. Each respiratory action
exerts a specific influence over its own sanguiferous system, and
the influence of the one action is the reverse of that of the
other, as the two currents they work flow in opposite directions.
The lungs, in inspiration, expand and receive the venous stream; in
expiration, collapse and expel the arterial stream. The expansion of
the lungs in inspiration is thus simultaneous with the dilatation of
the heart: during the inspiratory action both organs receive their
blood. The collapse of the lungs in expiration is simultaneous with
the contraction of the heart: during the expiratory action both organs
expel their blood.
406. We are thus enabled to form a clear and exact conception of the
mechanism and action of both parts of this complicated function. Almost
all the points connected with the systemic circulation were established
upwards of three hundred years ago (279), but many points connected
with the pulmonic circulation have been established only recently. Our
knowledge of the phenomena of both, and of their mutual relation and
dependence, has been slowly increasing, and is at length tolerably
complete; and now that we understand the exact office and working of
each, we see that the action of the one is not only in harmony with
that of the other, but co-operates with it, and renders it perfect.
407. But although the main points relative to the influence of
inspiration and expiration over the pulmonary circulation may be said
to be universally admitted, still physiologists are not agreed as to
the relative quantities of blood which are transmitted through the
lungs during these different respiratory states. All are agreed that
the state of inspiration is favourable to the passage of the blood
through the lungs: some maintain that this expansion of the lungs in
inspiration is essential to the pulmonary circulation. There is the
like general consent that the state of expiration retards the flow of
blood through the lungs; by many it is conceived that it completely
stops the current. By these physiologists it is supposed that, during
the action of expiration, the lungs are in a state of collapse; that
they contain a comparatively small portion of air; that in this state
the air vesicles are so compressed, and the pulmonary blood-vessels so
coiled up, that the lungs are absolutely impermeable, and consequently,
that when the blood arrives at the right chambers of the heart, it is
incapable of making its way to the left. This, according to a prevalent
theory, is the immediate cause of death in asphyxia, the state of the
system induced by suspended respiration, as in drowning, hanging, and
suffocation. Death takes place in this condition of the system, it is
argued, because the circulation of the blood is arrested at the right
side of the heart, cannot permeate the lungs, and consequently cannot
reach the left ventricle, to be sent out to supply the organs of the
body.
408. This opinion, which appears at first view to be favoured by
numerous observations and experiments, has been shown to be fallacious
by a series of decisive experiments, performed by Dr. Dill and myself,
undertaken, as has been stated (404), with the object of ascertaining,
in a more exact manner than had hitherto been done, the relation
between the circulation and respiration. The previously ascertained
fact that the heart continues to beat and the blood to flow several
minutes after the complete suspension of the respiration, or after
apparent death, afforded us the means of pursuing our research. The
details of these experiments are given elsewhere: it is sufficient to
state in this place the main results.
409. As a standard of comparison, the quantity of blood which flows
through the lungs after apparent death, when the lungs remain in a
perfectly natural state, was previously ascertained. It was found,
after death produced in an animal by a blow on the head, that blood
continued to be transmitted through the lungs for the space of
twenty-five minutes after the complete cessation of respiration. There
passed through the lungs in all five ounces and two drachms of blood.
410. Respiration was now suspended the instant after a perfectly
natural and easy _inspiration_; there flowed through the lungs four
ounces and five drachms of blood.
411. Respiration was next suspended the instant after a perfectly
natural and easy _expiration_; there flowed through the lungs two
ounces and seven drachms of blood.
412. When the trachea of an animal is closed by the pressure of a cord
in suspension, or when an animal is immersed under water, it makes a
succession of violent expirations, by which a large quantity of air is
forced out of the lungs. Hence, when the lungs of an animal that has
perished by hanging or drowning, are examined, they are always found
much reduced in bulk; so much reduced in bulk as to have suggested the
theory that the extreme collapse of the lungs and their consequent
impermeability, is the cause of death in this condition of the system.
On bringing this theory to the test of experiment, it was found that
blood continued to flow through the lungs after apparent death from
suspension, for the space of eleven minutes, and that there passed
through in all five ounces of blood. The comparatively larger quantity
transmitted in this case than when the inspiration and expiration were
perfectly natural, was owing to the larger size of the animal. In the
experiments made with a view to ascertain the relative proportions
of blood transmitted through the lungs in the states of natural
inspiration and expiration, the animals were chosen as nearly as
possible of the same size, and were much smaller than the former.
413. On examining the quantity of blood that passed through the lungs
after death from submersion, it was found to be very nearly the same
as that which was transmitted after death from suspension.
414. But the lungs may be brought to a much greater degree of collapse
than that to which they are reduced in hanging and drowning. By
introducing an exhausting syringe into the trachea, a much larger
quantity of air may be drawn out of the lungs than they are capable
of expelling by the most violent efforts of expiration. When, in this
mode, the lungs had been reduced to the greatest possible degree of
collapse, and had been exhausted of all the air that could be drawn out
of them, there flowed through them two ounces of blood.
415. Such are the results when the lungs are reduced successively
from the moderate degree of collapse incident to a perfectly natural
expiration, to the great degree of collapse incident to suspension
and submersion, and the most extreme degree of collapse which it is
possible to induce by exhaustion.
416. When the phenomena that take place in the opposite condition of
the lungs were investigated, results were obtained which present a
striking contrast to those which have been stated. On forcing into
the lungs the largest quantity of air which they are capable of
containing without the rupture of the air vesicles, and in this manner
communicating to them the greatest degree of dilatation compatible
with their integrity, it was found that in this state there passed
through them _only three drachms of blood_.
417. But on fully distending the lungs with water instead of air, the
pulmonary circulation was instantaneously and completely arrested; they
were incapable of transmitting a single drop of blood. On cutting the
aorta across, as in all the preceding experiments, not a particle of
blood was obtained, excepting what issued at a single jet, and which
consisted only of the blood contained in the vessel at the moment the
respiration was stopped.
418. From these experiments it follows—
1. That the state of inspiration is favorable to the passage of
the blood through the lungs. In the dilatation of inspiration they
transmitted nearly double the quantity that passed in the collapse of
expiration; or, as four ounces and five drachms are to two ounces and
seven drachms (410 and 411).
2. That no degree of collapse to which the lungs can be reduced is
capable of wholly stopping the flow of the blood through them. In the
collapse of suspension and submersion they transmitted as much blood,
with the exception of two drachms, as when death was produced by a blow
on the head (412 and 409). In the greatest degree of collapse capable
of being produced by an exhausting syringe, they transmitted half as
much as in the collapse of suspension and submersion (414 and 412).
