D. S. (David Samuel) Margoliouth.

The Popular science monthly (Volume 19) online

. (page 81 of 110)
Online LibraryD. S. (David Samuel) MargoliouthThe Popular science monthly (Volume 19) → online text (page 81 of 110)
Font size
QR-code for this ebook

state control, while the support of a national Church is altogether be-
yond the sphere of national authority.



IN vertebrates alone is there a closed circulation — a complete system
of tubes from whence the blood never escapes into the body-cavity.
We find an approach to it in the higher mollusks. Indeed, in power
and general efficiency, the circulation of the highest mollusks is great-
ly superior to that of the low vertebrates. Nevertheless, the perfectly
closed circulatory system of even the lowest vertebrates is of higher
type. Although the circulating system of the vertebrates is perfected
in principle, it still admits of very great and curious modifications.

There exist in vertebrates three sets of capillary blood-vessels,
which are usually spoken of as three systems, although together they
constitute but a single circuit. They are distinguished as the body or
systemic circulation, the respiratory or pulmonary circulation, and the
liver or portal circulation. Connected Avith the blood-system by the
thoracic duct is the lymphatic circulation.

Tlie lymphatic system, which has previously been mentioned as the
second source of blood material, deserves some notice on account
of its intimate relation with the blood system of the vertebrates.
The lymphatics are minute capillary vessels, found in all parts of
the body of vertebrates, excepting, perhaps, the bulb of the eye, the
cartilages, and the bones. They unite to form, with the lacteals, the
thoracic duct, which was described in the article on digestion, in the
September number of the " Monthly."

The office of the lymphatics is to collect the waste matter of the
tissues and return it to the blood, to be again used elsewhere, or, if
wholly useless, to be excreted from the body. They also collect the
blood which may be poured upon the tissues in excess of their needs.
The fluid which the lymphatics carry is called lymph. It is color-
less, and contains corpuscles resembling the white corpuscles of the

The lacteals, which take the new food from the intestines, are
lyniphatics modified for a special purpose, and, when they are not busy
with the chyle, they also carry lymph.

The lymphatic tubes are provided with valves to keep the lymph
flowing toward the larger trunks.

* Concluded from page 468.



This lymphatic sysiem of the vertebrated animals is, however,
expressed in technical language, only the differentiated interstitial
sinuses of the lower animals, which has, in the latter, a share in the
venous circulation. Indeed, in the lower ver-
tebrates the lymphatic tubes frequently as-
sume the form of large sinuses, and connect
with the veins. They are even found in the
birds. In the frog four of these sinuses
have muscular walls, and rhythmically con-
tract. These are known as lymphatic hearts.

In various parts of the body the lym-
phatics form glands, such as the thymus,
thyroid gland, and the spleen.

Fishes have a heart resembling that of
the mollusks. It is a double force-pump,
consisting of a receiving-chamber (auricle),
and a propelling chamber (ventricle), with
all the valves necessary to prevent a back-
ward flow of the blood. But this heart is
respiratory —it sends the blood directly to the
breathing organs ; consequently, it passes
only impure blood. When the blood has
traversed the gills and is purified, it passes
around the circuit of the body through the
systemic and portal capillaries, and back to
the heart without any further propulsion.

The low, worm-like fish, lancelet, or am-
phioxus, has no special heart, but a number
of contractile bulbs in the veins. The eel
has such an auxiliary heart in its tail, while
the hag has the circulation aided by the contractility of the

Lepidosiren, one of the mud-fishes, approaches the amphibians in
tlie possession of two auricles ; for, in addition to gills, it has true
hmgs. The vein conveying the purified blood from the lungs joins
the left auricle.

Fig. 1.— Diagram of the CrecuLA-
TiON IS A Fisu. (The portion of
the system coiitaininErpnre blood
i!> black; the part containing im-
pure blood is white.) a. auricle,
receiving venous blood from the
biKiy; r. ventricle; w. bnibcsar-
teribans ; n. branchial artery, car-
rying venous blood to the gills
(h.b); c. aorta, carrying arteri-
alized blood to all parts of tlie


iG. 2. — Diagram op the Lancklkt (AmpfiiOTUfi). m, moalh, surrounded by cartilaginons cirri ;
p. greatlv-dilated pharynx, perforated by ciliated clefts; /, intestine ; a, anus ; A. blood-sys-
tem, with pulsating dilatations ; ch, notochord ; n, nervous cord.



