the animal[348]: every object of creation, therefore, that is gifted
with animal life, we may conclude, in some sense, has blood, which in
this large sense may be defined--_The fluid that visits and nourishes
every part of a living body_[349]. But the GREAT AUTHOR of nature has
varied the _machinery_ by which this nutritive fluid is formed and
distributed, gradually proceeding from the most _simple_ to the most
_complex_ structure; in which he seems to have seen it fit to _invert_
the process observable in the systems of sensation and respiration,
where the ascent is from the most _complex_, to the most _simple_
structure. In the lowest members of the animal creation, the blood
seems the portion they imbibe of the fluid medium in which they reside,
which when chylified, distributes new molecules to all parts of their
frame[350]. In others, as in insects, it is formed by the chyle that
transpires through the intestinal canal into the general cavity of the
body, where it receives oxygen from the air-vessels, and is fitted for
nutrition[351]. In these animals it is accompanied by a long dorsal
vessel, the first step towards a _heart_, which alternately contracts
and dilates with an irregular systole and diastole, but appears to
have no vascular system connected with it, though in their preparatory
states it has an _extra-vascular_ circulation which ceases in the
perfect insect. Again: in others, as the _Tubicoles_, _Annelida_, &c.,
a real _circulation_ has been discovered; that is to say, a system of
veins and arteries, but unaccompanied by a muscular heart[352]. In
the _Arachnida_ and _Branchiopod Crustacea_ the long dorsal vessel is
also found; but in these it is connected with an arterial and venous
system, which receives, distributes, and returns the blood[353].
It has therefore now become a true _heart_, and there is a regular
_circulation_; and in the _Decapod Crustacea_ the dorsal vessel is
contracted into an oval form, and placed nearly in the centre of the
trunk[354]. In the great majority of invertebrate animals the blood
is _white_, but in the _Annelida_, to which Class the common dew-worm
belongs, a curious anomaly takes place--for it is _red_[355]. Thus a
gradual ascent is made to the circulating system of the vertebrate and
red-blooded animals. In all, however, the _blood_ is the principal
instrument of nutrition and accretion; and is on that account properly
so denominated, though not connected with a circulating system.
Having given you this general outline of the means by which the
blood is distributed in the different Classes of animals, I shall now
confine myself to the case of insects and _Arachnida_, beginning with
the _former_.
I. If you examine attentively the back of any smooth caterpillar
with a transparent skin, you will perceive in that part an evident
pulsation, as though a fluid were pushed at regular intervals
towards the head, along a narrow tube which seems to run the whole
length of the body. Accurate dissections have proved that this
appearance is real, that there is actually present in the back of
most insects, placed immediately under the skin and furnished with
numerous air-vessels, a longitudinal vessel[356] originating in
the head near the mouth[357], running parallel with the alimentary
canal nearly to the anus, containing a fluid which is propelled in
regular pulsations of from 20 to 100 per minute, more or less as
the weather is colder or warmer[358], causing a sensible alternate
systole and diastole from the anal extremity towards the head. In
the _Cossus_ these pulses were observed by Lyonet to begin in the
_eleventh_ segment, from which they passed from segment to segment,
till they arrived at the _fourth_, where they terminated[359]. This
vessel is what Malpighi, who first discovered it, termed a _heart_,
or rather series of hearts[360]; but which Reaumur, who injected it,
regarded as a simple _artery_ without striking contractions[361]:
but to steer clear of any hypothesis, I shall merely call it the
_dorsal vessel_ (_Pseudo-cardia_). When carefully taken out of the
body it is found to be a membranous tube, appearing to be closed
at each end[362], in many larvæ of equal diameter every where, but
in perfect insects usually widest at the _anal_ extremity[363], and
attenuated into a very slender filament towards the head. In some
insects, however, as in the larva of the chamæleon-fly (_Stratyomis
Chamæleon_), it is attenuated at _both_ ends, and in the _Ephemera_
is alternately constricted and dilated as Malpighi describes that of
the silkworm[364], a dilated portion belonging to each segment[365].
In the _Cossus_, and probably others, after the _third_ segment,
it is furnished with nine pair, the three posterior pair being the
largest, of triangular transverse bundles of muscular fibres, which
Lyonet denominates its _wings_[366], the action of which produces its
systole and diastole, and their propagation from the tail towards the
head[367]. Under the last pair of these wings it is strengthened by
a large number of circular muscular fibres[368]. I have stated it as
_appearing_ to be closed at each extremity, because Cuvier and most
writers have so regarded it, and probably it is so closed in the
perfect insect; but from Lyonet's words it should seem that, in the
larva of the _Cossus_, he considered it as open and expanded at its
anterior end[369]. He seems also to suspect, that, by means of what
he calls the frontal ganglions, a fluid is derived from the dorsal
vessel to the spinal marrow. He likewise describes a large nerve as
passing through it and becoming recurrent[370]. Carus, as we shall
soon see, has also proved that this tube is not closed in larvæ.