3. That it is only a moderate degree of dilatation that is favorable
to the transmission of the blood through the lungs. When the lungs
are over-distended with air, they are capable of transmitting only
an exceedingly small quantity of blood (416); when they are fully
distended with water, they are incapable of transmitting a single drop
of blood (417). In fact they can contain only a certain quantity of
air and blood; and when either of these fluids preponderates, it can
only be by the proportionate exclusion of the other. It will appear
hereafter that these results are capable of applications of the highest
interest and importance in the explanation of numerous phenomena of
health and of disease.
419. Physiologists have laboured with great diligence to determine the
exact quantity of air and blood which enters and which flows from the
lung at each of the actions of respiration, and they have succeeded in
obtaining tolerably precise results.
420. The quantity of air capable of being received into the lungs of
an adult man, in sound health, at an inspiration, is determined with
correctness by an instrument constructed by Mr. Green, analagous to
one suggested by Mr. Abernethy. It consists of a tin trough, about a
foot square, and six inches deep, three parts of which are filled
with water. Into this trough is placed a three-gallon glass jar, open
at the bottom, and graduated at the side into pints, half-pints, &c.
To the upper end of the jar a flexible tube is affixed, having at its
connexion a stop-cock. The lungs being emptied, as in the ordinary
action of expiration, and the mouth applied to the end of the flexible
tube, the nostrils being closed by the pressure of the fingers, the
air is drawn out of the jar into the lungs by the ordinary action of
inspiration. When as much air is thus drawn into the lungs as the air
vesicles will hold, the stop-cock is closed, and the quantity of air
inspired is ascertained by the rise of the water, the level of the
water corresponding with the indications marked on the side of the jar.
421. The quantity of air which a person by a voluntary effort can
inspire at one time is found, as might have been anticipated, to be
different in every different individual. These varieties depend, among
other causes, on the greater or less development of the trunk, on the
presence or absence of disease in the chest, on the degree in which the
lung is emptied of air by expiration previously to inspiration, and
on the energy of the inspiratory effort. The greatest volume of air
hitherto found to have been received by the lung, on the most powerful
inspiration, is nine pints and a quarter. The average quantity which
the lungs are capable of receiving in persons in good health, and free
from the accumulation of fat about the chest, appears to be from five
to seven pints. The latter is about the average quantity capable of
being inspired by public singers.
422. But these measurements relate to the greatest volume of air which
the lungs are capable of receiving, on the most forcible inspiration
which it is possible to make, after they have been emptied by forcible
expiration, and consequently express the quantity received in
extraordinary, not in ordinary inspiration. The quantity received at an
inspiration easy, natural, and free from any great effort, may be two
pints and a half, but the quantity received at an ordinary inspiration,
made without any effort at all, is, according to former observations
which referred to Winchester measure, about one pint.
423. The quantity of air expelled from the lung by an ordinary
expiration is probably a very little less than that received by an
ordinary inspiration (456).
424. No one is able by a voluntary effort to expel the whole contents
of the lungs. Observation and experiment lead to the conclusion that
the lungs, when moderately distended, contain at a medium about twelve
pints of air. As one pint is inhaled at an ordinary inspiration,
and somewhat less than the same volume is expelled at an ordinary
expiration (456), there remain present in the lungs, at a minimum,
eleven pints of air. There is one act of respiration to four pulsations
of the heart; and, as in the ordinary state of health there are
seventy-two pulsations, so there are eighteen respirations in a minute,
or 25,920 in the twenty-four hours.
425. About two ounces of blood are received by the heart at each
dilatation of the auricles; about the same quantity is expelled from
it at each contraction of its ventricles; consequently, as the heart
dilates and contracts seventy-two times in a minute, it sends thus
often to the lungs, there to be acted upon by the air, two ounces of
blood. It is estimated by Haller that 10,527 grains of blood occupy
the same space as 10,000 grains of water, so that if one cubic inch of
water weigh 253 grains, the same bulk of blood will weigh 266⅓ grains.
426. It is ordinarily estimated that on an average one circuit of the
blood is performed in 150 seconds; but it is shown (451 and 452) that
the quantity of air always present in the lungs contains precisely a
sufficient quantity of oxygen to oxygenate the blood, while flowing
at the ordinary rate of 72 contractions of the heart per minute, for
the exact space of 160 seconds. It is therefore highly probable that
this interval of time, 160 seconds, is the exact period in which the
blood performs one circuit, and not 150 seconds, as former observations
had assigned. If this be so, then 540 circuits are performed in the
twenty-four hours; that is, there are three complete circulations of
the blood through the body in every eight minutes of time.
427. But it has been shown (425) that the weight of the blood is to
that of water as 1.0527 is to unity, and that consequently 10,527
grains of blood are in volume the same as 10,000 grains of water.
428. From this it results that if in the human adult two ounces of
blood are propelled into the lungs at each contraction of the heart,
that is, 72 times in a minute, there are in the whole body precisely
384 ounces, or 24 pounds avoirdupois, which measure 692.0657 cubic
inches, or within one cubic inch of 20 imperial pints, which measure
693.1847 cubic inches.
429. By an elaborate series of calculations from these data Mr.
Finlaison has deduced the following general results:—
1. As there are four pulsations to one respiration (424), there are 8
ounces of blood, measuring 14.418 cubic inches, presented to 10.5843
grains of air, measuring 34.24105 cubic inches.
2. The whole contents of the lungs is equal to a volume of very nearly
411 cubic inches full of air, weighing 127 grains, of which 29.18132
grains are oxygen.
3. In the space of five-sixth parts of one second of time, two ounces,
or 960 grains weight of blood, measuring 3⅗ or 3.60451 cubic inches,
are presented for aëration.
4. Therefore the air contained in the lungs is 114 times the bulk of
the blood presented, while the weight of the blood so presented is 7½
times as great as the weight of the air contained.
5. In one minute of time the fresh air inspired amounts to 616⅓ cubic
inches, or as nearly as may be 18 pints, weighing 190½ grains.
6. In one hour the quantity inspired amounts to 1066⅔ pints, or 2
hogsheads, 20 gallons, and 10⅔ pints, weighing 23¾ ounces and 31 grains.
7. In one day it amounts to 57 hogsheads, 1 gallon, and 7¼ pints,
weighing 571½ ounces and 25 grains (454).
8. To this volume of air there are presented for aëration in one minute
of time 144 ounces of blood, in volume 259½ cubic inches, which is
within 18 cubic inches of an imperial gallon.