Amphibians and reptiles exist under conditions incompatible with
a high temperature of the body. In the adult state they are air-
breathers, and, if their circulation were complete, they would be
" warm-blooded." But the temperature is subdued by imperfect cir-
culation, which results from the arrangement of the heart-chambers.
There is but one ventricle for the two auricles, hence the pure blood
from the breathing organs and the impure blood from the body are
mingled, so that, besides the venous and arterial, they have a mixed
blood. The blood which goes to the lungs is never wholly impure,
and that which goes to the body is never entirely pure. However, by
a complex and beautiful action of the parts and valves of the heart —
too complex to be here described — the mingling of venous and arterial
blood is not complete.

The change which the amphibians undergo in outward form, from
the tadpole or larval state to the frog-like condition, is accompanied

Pig. S.— Diagram op the Circulation in a Reptile. (The part contaiiiinp: pure blood is black ;
that containing impure blood is white : and that containing mixed blood is crosg-ehaded.) a,
light auricle, receiving impure blood from the body : «', left auricle, receiving pure blood from
the lunirs ; v. ventricle. contBining mixed blood, which is carried by the pulmonary artery (j))
to the lungs, and by the aorta (ol to the body.

Fig. 4.— Diagram of the Circli-ation of a Bird or Mammal. (The venous pystem is black :
the arterial system is white.) a, right auricle ; e, rieht ventricle ; /). pulmonary artery, carry-
ing venous blood to the lungs ; p p, pulmonary veins, carrying arterial blood frt)ra the lungs ;
o', left auricle ; v\ left ventricle ; 6, aorta, carrying arterial blood t« the body; c, vena cava,
carrying venous blood to the lungs.

by a remarkable inward change in the cii'culation. In the larval
stage, with respiration by gills, the heart and circulation resemble
that of the fishes — a single auricle and ventricle and complete purifi-
cation of the blood. But, as the gills disappear and the lungs devel-



op, and the blood is diverted from the former to the latter, there is a
corresponding change in the carrying capacity of the blood-vessels,
resulting in the final disappearance of the vessels connected with the
gills. Moreover, while the blood was not returned directly from the
gills to the heart, it is returned directly from the lungs, and a second
auricle is developed. But the aerial respiration of the frog, with its
mixed circulation, is more rapid than the aquatic respiration with the
])erfect circulation of the tadpole.

In the reptiles circulation is essentially the same as in the amphibi-
ans ; but the ventricle is more or less divided by a partition into two
chambers. This membranous partition is perfect only in the croco-
dile, where we find a right and left ventricle without communication,
and the heart structurally like that of a bird or mammal. But the
circulation is still the same as in the lower reptiles, for the pure and
impure blood are somewhat mingled by a communication between the
two arteries near their point of origin.

Although birds in their general organization are closely allied to
reptiles, their circulation is similar to that of the mammals. In these

Fio. 5.- MuscrLAR Fibers of" the Ventricxes. 1, -iiptrticinl flbor», common to both ventricles :
2. fibers of the iett ventricle : ."i. deep fliers, pussing upward toward the base of the heart ;
4. libers penetrating the left ventricle.

two highest classes of the animal kingdom, there are always two auri-
cles and two ventricles, and the right and left sides of the heart are
entirely distinct. Functionally these are two hearts : a systemic heart.


forcing pure blood to the body ; and a pulmonary, forcing impure
blood to the lungs. The pure and impure blood arc never mingled,
and all the blood has to pass through the lungs and be oxygenated
every time it makes the complete circuit. This perfect circulation,
with aerial respiration, produces more rapid chemical changes in
the blood and tissues, and consequently the higher temperature of
the " warm-blooded " animals. In the embryonic ctages of the heart,
the septum dividing the auricles is slowly formed, and an aper-
ture exists for a time, called the foramen ovale. Cases rarely occur
of human subjects in which the opening persists. Such persons are
physiologically reduced to the condition of a reptile. It is stated that
human infants have lived several days with a circulation as mixed as
that of a frog.