The _fluid_ which this vessel contains is very abundant; in the
animal it appears colourless and transparent like water, but
when collected in drops it becomes more or less yellow, and even
orange[371]. Examined under the microscope it appears filled
with a prodigious number of transparent globules, of incredible
minuteness[372]. When mixed with water, which it does readily, its
globules lose all their transparency, and coagulate into small clammy
masses. After evaporation it becomes hard, and cracks like gum, as
blood does also. This gummy substance is so abundant, that the fluid
contained in the dorsal vessel of the caterpillar of the _Cossus_
yields a mass of it of the size of a grey pea[373].
From the situation of this dorsal vessel, which is precisely the
same with that of the heart in _Arachnida_ and the Branchiopod
_Crustacea_, and from the systole and diastole which keep its fluid
contents in constant motion, who can wonder that the physiologists
who first discovered it, reasoning analogically, maintained that it
was a true _heart_? But modern comparative anatomists, and those
of the highest name, from the absence of a vascular system for a
circulation, have contended that it is not a true heart, but an organ
appropriated to other purposes: a third hypothesis, and intermediate
between these two, has very recently been promulgated, that the
organ in question, namely, is a real heart, and in the preparatory
states of insects, the centre of a real circulation, which, in the
imago state, ceases with the full development of the wings; but that
this circulation is _extravascular_, or without peculiar vessels
analogous to veins and arteries.
I shall now enlarge a little upon each of these hypotheses, beginning
with the first or original one.
No one will deny that the argument from analogy is strongly in favour
of this: I need not therefore dwell upon it, but proceed to others.
Swammerdam, to whose exactness in observing, and scrupulous accuracy,
every reader of his immortal work will bear testimony, expressly
asserts that he has seen vessels issuing from the dorsal vessel in
the silkworm, and even succeeded in injecting them with a coloured
fluid[374]. Now it seems extremely improbable that so practised and
expert an anatomist should have been deceived, especially upon a
point which would naturally excite his most earnest and undivided
attention. Without this _recorded_ experiment, perhaps, it might
be thought, though this was very unlikely, that he had mistaken
_bronchiæ_ for veins and arteries: but how could they have been
_injected_ from the supposed heart? Another great physiologist,
Reaumur, in the caterpillar of the saw-fly of the rose (_Hylotoma
Rosæ_) observed, besides the _dorsal_ vessel, a _ventral_ one of
similar form, in which also was a pulsation, but slower than that
of the other. This he supposes may be the principal trunk of the
veins[375]. Bonnet thought he discovered a similar vessel in a large
caterpillar, but with all his attention could perceive no motion
in it[376]. Reaumur also fancied he perceived in the grub of _Musca
vomitoria_, in which he in vain looked for the dorsal vessel, a
fleshy part which exhibited alternate pulsations; and when with a
pair of scissors he made a lateral incision in the insect, amongst
other parts that came out, there was one that had movements of
contraction and dilatation for several minutes,--this experiment
was repeated with the same result upon several grubs[377]. De Geer,
whose love of truth and accuracy no one will call in question,
saw the appearance of blood-vessels in the leg of the larva of a
_Phryganea_ L. (as Lyonet did in those of a flea[378]); and in the
transparent thigh of _Ornithomia avicularia_ he discovered a pulse
like that of an artery[379]. Baker, whose only object was to record
what he _saw_, speaks of the _current_ of the blood being remarkably
visible in the legs of some small _bugs_[380]: what he meant by
that term is uncertain, but they could not be _spiders_, which he
had just distinguished. This author has likewise seen a green fluid
passing through the vessels of the wings of grasshoppers[381]; and M.
Chabrier is of opinion that insects possess the power of propelling
a fluid into the nervures of their wings and withdrawing it at
pleasure, as they are elevated or depressed[382]; but this last fact
may be independent of a circulation.
But though these arguments, which I have stated in their full force,
appear strong, and at first sight conclusive, those which may be
urged for the more modern opinion--that no circulation exists in
insects, properly so called,--appear to have still greater weight.