9. In one hour 540 pounds avoirdupois, measuring 449¼ pints, or 1
hogshead and 1¼ pints;—and
10. In the twenty-four hours, in weight 12,960 pounds; in bulk 10,782½
pints, that is, 24 hogsheads and 4 gallons.
11. Thus, in round numbers, there flow to the human lungs every minute
nearly 18 pints of air (besides the 12 pints constantly in the air
vesicles) and nearly 8 pints of blood; but in the space of twenty-four
hours, upwards of 57 hogsheads of air and 24 hogsheads of blood.
430. Provision cannot have been made for bringing into contact such
immense quantities of air and blood, unless important changes are to be
produced in both fluids; and accordingly it is found that the air is
essentially changed by its contact with the blood, and the blood by its
contact with the air.
431. Chemistry has demonstrated the changes effected in the air.
Common atmospheric air is a compound body, consisting of pure air and
of certain substances diffused in it. Pure air is composed of two
gases, azote and oxygen, always combined in fixed proportions. The
substances diffused in pure air, and which are in variable quantity,
are aqueous vapour and carbonic acid gas. These latter substances form
no part of the chemical agents essentially concerned in the process of
respiration. The only constituents of the air which are essentially
concerned in the process of respiration are the two gases, azote and
oxygen, the union of which, in definite proportions, constitutes pure
air. But of these two gases each does not perform the same part in the
function of respiration, nor is each equally necessary to the support
of life.
432. If a living animal be placed in a vessel full of atmospheric
air, and if all communication of the atmosphere with the vessel be
prevented, the animal in a given time perishes. If an animal be placed
in a vessel full of azote, after a given time it equally perishes;
but if an animal be placed in a vessel full of oxygen, not only is
the function of respiration carried on with far greater energy than
in atmospheric air, but the animal lives a much longer time than in
the same bulk of the latter fluid. If twenty cubic inches of pure
oxygen be capable of sustaining the life of an animal for the space of
fourteen minutes, it can support life in the same bulk of atmospheric
air only six minutes; and if its respiration be confined to either of
these gases, after they have been already respired by another animal
of the same species, the former will live only four minutes; that is,
not longer than when entirely deprived of air. It follows that the gas
which gives to atmospheric air its chief power of sustaining life is
oxygen.
433. Accordingly it is proved that no animal, from the lowest to the
highest, is capable of sustaining life unless a certain proportion of
oxygen be present in the fluid which it respires. Whether it breathe
by the skin, by gills, or by lungs, whether the respiratory medium be
water or air, the presence of oxygen is alike indispensable. Yet the
life of no animal can be sustained by pure oxygen. If azote be not
mixed with oxygen, evils are produced in the economy which sooner or
later prove fatal. On the other hand, if the proportion of oxygen
be diminished beyond a certain point, drowsiness, torpor, and death
result. Not oxygen alone, then, but oxygen combined with azote, in the
proportion in which nature has united these two fluids to form the
atmosphere of the globe, is indispensable to animal existence.
434. When the same portion of atmospheric air is repeatedly respired
by an animal, the oxygen contained in it gradually disappears, the gas
lessening with every successive respiration, until at last so small a
quantity remains that it is no longer capable of sustaining the life of
an animal of that class. When respiration has deprived the air of its
oxygen to such an extent, that it can no longer support animal life,
the air is said to be consumed; but, correctly speaking, it is merely
changed in composition, in the proportions in which its constituents
are combined; consequently the effect of respiration is to alter the
chemical composition of the air.
435. The essential change that takes place consists in the diminution
of the oxygen and the increase of the carbonic acid. When inspired,
atmospheric air goes to the lungs loaded with oxygen; when expired, it
returns loaded with carbonic acid. That the air which returns from the
lungs is loaded with carbonic acid, may be rendered manifest even to
the eye. If a person breathe through a tube into water holding lime in
solution, the carbonic acid contained in the expired air will unite
with the lime and form a white powder analogous to chalk (carbonate of
lime), which being insoluble, becomes visible.
436. On the other hand, the diminution of oxygen is demonstrated by
chemical analysis. If 100 parts of atmospheric air be successively
respired, until it is no longer capable of supporting life, and if
it be then subjected to analysis, it is found that in place of being
composed of 79 parts azote, 21 oxygen, and a variable quantity of
carbonic acid, sometimes amounting to half a grain per cent., it
consists of 77 parts azote, and 23 carbonic acid. The oxygen is gone,
and is replaced by 23 parts of carbonic acid; at least this is the
ordinary estimate; but different experimentalists differ somewhat in
their account of the absolute quantity of oxygen that disappears, and
of carbonic acid that is generated.
437. Whatever estimates of the oxygen consumed, and of the carbonic
acid generated, be adopted, they can be taken only as medium
quantities. Dr. Edwards has demonstrated that the absolute quantity
of oxygen consumed in a given time is constantly varying, not only
in animals of different species, but even in the same animal under
different circumstances; insomuch, that there are scarcely two hours
in the day in which the same individual expends precisely the same
quantity. The nature and degree of the exercise taken during the
observation, the condition of the mind, the state of the health,
the kind of food, the temperature of the air, and innumerable other
causes materially influence the quantity of oxygen consumed. When,
for example, the hourly consumption of oxygen, at the temperature
of 54° Fahrenheit, amounted to 1345 cubic inches,[1] it fell, at
the temperature of 79°, to 1210 cubic inches. During the process of
digestion more is consumed than when the stomach is empty; more is
required when the diet is animal than when it is vegetable, and more
when the body and mind are active than when at rest.
[1] The ordinary consumption of oxygen is, for an adult, 1905 cubic
inches per hour (444).
438. With regard to the carbonic acid, Dr. Prout has recently made the
remarkable discovery, not only that the generation of this gas differs
according to different circumstances, and more especially according
to particular states of the system; but that the quantity of it which
is produced regularly varies at particular periods of the day. The
quantity generated is always more abundant during the day than during
the night. About daybreak it begins to increase; continues to do so
until noon, when it comes to its maximum, and then decreases until
sunset. The maximum quantity generated at noon exceeds the minimum by
about one-fifth of the whole. If from any cause the relative quantity
be either increased or diminished above or below the ordinary maximum
or minimum, it is invariably diminished or increased in an equal
proportion during some subsequent diurnal period. The absolute quantity
generated is materially diminished by the operation of any debilitating
cause, such as low diet, protracted fasting, or long-continued
exercise, depressing passions and the like. Few circumstances of any
kind increase the quantity produced, and those only in a slight degree.