To economize space and muscular effort, these two hearts are
formed of the same circular muscles, and are inclosed by a lubricating
membrane called the pericardium. In the dugong, however, the two
ventricles are quite separate, showing a structural distinction corre-
sponding with the functional difference.

_ On account of the structural union, the

two hearts contract and dilate in unison, pro-
ducing the " beating " of the heart. The
cause of the first sound in the heart-boat is
uncertain, but it occurs at the time of the
ventricle contraction. The second sound is
produced when the ventricles dilr.te, by the
flapping back of the semilunar valves, those
placed at the origin of the arteries to pre-
vent the regurgitation of the blood.

Each half of the heart of birds or mam-
mals is, like the entire heart of the fishes, a

Heart ot^'t^z jy^o^2\HSi- doublc forcc-pump, with perfection of val

^E^."Ho^tKl":;IS'LTtoS and tubes and surpassing efficiency. The
vL^ride^zTie'ffvenfrkie""''' P^^^^ is enormous. It has been estimated
that, while an engine can lift its own weight
three thousand feet in an hour, and an active climber can ascend four
thousand feet, the human heart performs hourly a labor equal to lifting
its own weight twenty thousand feet. Its daily work is also estimated
at seventy-five thousand kilogramme-metres. We can otherwise gain
an idea of the power of Nature's enginery, by observing what the heart
actually performs. The quantity of blood in the human body is at least
six quarts. In its course it has to traverse many feet of tubing and
two sets of capillaries, and, notwithstanding the friction and loss of
power, all the blood completes the circulation in about thirty-two
heart-beats. We should further observe that the heart never rests,
but is ceaseless from birth to death. Its cessation is death.

The necessity of uninterrupted action of the heart requires that it


should be involuntary, and so its action is placed beyond our control.
It is said that an individual once lived who could stop for an instant,
at will, the beating of his heart. But, it is also stated in connection,
that he died as the result of a too successful attempt.

The flow of blood through the arteries by successive impulses is
facilitated by their branching at acute angles. Veins, on the contrary,
branch at greater angles, which is compatible with a steady and
slower flow. As the veins carry in any given time the same amount
of blood as the arteries, while the rate of flow is slower, it follows that
their diameter or capacity is greater.

The pressure of the blood-current diminishes from the heart. In
the carotid artery of man it is probably equal to the weight of one
hundred and fifty to two hundred cubic millimetres of mercury. The
pressure in the pulmonary artery is only thirty to forty cubic milli-

There is much disagreement among writers regarding the velocity
of the blood. In the carotid artery of the horse, it probably flows at
the rate of about three hundred millimetres per second; in the dog, at
the rate of three hundred to five hundred millimetres. The velocity
in the large arteries of man can hardly be over twenty inches per sec-
ond, but varies greatly at different times. The length of the capilla-
ries is about one half of a millimetre, and the blood passes through them
in about one second. In the human retina the corpuscles travel at the
rate of '75 millimetre per second. The small arteries pulsate within
one sixth of a second after the main trunks ; but the rate of flow is
much slower than the wave-progression.

In vertebrates, the rapidity of the circulation is generally propor-
tionate to the activity of the animal. The pulse of aerial birds is about
150 per minute; of the cat, 115; dog, 95; man, 72 ; ox, 35. But
this generalization does not hold with the invertebrates. Insects, the
most active of all creatures, have a very sluggish and imperfect circu-
lation, for in this class the air is so freely admitted into the body as to
obviate the necessity of great movement of the blood.

The human pulse is somewhat more rapid in childhood, and again
in old age ; slightly faster in the evening than in the morning, in
summer than in winter, and probably increases with geographical alti-
tude. In fever the circulation is very greatly and mysteriously quick-

All the blood of a man probably completes the round of the circu-
lation in about thirty-two heart-beats, or in less than half a minute.
The blood of a horse, it is estimated, completes the circuit in thirty
si'conds, that of a dog in fifteen, and that of a rabbit in seven sec-

The velocity of the blood decreases from the ventricles toward the
• apillaries, and then increases from the capillaries toward the auricles.
The velocity being necessarily the reverse of the carrying capacity, or


sectional area. The capillaries have a sectional area several times
that of the aorta, the purpose of this being to delay the blood at the
time it is brought into most intimate contact with the tissues.