439. The changes produced by respiration on the other constituent of
the air, azote, appear at first view to be extremely variable. By
numerous and accurate experiments it is established that the quantity
of this gas is at one time increased; at another diminished, and at
another unchanged. It is probable that there is a constant absorption
and exhalation of it; and that the apparent irregularity is the result
of the preponderance of the one process over the other. When absorption
preponderates, a smaller quantity is found in the air expired than
in that inspired: when exhalation preponderates, a larger quantity
is expired than inspired; and when the absorption and exhalation are
equal, just as much is expired as inspired, and consequently there
appears to be no absorption at all.
440. Such are the phenomena of respiration, as far as the labours of
physiologists has succeeded in ascertaining them, up to the present
time. But as the estimates of the quantity of air and blood contained
in the lungs were rather matters of conjecture than of demonstration,
and as the quantity of oxygen consumed, of carbonic acid generated, and
of azote absorbed, appeared still not to be determined with exactness,
I requested Mr. Finlaison to apply his power of calculation to the
investigation of this subject, taking as the basis of his calculations
the facts positively and precisely ascertained by experiment and
analysis. This he has done with great care, and has obtained the
following results.
441. It was formerly estimated that the weight of pure atmospheric air
is 305,000 grains troy for one million of cubic inches; but the latest
authorities assign it to be 310,117 grains. Of this weight of one
million of cubic inches of pure air,
The weight of the oxygen is 71,809.3
The weight of the azote is 238,307.7
—————————
Total 310,117.0
442. But common atmospheric air in its ordinary state contains in 1000
cubic inches,
Of pure air 989
Of the vapour of water 10
Of carbonic acid gas 1
Ten inches of pure air are equal in weight to nine of oxygen.
Eight inches of azote are equal in weight to seven of oxygen.
The specific gravity of carbonic acid is to pure air at the rate of
15,277 to 10,000.
The specific gravity of the vapour of water is to pure air as 6,230
to 10,000. It follows that a million of cubic inches of air in its
ordinary state weigh 309,111½ grains.
Carbonic acid gas is composed of oxygen and pure carbon in the
proportion of eight grains of oxygen to three of carbon out of every
eleven grains of carbonic acid.
443. Though during particular portions in the twenty-four hours, under
circumstances which influence variously the actions of life (437 and
438), the quantity of the oxygen consumed, of carbonic acid generated,
and of azote absorbed, vary (436 to 439), yet it is probable that
the daily consumption, reproduction, and absorption of these gases,
is pretty much the same one day with another. The experiments of
Dr. Edwards clearly show that while these quantities vary to such
an extent, when the observation embraces only a short interval, as
to be scarcely ever the same hour by hour, yet that they lessen as
the interval extends, until at length a nearly exact equilibrium is
established.
444. Experimental philosophers have not obtained precisely the
same results as to the quantities consumed and reproduced of these
respective gases. At present, therefore, we can only approximate to
the exact amount by taking the average of their observations. The
following are the results of the principal experiments which have
been instituted. The quantity of oxygen consumed by an adult man in
twenty-four hours is, according to
Menzes 51,840
Lavoisier 46,048
Davy 45,504
Allen and Pepys 39,534
The mean of all which is, 45,731.5 inches.
445. In like manner the quantity of carbonic acid generated in the same
time is, according to
Davy 38,304 cubic inches.
Allen and Pepys 38,232 “
The mean of which is, 38,268 “
The weight of 38,268 inches of carbonic acid gas is 18,130.1474 grains
troy; and the weight of 45,731½ inches of oxygen is 15,757.9131 grains
troy.
Now this weight of oxygen must have been derived from the decomposition
of 221,882 cubic inches of common atmospheric air.
446. It has been shown that, in the state of health, one contraction
of the heart propels to the lungs two ounces of blood; that this
action of the heart is repeated 72 times in one minute; that to every
four actions of the heart there is one action of respiration; that
consequently there are 18 respirations in a minute, and 25,920 in the
twenty-four hours.
447. From these premises it results that at each action of the heart
there is decomposed of the air inspired, 8.5603 cubic inches, that is,
a quarter of a pint within one-tenth of a cubic inch,—the quarter of a
pint imperial measure being 8.6648 cubic inches.
448. Previous observation had assigned one pint as the volume of
air ordinarily inhaled at a single inspiration. We now see that the
quantity decomposed is a quarter of a pint. It is, then, an absolute
truth, that of the whole volume of air inspired, one-fourth part only
is decomposed, and that three-fourths, after having been diffused
through the air vesicles of the lungs, are expired without change.
449. Observation had also assigned 12 pints
of air as the volume constantly present in the
lungs,—that is, 415.9108 cubic inches.
The truth seems to be,
that forty-eight times the
quantity decomposed is
constantly present, namely, 410.8926 cubic inches.
The difference is only 4.0182 cubic inches,
which difference weighs less than 1¼ grains troy.
450. It is then concluded that the real contents of the lungs is a
volume of 410.8926 cubic inches, which is exactly the 540th part of
221,882 cubic inches, being the whole volume decomposed in twenty-four
hours. But 160 seconds is also exactly the 540th part of the number of
seconds in twenty-four hours.
451. Of the whole weight of oxygen consumed
in twenty-four hours 15,757.9131 grains,
the 540th part, or the proportion
of 160 seconds, is 29.18132 “
and 410.8926 cubic inches of
atmospheric air, which, as
above, is the contents of the
lungs, contain of oxygen the
same weight 29.18132 “
452. Then, if respiration were suddenly stopped, provision is made by
the quantity of air always retained in the lungs for the oxygenation of
the blood while flowing at the ordinary rate of 72 strokes per minute,
for the exact space of 160 seconds, and for not one instant longer.
453. This interval of time, then, as has been stated (426), is very
probably the time in which the blood performs one circuit, not 150
seconds. Then 540 circuits are performed in the twenty-four hours, or
3 circuits in every eight minutes. From this estimate has been deduced
the quantity of blood contained in the whole body of the human adult
(428).
454. The air inspired in twenty-four hours contains as under:—
Bulk in Weight in Ingredients.
cubic inches. grains troy.
Undecomposed, and to be
returned unchanged 665,646 205,758.833, Common air,
To be decomposed,
containing in solution
{ Pure atmospheric air 219,441 { 15,757.913, Oxygen,
{ { 52,294.509, Azote,
{ Vapour of water 2,219 428.726, Vapour,
{ Carbonic acid gas 222 105.130, Carbonic acid,
Total 887,528 274,345.111, Of all kinds.
This is, in bulk, 25,607¼ imperial pints, or 57 hogsheads, 1 gallon,
and 7¼ pints, and in weight 571½ ounces and 25 grains.
455. Now, although the air expired, in consequence of its
recomposition, may have undergone changes in bulk, yet it seems
agreeable to all analogy to suppose that its weight will remain the
same as the weight inhaled. This, however, is not asserted as a truth,
but only assumed, in order to show the result of such a theory.
456. Then the air expired in twenty-four hours will be as follows:—
Bulk in Weight in
cubic inches. grains troy.
Given out undecomposed
as before 665,646 205,758.833
Recomposed carbonic
acid gas 38,268 18,130.147
Azote liberated 165,927 50,027.405
Vapour of water as before 2,219 428.726
——————— ——————————-
Total 872,060 274,345.111
weighing as before, but less in bulk by 446¼ pints: so that for every
100,000 inches expired there were inspired 101,774 cubic inches.
457. When from the weight of
carbonic acid gas thus expired, viz., 18,130.147
we deduct the small portion inhaled
in solution with the air 105.130
——————————
The remainder is 18,025.017
The constituent parts of which are,
oxygen derived from the air 13,109.104
——————————
And pure carbon derived from the
blood being the difference 4,915.913
Thus in the compass of twenty-four hours the blood has produced 10
ounces and 116 grains very nearly of pure carbon.
458. Now, from the oxygen consumed Grains.
in twenty-four hours as above 15,757.913
Deduct the weight restored in the
form of carbonic acid gas 13,109.104
——————————
The remainder must have been absorbed
into the blood 2,648.809
But the weight of carbon given out
being as above 4,915.913
—————————
There is still an excess given outweighing 2,267.104
459. Some azote, however, is absorbed into the blood (439) as well as
the above ascertained quantity of oxygen.
The weight of azote so absorbed must
be precisely 2,267.104
if the theory be true, that equal weights
are expired and inspired. In
which case, as the weight of the
azote of the air inspired was, as
shown above 52,294.509
While the azote expired could only
have weighed 50,027.405
—————————-
The difference would have been absorbed 2,267.104
And thus the weight of carbon discharged by the blood is precisely
compensated by the united weight of the oxygen and azote which it has
absorbed.
460. Since it appears to be a general truth that one quarter of the air
respired is decomposed, and that the volume of air continually present
in the lungs is sufficient for that consumption of oxygen which is
requisite in 160 seconds of time, _if that volume be_, as is apparent,
48 _times the quantity decomposed_ out of a single respiration, no
error in the quantity of oxygen consumed in the twenty-four hours,
which we have assumed, will affect the time of 160 seconds. For there
being 18 × 60 × 24 respirations, and 60 × 60 × 24 seconds of time in
the twenty-four hours, the 48th part of the first, and the 160th part
of the last product is equally the 540th part of the whole, whatever it
may be.
461. But if the time in which a circuit of the blood is performed
be, as is most evident, identical with the time in which the whole
volume of air in the lungs is decomposed, and if such period of time
were, as the old observers have assigned, 150 seconds, then it would
follow that only 45 times the quantity of air decomposed at a breath
is present in the lungs, amounting to 385¼ cubic inches, and that the
whole blood in the body is 24 ounces less than on the supposition
of 160 seconds, that is to say, only 360 ounces, or 22½ pounds
avoirdupois. Because the 45th part of 18 × 60 × 24 is the same as the
150th part of 60 × 60 × 24; in each it is the 567th part of the whole.
462. From the whole of these observations and calculations the
following general results are deduced:—
1. The volume of air ordinarily present in the lungs is very nearly
twelve pints (449).
2. The volume of air received by the lungs at an ordinary inspiration
is one pint (422).
3. The volume of air expelled from the lungs at an ordinary expiration
is a very little less than one pint (456).
4. Of the volume of air received by the lungs at one inspiration, only
one-fourth part is decomposed at one action of the heart (447).
5. The fourth part of the volume of air received by the lungs at
one inspiration, and decomposed at one action of the heart, is so
decomposed in the five-sixth parts of one second of time (429.3).
6. The time in which a circuit of blood is performed is identical with
the time in which the whole volume of air in the lungs is decomposed
(461).
7. The whole volume of air decomposed in twenty-four hours is 221,882
cubic inches, exactly 540 times the volume of the contents of the
lungs; 160 seconds being also exactly the 540th part of the number of
seconds in twenty-four hours (450).
8. The quantity of the blood that flows to the lungs to be acted upon
by the air at one action of the heart is two ounces (425).
9. This quantity of blood is acted upon by the air in the five-sixth
parts of one second of time (429.3).
10. One circuit of the blood is performed in 160 seconds of time. Three
circuits are performed every eight minutes; 540 circuits are performed
in the twenty-four hours (453).
11. The quantity of blood in the whole body of the human adult is 24
pounds avoirdupois, or 20 pints imperial measure (428).
12. In the space of twenty-four hours, 57 hogsheads of air flow to the
lungs (429.7).
13. In the same space of time 24 hogsheads of blood are presented in
the lungs to this quantity of air (424.10).
14. In the mutual action that takes place between these quantities of
air and blood, the air loses 15,757.9131 grains, or 328¼ ounces of
oxygen, and the blood 10 ounces and 116 grains of carbon (445).
15. The blood, while circulating through the lungs, permanently retains
and carries into the system—of oxygen, 2,648,809 grams; and of azote,
2,267,104 grains (458).
16. The ultimate results are two:—
1st. While the chemical composition of the blood is essentially
changed, its weight amidst all these complicated actions is maintained
steadily the same; for the weight of carbon which is discharged by the
blood is precisely compensated by the united weight of the oxygen and
azote which it absorbs (459).
2ndly. The distribution of quantities is universally by proportions
or multiples. Thus, of the air inspired, one measure is decomposed
and three measures are returned unchanged: of the air decomposed at a
single inspiration, there are always in store in the lungs precisely
forty-eight measures; and so on in many other cases. The proportions
are not arithmetical, but geometrical. When we compare arithmetical
quantities with each other, we say that one quantity is by so much
greater than another; when we compare geometrical quantities, we say
that one quantity is so many times greater than another. From this
adoption in the distribution of quantities of geometrical proportions
it results that whatever be the size of the animal the ratios remain
uniformly the same, and that thus one and the same law is adapted to
the vital agencies of living beings under every possible diversity of
magnitude and circumstance.