The walls of the capillaries are of extreme tenuity, and easily per-
meable under the physical action called osmosis. Even the corpuscles
can pass outward through the walls.

Fig. 7.— Diagram of the Four Cavities op the Human Heart. d, right auricle ; r d, right
ventricle ; og, left auricle ; v g, left veutricle. The arrows indicate the course of the blood.

To what degree the heart is aided by other forces is yet a matter of
investigation. Probably there are several forces assisting. The elas-
ticity of the arteries increases their caiTying capacitj'. They are firm,
elastic tubes, which expand under the pressure from each heart-con-
traction, and then by their own elasticity contract and help the on-
ward flow of the blood. In the smaller arteries the flow loses the in-
termittent character it possesses in the larger arteries, and becomes a
steady stream. The elasticity of the arteries serves precisely the same
purpose as the air-chamber of any force-pump, that of equalizing the
flow, and so increasing the amount delivered. The whole force is de-
rived from the" heart : the arteries cause the force to act continuously.

The veins are lax tubes, somewhat larger than the arteries, and
capable of holding all the blood of the body. They convey the same
amount of blood as the latter, but more slowly. In the larger veins,
however, near the auricles, the velocity may be two hundred millimetres
per second. They are provided with valves which effectually prevent
the blood from flowing backward toward the heart. Any compres-
sion, produced by muscular contraction, or otherwise, will therefore
assist the forward flow of venous blood. This is one explanation uhy



exercise hastens the circulation. The movement of the chest in breath-
ing probably aids the pulmonary circulation, the blood, as well as
the atmosphere, tending to fill the vacuum during inspiration.

Physical capillary force is not generally regarded as an active
force in the circulation. But there is an admitted force in the capil-
laries, resulting from the attraction of the tissues for the arterial
blood, containing the required oxygen and nutriment. " The vital
condition of the tissue becomes a factor in the maintenance of the
circulation." It is this force, primarily, which adapts the amount of
blood to the varying needs of any organ ; the nervous system regu-
lates the supply by varying the caliber of the vessels.

The force in the capillaries, or some other force, carries the blood,
after death, from the arteries, where the heart leaves it, into the veins.

FiQ. 6 —Circulation iv the Web op a Frog's Foot. The black sputs, eome of them star-
Bhflped, are pisrment, or colorins^-matter. a, a venous trunk, composud of three principal
branches (6, 0. 6), and covered with a plexus of smaller vessels {c, c).

Finding the arteries empty after death gave rise to the idea that they
conveyed only air ; whence the name. It was this belief which Harvey
overthrew in 1620.

In bats the heart is aided by the rhythmic contraction of the veins
in the wing. Other accessory hearts of the lower animals have already
been mentioned. In some of the lowest creatures, the cause of the
circulation may be wholly the movements of the body, as in the jelly-
1ish and anemone ; or by cilia, as in the sponge, where the sea- water
iiiswers to blood.

Under the influence of nerve-force, the walls of the arteries and
•apillaries are usually somewhat contracted. The withdrawal of nerve
stimulus allows the tubes to relax, which consequently permits more
blood to pass through, or to accumulate, and perhaps add color to


the skin. This is the physiology of blushing. Congestion is produced
by a permanent expansion of the capillaries. "We might call blushing
a momentary congestion.

In an emergency the arteries are capable of great expansion. They
are connected by branches, or loops ; and, in case of stoppage of the
circulation in a large artery, either by disease or a surgical operation,
another will, after a time, perhaps a few hours, expand to a size requi-
site to carry sufficient blood. This variation in the carrying capacity
of the arteries is the important secondary means of adapting the
amount of blood to the wants of any part of the body.

In man there is a greater and more direct supply of blood to the
right arm, corresponding to the greater use of that limb. But, in
birds, equality of supply is necessary for the equality of strength
needed in steady flight.