463. Such are the interesting and important properties and relations
deducible from the phenomena of respiration. The disappearance of
oxygen and azote from the air inspired, and the replacement of the
oxygen that disappears by the production of carbonic acid, and of the
azote by the exhalation of azote, in which, as we have seen, the great
changes wrought by respiration on the air consist, are essentially the
same in all animals, whatever the medium breathed, and whatever the
rank of the animal in the scale of organization. In all, the proportion
of the oxygen of the inspired air is diminished;—in all, carbonic acid
gas is produced. Comparing, then, the ultimate result of the function
of respiration in the two great classes of living beings, it follows
that the plant and the animal produce directly opposite changes in the
chemical constitution of the air. The carbonic acid produced by the
animal is decomposed by the plant, which retains the carbon in its own
system and returns the oxygen to the air. On the other hand, the oxygen
evolved by the plant is absorbed by the animal, which in its turn
exhales carbonic acid for the re-absorption of the plant.
464. Thus the two great classes of organized beings renovate the air
for each other, and maintain it in a state of perpetual purity. The
plant, it is true, absorbs oxygen during the night as well as the
animal; but the quantity which it gives off in the day more than
compensates for that which it abstracts in the absence of light. This
interesting fact has been recently established by an extended series
of experiments instituted by Professor Daubeney[2] for the express
purpose of investigating this point.
[2] On the Action of Leaves upon Plants, and of Plants upon the
Atmosphere, by Charles Daubeney, M.D. F.R.S., Professor of Chemistry
and Botany in the University of Oxford. Philosophical Transactions of
the Royal Society of London, for the year 1836. Part I. succession:
the amount of oxygen now evolved was increased from twenty-one to
thirty-nine per cent., and probably had not even then attained the
limit to which the increase of this constituent might have been
brought. From the proportions of the constituent elements of carbonic
acid gas (442) it necessarily follows that, by the mere process of
decomposition, out of every eleven grains of carbonic acid gas eight
grains of oxygen must be liberated, three grains of carbon being
retained by the plant, and consequently that eight grains of oxygen
must be restored to the atmosphere, less only by so much as the plant
itself may absorb. How great, then, must be the production of oxygen
by an entire tree under favourable circumstances; that is, when animal
respiration and animal putrefaction present to it an abundant supply
of carbonic acid on which to act!
465. From the general tenor of these experiments, it appears that,
in fine weather and as long as the plant is healthy, it adds to the
atmosphere an amount of oxygen not only sufficient to compensate for
the quantity it abstracts in the absence of light, but to counterpoise
the effects produced by the respiration of the whole animal kingdom.
The result of one of these experiments will convey some conception
of the amount of oxygen evolved. A quantity of leaves about fifty in
number were enclosed in a jar of air; the surface of all the leaves
taken together was calculated at about three hundred square inches; by
the action of these leaves on the carbonic acid introduced into the
jar, there was added to the air contained in it no less than twenty-six
cubic inches of oxygen. As there was reason to conclude that the
evolution of oxygen, in the circumstances under which this experiment
was performed, was considerably less than it would have been in the
open air, several plants were introduced into the same jar of air in
pretty quick
466. This influence, says Professor Daubeney, is not exerted
exclusively by plants of any particular kind or description. I have
found it alike in the monocotyledonous and dycotyledonous; in such as
thrive in sunshine and those which prefer the shade; in the aquatic
as well as in those of a more complicated organization. How low in
the scale of vegetable life this power extends is not yet exactly
ascertained; the point at which it stops is probably that at which
there ceases to be leaves.
467. From the whole, then, it appears that the functions of the plant
have a strict relation to those of the animal; that the plant, created
to afford subsistence to the animal, derives its nutriment from
principles which the animal rejects as excrementitious, and that the
vegetable and animal kingdoms are so beautifully adjusted, that the
very existence of the plant depends upon its perpetual abstraction of
that, without the removal of which the existence of the animal could
not be maintained.
468. The changes produced upon the blood by the action of respiration
are no less striking and important than those produced upon the air.
The blood contained in the pulmonary artery, venous blood (fig.
140-7.), is of a purple or modena red colour: the moment the air
transmitted to the blood by the bronchial tubes comes into contact with
it, in the rete mirabile (fig. 140-10.), this purple blood is converted
into blood of a bright scarlet colour. Precisely the same change is
produced upon the blood by its contact with the air out of the body.
If a clot of venous blood be introduced into a vessel of air, the clot
speedily passes from a purple to a scarlet colour; and if the air
contained in the vessel be analyzed, it is found that a large portion
of its oxygen has disappeared, and that the oxygen is replaced by a
proportionate quantity of carbonic acid. If the clot be exposed to pure
oxygen, this change takes place more rapidly and to a greater extent;
if to air containing no oxygen, no change of colour takes place.
469. The elements of the blood upon which a portion of the air exerts
its action are carbon and hydrogen. The oxygen of the air unites with
the carbon of the blood and forms carbonic acid, and this gas is
expelled from the system by the action of expiration. The constituent
of the blood which affords carbon to the air would appear to be chiefly
the red particles. The other portion of the oxygen of the air unites
with the hydrogen which is expelled with the carbonic acid in the form
of aqueous vapour. The direct and immediate effect of the action of
respiration upon the blood is then to free it from a quantity of carbon
and hydrogen.
470. Physiologists are not agreed whether the union of the oxygen of
the air with the carbon of the blood takes place in the lungs or in the
system. Some experimentalists maintain that the oxygen which disappears
from the air, and that which is contained in the carbonic acid, are
exactly equivalent, so that no oxygen can be absorbed. According to
this view, which has been clearly shown to be incorrect (459), the
effect of respiration is merely to burn the carbon of the blood, just
as the oxygen of the air burns wood in a common fire, the result
of this combustion being the generation of carbonic acid, which is
expelled from the system the moment it is formed.
471. The theory of Dr. Crawford is essentially the same, which supposes
that venous blood contains a peculiar compound of carbon and hydrogen,
termed _hydro-carbon_, the elements of which unite in the lungs with
the oxygen of the air, forming water with the one and carbonic acid
with the other. Mr. Cooper, for many years past, has taught the same
doctrine in his lectures, without any knowledge of the fact that
Crawford had suggested a similar modification of his theory.
472. It is now established that more oxygen disappears than is
accounted for by the amount of carbonic acid that is generated. The
experiments of Dr. Edwards had already shown this in so decisive
a manner that physiologists almost universally admitted it as an
ascertained fact. The calculations of Mr. Finlaison, to whom the
opinions of physiologists on this point were unknown, have now
determined the precise amount of oxygen (444 _et seq._), and the
probable amount of azote (459) absorbed. By many physiologists it is
supposed that the oxygen retained by the lungs, as long as it remains
in this organ, enters only into a state of loose combination with the
blood; that in this state of loose combination, it is carried from
the lungs into the general system; and that it is only in the system
that the union becomes intimate and complete. According to this view,
the lungs are merely the portal by which the substances employed in
respiration are received and discharged, the essential changes induced
taking place in the system. That it is through the lungs that the
oxygen required by the system is received, is an opinion founded on
experiments no less exact than decisive; it is in accordance with the
most probable theory of the production and distribution of animal heat
(chap. ix.); and the preponderance of evidence in its favour is so
great that, in the present state of our knowledge, it may be considered
as established; but it will appear hereafter that the lungs are by no
means passive in the process, and that, physiologically considered,
they as truly constitute a gland secreting carbonic acid gas as the
liver is a gland secreting bile.
473. Such are the main facts which have been ascertained relative to
respiration, as far as this function is performed by the lungs. But
the liver is a respiratory organ as well as the lungs. It decarbonizes
the blood. It carries on this process to such an extent, that some
physiologists are of opinion that the liver is the chief organ by
which the decarbonization of the blood is effected. The following
considerations show that whatever be the relative amount of its action,
the liver powerfully co-operates with the lungs in the performance of a
respiratory function.
1. The liver, like the lungs, is a receptacle of venous blood; blood
loaded with carbon. The great venous trunk which ramifies through the
lungs is the pulmonary artery, containing all the blood which has
finished its circuit through the system. The great venous trunk which
ramifies through the liver is the vena portæ, containing all the blood
which has finished its circuit through the apparatus of digestion. The
liver is a secreting organ, distinguished from every other secreting
organ by elaborating its peculiar secretion from venous blood. Carbon
is abstracted from the venous blood that flows through the lungs in the
form of carbonic acid; carbon is abstracted from the venous blood that
flows through the liver in the form of bile.
2. All aliment, but more especially vegetable food, contains a large
portion of carbon, more it would appear than the lungs can evolve. The
excess is secreted from the blood by the liver, in the form of resin,
colouring matter, fatty matter, mucus, and the principal constituents
of the bile. All these substances contain a large proportion of carbon.
After accomplishing certain secondary purposes in the process of
digestion, these biliary matters, loaded with carbon, are carried out
of the system together with the non-nutrient portion of the aliment.
In the decarbonizing process performed by the lungs and the liver,
the chief difference would seem, then, to be in the mode in which the
carbon that is separated is carried out of the system. In the lungs
it is evolved, as has been stated, in union with oxygen in the form
of carbonic acid; in the liver, in union with hydrogen in the form of
resin and fatty matter.
3. Accordingly, in tracing the organization of the animal body from the
commencement of the scale, it is found that among the distinct and
special organs that are formed, the liver is one of the very first. It
would appear to be constructed as soon as the economy of the animal
requires a higher degree of respiration than can be effected by the
nearly homogeneous substance of which, very low down in the scale,
the body is composed. Invariably through the whole animal series, the
magnitude of the liver is in the inverse ratio to that of the lungs.
The larger, the more perfectly developed the lungs, the smaller the
liver; and conversely, the larger the liver the smaller and the less
perfectly developed the lungs. This is so uniform that it may be
considered as a law of the animal economy. In the highly organized
warm-blooded animal, with its large lungs, divided into numerous lobes,
and each lobe composed of minute vesicles respiring only air, the
magnitude of the liver compared with that of the body is small. In the
less highly organized animal of the same class, with its smaller and
less perfectly developed lung, respiring partly air and partly water,
the liver increases as the lung diminishes in size. In the reptile
with its little vesicular lung, divided into large cells, the liver is
proportionally of greater magnitude. In the fish which has no lung,
but which respires by the less highly organized gill, and only in the
medium of water, the proportionate size of the liver is still greater;
but in the molluscous animal, in which the lung or the gill is still
less perfectly developed, the bulk of the liver is prodigious.
4. In all animals the quantity of venous blood which is sent to the
liver increases, as that transmitted to the lung diminishes. In the
higher animal the great venous trunk which ramifies through the liver
(the vena portæ) is formed by the veins of the stomach, intestines,
spleen, and pancreas, which are the only organs that transmit their
blood to the liver. In the reptile, besides all these organs, the hind
legs, the pelvis, the tail, the intercostal veins forming the vena
azygos and in some orders of this class, even the kidneys also send
their blood to the liver; but in the fish, in addition to all the
preceding organs, the apparatus of reproduction likewise transmits its
blood to the liver. The very formation of the venous system in the
different classes of animals seems thus to point to the liver as a
compensating and supplementary organ to the lung.
5. The permanent organs in the lower animal are a type of the
transitory forms through which the organs of the higher animal pass
in the progress of their growth. Thus the liver of the human fœtus is
of such a disproportionate size, as to approximate it closely to that
of the fish or of the reptile. After the birth of the human embryo,
respiration is effected in part by the lung; but before birth the lung
is inactive, no air reaches it; it contributes nothing to respiration;
the decarbonizing action of the blood is accomplished, not by the lung,
but by the liver; hence the prodigious bulk of the fœtal liver and its
activity in the secretion of bile, and especially towards the latter
months of pregnancy, when all the organs are greatly advanced in size
and completeness.
6. Pathology confirms the evidence derived from comparative anatomy and
physiology. When the function of the lung is interrupted by disease,
the activity of the liver is increased. In inflammation of the lung
(pneumonia); in the deposition of adventitious matter in the lung
(tubercles), by which the air vesicles are compressed and obliterated,
the lung loses the power of decarbonizing the blood in proportion to
the extent and severity of the disease with which it is affected. In
this case the secretion of bile is increased. In diseases of the heart
the liver is enlarged. In the morbus cæruleus (516) the liver retains
through life its fœtal state of disproportion.
7. In the last place, there is a striking illustration of the
respiratory action of the liver, in the vicarious office which it
performs for the lung, during the heat of summer in cold, and all the
year round in hot climates. In the heat of summer, and more especially
in the intense and constant heat of a warm climate, in consequence of
the rarefaction of the air, respiration by the lung is less active
and efficient than in the winter of the cold climate. During the
exposure of the body to this long-continued heat, there is a tendency
to the accumulation of carbon in the blood. An actual accumulation is
prevented, by an increased activity in the secretion of bile, to which
the liver is stimulated by the heat. In order to obtain the material
for the formation of this unusual quantity of bile, it abstracts
carbon largely from the blood; to this extent it compensates for the
diminished efficiency of the lung, and thus removes through the vena
portæ that superfluous carbon which would otherwise have been excreted
through the pulmonary artery.