For protection, the arteries are as deep-seated as jjossible, lying
beneath the muscles, and appearing rarely at the surface. At the
joints they form loops, so that the circulation may not be stopped by
compression of a single trunk. A fine example of adaptation is seen
in the arm of the lion, where the main artery, to be protected by the
]>o\verful muscles, passes through a perforation in the bone.



FEW realize the great practical importance of extreme accuracy
in standards of weight and extension, and it is not generally
known what degree of accuracy has been attained in the measure-
ment of the standards of length now in use in different countries.
The carpenter's foot-rule and the tailor's yard are familiar articles,
but, if asked, probably neither the carpenter nor the tailor could
tell whether there is any means by which the true length of a foot
or a yard can be determined. It is clear, howevei*, that there must
be a standard with which the common measures should be made to
agree, in order to have the same absolute value. But we may reflect
that the constant use of any measure will change its length, and that
it will eventually become worn out. We can, then, readily under-
stand the great value of an accepted standard, from which copies can
be made, thus preventing any gradual alteration in our measures.
Such standards of reference are properly held in the custody of
national governments, scientific societies, and institutions.

It is by no means a simple process to compare one measure with
another, and to determine the variation between the two. On the
contrary, the utmost skill and long experience are required for such


work, as well as the most elaborate and costly apparatus. Allowance
must be made for errors that are so small as to be almost inappreci-
able, but which can not be eliminated until they have been subjected
to future investigations of a very delicate nature. Every careful ob-
server will obtain results which are almost marvelously accordant inter
se ; but the results obtained by two observers, with different instru-
ments, will probably not agree. The "personal equation "has not
yet been eliminated from work of this kind.

In the comparison of weights and measures, science demands the
utmost accuracy, and it would not be possible, even if it were
desirable, in an article like this, to more than allude to a few of
the steps which have resulted in the final adoption of national and
international standards. Professor W. A, Rogers, of Harvard Ob-
servatory, has devoted himself to a critical study of measures of
length, and to him we are indebted for many observations on the sub-
ject, of great scientific importance, and for some very ingenious de-
vices for making accurate comparisons of spaces. He has recently
published a valuable contribution to the literature of the subject,* in
which the present state of the question of standards of length is dis-
cussed with considerable detail.

As the comparisons of measures of length are made with micro-
scopes, the results are affected by the magnification and by the method of
illumination employed. The measurements are made so carefully that
the standard metal bar upon which the graduations are made must
be carefully supported on rollers, mutually connected by a system of
levers, so that no flexure can take place, or else a bedplate must be so
carefully adjusted in an horizontal plane that no effect of flexure can
be detected. Professor Rogers has adopted the latter plan, and he
believes that no part of the bedplate of his comparator is more tlian
•00002 of an inch from the true level.

In constructing a standard, the shape of the bar and the material
of which it is composed require careful consideration. In a few cases,
standard bars seem to have undergone some molecular change by
which their length has been altered. As an example, we may cite this
instance : A Russian standard, which was used at one time in geodetic
surveys, after it had been transported for a distance of about eight
thousand miles, was found to have shortened in length by about one
six-thousandth of an inch. This bar was of iron, about seven feet

The influence of temperature upon the length of a metal bar is
very noticeable, when careful measurements are made. Xot long ago,
before our kno.vledge of this subject v/as as complete as it is now, it
was assumed that, if two bars Avere allowed to remain in a liquid
maintained at a certain temperature, they would soon acquire their

* " Proceedings of the American Academy of Arts and Sciences," vol. vii, New Series,
p. 273.


true length for that temperature. Is has lately been shown, however,
by the experiments of Professor Rogers, that if two steel bars, one of
which is nickel-plated, be subjected to a gradual change of tempera-
ture, they will acquire their true length after the temperature has been
maintained constant for about twelve hours ; but, if the change be an
abrupt one, it is not safe to compare them until after the lapse of
from forty-eight to sixty hours.

Enough has been said to indicate what great precautions must be
taken in order to obtain accurate copies of a given standard of length.
We may now consider how the standard measures at present in use
were originally obtained, and how they are related to each other.
We will confine our attention to the measures of France and Eng-

Online LibraryD. S. (David Samuel) MargoliouthThe Popular science monthly (Volume 19) → online text (page 81 of 110)