474. Taking life in its most extended sense, as comprehending both the
circles it includes, the organic and the animal (vol. i. chap. 2), it
may be said to have three great centres, of which two relate to the
organic, and the third to the animal life (vol. i. chap. 2). The two
centres which relate to the organic life are the systems of respiration
and circulation; the third, which relates to the animal life, is the
nervous system. Of the organic life, the lungs and the heart are the
primary seats; of the animal, the brain and the spinal cord. Between
each the bond of union is so close, that any lesion of the one
influences the other, and neither can exist without the support of
all. They form a triple chain, the breaking of a single link of which
destroys the whole.
475. But of these three great centres of life, upon which all the
other vital phenomena depend, the most essential is respiration; hence,
to consider the relation of this function to the others, is to take the
most comprehensive view of the uses which respiration serves in the
economy.
476. The first and most important use of the function of respiration
is to maintain the action of the organs of the animal life. It has
been shown (vol. i. chap. 2) that the organic is subservient to the
animal life, and that to build up the apparatus of the latter, and to
maintain it in a condition fit for performing its functions, is the
final end of the former. The direct and the immediate effect of the
suspension of respiration is the abolition of both functions of the
animal life—sensation and voluntary motion. If a ligature be placed
around the trachea of a living animal so as completely to exclude all
access of air to the lungs, and if the carotid artery be then opened,
and the blood allowed to flow, the bright scarlet-coloured blood
contained in the artery is observed gradually to change to a purple
hue. The exact point of time at which this change begins may be noted.
It is seen to assume a darker tinge at the end of half a minute; at
the end of one minute its colour is still darker, and at the end of
one minute and a half, or at most two minutes (426), it is no longer
possible to distinguish it from venous blood. As soon as this change of
colour begins to be visible the animal becomes uneasy; his agitation
increases as the colour deepens; and when it becomes completely dark,
that instant the animal falls down insensible. If in this state of
insensibility air be readmitted to the lungs, the dark colour of the
blood rapidly changes to a bright scarlet, and instantly sensation
and consciousness return. But if, on the contrary, the exclusion of
the air be continued for the space of three minutes from the first
closing of the trachea, the animal not only remains to all appearance
dead, but in general no means are capable of recovering him from the
state of insensibility; and if the exclusion of the air be protracted
to four minutes, apparent passes into real death, and recovery is no
longer possible. It follows that one of the conditions essential to the
exercise of the function of the brain is, that this organ receive a due
supply of arterial blood.
477. The second use of the function of respiration is to afford
blood capable of maintaining the muscles in a condition fit for the
performance of their peculiar office, that of contractility. The
closure of the trachea not only abolishes sensation, but the power
of voluntary motion: sensation and motion are lost at once: on the
re-admission of air to the lungs, both functions are regained at
once: it follows that the process of respiration is as essential
to the action of the muscle as to that of the brain. “By arterial
blood,” says Young, “the muscles are furnished with a store of that
unknown principle by which they are rendered capable of contracting.”
“The oxygen absorbed by the blood,” says Spalanzani, “unites with
the muscular fibres and endows them with their contractility.” It
is more correct to say, respiration takes carbon from the blood and
gives it oxygen, and by this means endows the blood with the power of
maintaining the contractility of the muscular fibre.
478. But respiration is as essential to the action of the organs of
the organic life as to those of the animal. In a short time after
the respiration ceases, the circulation stops. When the blood is no
longer changed in the lungs, it soon loses all power of motion in the
system; because venous blood paralyses the muscular fibres of the heart
as of the arm. When the left ventricle of the heart sends out venous
blood to the system, it propels it into its own nutrient arteries,
as well as into the other arteries of the body; into the coronary
arteries, as well as into the other branches of the aorta; the heart
loses its contractility, for the same reason as every muscle under
the like privation; because venous instead of arterial blood flows in
its nutrient arteries; and the circulation stops when the heart is no
longer contractile, because the engine is destroyed that works the
current.
479. Venous blood consists of chyle, the nutritive fluid formed from
the aliment; of lymph, a fluid composed of organic particles, which
having already formed an actual part of the solid structures of the
body, are now returning to the lungs to receive a higher elaboration;
and of blood which, having completed its circuit through the system,
and there given off its nutrient and received excrementitious matter,
is now returning to the lungs for depuration and renovation. These
commingled fluids, on parting in the lungs with carbonic acid and
water, and on receiving in return oxygen and azote, are converted into
arterial blood; that is, blood more coagulable than venous, and richer
in albumen, fibrin, and red particles, the proximate organic principles
of all animal structures. The rich and pure stream thus formed is sent
out to the various tissues and organs, from which, as it flows to
them, they abstract the materials adapted to their own peculiar form,
composition, and vital endowments. By the reception of these materials
the organs are rendered capable of performing the vital actions which
it is their office to accomplish. And thus the processes of digestion,
absorption, secretion, nutrition, formation, reproduction, all the
processes included in the great organic circle, no less than muscular
action and nervous energy, depend on receiving a due supply of arterial
blood. All these actions, like the faculties of the animal life, cease
totally and for ever in a few minutes after the formation of this vital
fluid has been stopped by the suspension of respiration.
480. In the last place, the depurating process effected by respiration
is necessary to prevent the decomposition of the blood, and eventually
that of the body. The first step in the spontaneous decomposition
of animal matter consists in the loss of a portion of its carbon,
which, uniting with the oxygen of the atmosphere, forms carbonic
acid; precisely the same thing that takes place in the process of
respiration. The bodies of all animals, of worms, insects, fishes,
birds, and mammalia, deoxidate the air and load it with carbonic
acid after death, some of them nearly as much as during life; and
this before any visible marks of decomposition can be traced. It is
probable that the cause which more immediately operates in preventing
the decomposition of the body is the abstraction of a part of the
carbon of the blood; that were these carbonaceous particles allowed
to accumulate, they would produce a tendency to decomposition, which
would terminate in complete disorganization; and consequently, that one
main object of the process of respiration is to afford blood not only
capable of nourishing and sustaining the organs, but of maintaining
their integrity, by removing noxious matter, the presence of which
would subvert their composition and lead to their entire decomposition.
481. The ultimate object of respiration, then, is to prepare and to
preserve in a state of purity a fluid capable of affording to all the
parts of the body the materials necessary to maintain their vital
endowments. By the exhalation of oxygen and water, and the absorption
of carbon, under the agency of light, the plant elaborates such a fluid
from its nutritive sap, and out of this elaborated sap forms terniary
combinations, the organic elements of all vegetable solids. By the
absorption of oxygen and azote, and the exhalation of carbonic acid
and water, probably under the influence of electricity, conducted and
regulated by the nervous system, the animal elaborates such a fluid
from its aliment, and out of this elaborated fluid forms quaternary
combinations, albumen, and fibrin, the organic elements of all animal
solids.