Lyonet, whose piercing eye and skilful hand traced the course of so
many hundred nerves and _bronchiæ_ long after they became invisible
to the unassisted eye, and which were a thousand times smaller
than the principal blood-vessels, opening into so large an organ
as the supposed heart of insects, might be expected to be, could
never discover any thing like them. His most painful researches,
and repeated attempts to inject them with coloured liquors, were
unable to detect the most minute opening in the dorsal vessel,
or the slightest trace of any artery or vein proceeding from or
communicating with it[383]. And Cuvier, whose unrivalled skill in
Comparative Anatomy peculiarly qualified him for the investigation,
repeated these inquiries, and tried all the known modes of injection,
with equal want of success; and is thus led to the conclusion,
that insects have no circulation, that their dorsal vessel is no
heart, and therefore ought not to be called by that name: that it is
rather a secretory vessel, like many others of that kind in those
animals. As to the nature of the fluid that it secretes, and its
use, he thinks it impossible, from our present information on the
subject, to form any satisfactory conclusion[384]. Marcel de Serres
informs us--which further seems to prove that it can be no real
heart--that this vessel may be totally removed without causing the
immediate death of the insect[385]. This opinion receives additional
confirmation from the mode in which _respiration_ is performed in
insects. In those animals that have a circulation, this takes
place by means of _lungs_ or _gills_;--thus we find, even in the
_Crustacea_ and _Arachnida_ so nearly related to insects, that the
organs of this function are true _gills_; whereas in insects, though
in some of their states their respiratory tubes are branchiform, yet
they are _not_ gills, and the respiration is by tubes and spiracles.
And these tubes, as you have seen, are so numerous and so infinitely
ramified and dispersed, as to occupy the place of arteries and veins,
and to imitate their distribution,--and thus to oxygenate what may be
deemed the real analogue of the blood, which bathes every internal
part of the body of an insect. Those animals likewise that have a
circulation are furnished with a _liver_, as is the case with the
_Arachnida_ and even many aggregate animals that have a heart; but
in insects there are only hepatic ducts. M. Cuvier has also proved
that the _conglomerate glands_, which exist in all animals that have
a heart and blood-vessels, do not exist in insects, in which they
are replaced by long slender secretory tubes, which without being
united float in the interior of the body: from this circumstance,
he is led to conclude that their nutrition is by _imbibition_ or
immediate absorption, as in the _Polypi_ and other zoophytes, the
chyle transpiring through the alimentary canal, and running uniformly
to all parts of the body[386].
These arguments appear so satisfactory, that Physiologists in
general seem to have been convinced by them that no circulation, at
any time, takes place in insects, and that their supposed heart is
merely a secretory vessel, though of what kind they were at a loss
to conjecture[387]. But, convincing as they seem, they appear to
have been founded in _error_, and on the idea that a _circulation_,
as well as a _heart_, necessarily implies a _vascular system_
consisting of veins and arteries; for by the recent discoveries
of M. Carus, it has been satisfactorily proved that insects in
their preparatory states, have an _extravascular_ circulation, the
arterial and venose currents not being confined by _parietes_. The
observations upon which M. Carus' hypothesis is founded, were made
in the Autumn of 1826; and an abstract of their results presented
to the Union of German Naturalists and Physicians, which then held
its meeting at Dresden, many of the members of which, as MM. Oken,
Husche, Heyne, Purkinje, Otto, Weber, and Müller, had ocular proofs
of the reality of the phenomena.
His first observations were made on the larva of _Agrion Puella_,
which swims by means of three vertical laminæ attached to the
tail; which, when the wings first appear as rudiments, begin to be
exsiccated and are finally detached. Each of these laminæ, in its
natural vertical position, presents an inferior abdominal and a
superior dorsal edge, has two tracheæ running along its centre with
ramifying bronchiæ, and consists of granular substance contained
between two strata of the external integuments. A current of
blood-globules enters each lamina somewhat nearer to its abdominal
than to its dorsal edge, and running through the greater part of
its length suddenly turns and bends its course back towards the
body, somewhat nearer to the dorsal than to the abdominal margin of
the lamina. The channel thus formed in the midst of the granular
substance is perfectly transparent, except where it is occupied by
the blood-globules, or crossed by the bronchiæ. The parietes of
the channel are not strictly defined, nor formed by any thing like
the coats of a vessel, the blood circulating through the granular
_Parenchyma_; a circumstance however which is not peculiar to
this case, but also occurs generally in the first states of the
circulation, as it presents itself for instance in the embryo of
_Fishes_, and in the _figura venosa_ of the incubated egg[394]. The
blood-globules are elongated like a grain of wheat, considerably
larger than those of the human blood, and float in a fluid which is
invisible because of its transparency, but the existence of which
is proved by the variations in the position of the globules in the
current, sometimes following its direction, at others crossing it
transversely, or more or less obliquely.
When the animal is vigorous, the current is uninterrupted, although
its velocity is accelerated at regular intervals; and that not only
in the excurrent (_arterial_), but also in the recurrent (_venous_)
part of its course through the lamina. When the animal becomes
exhausted, or the laminæ exsiccated, the circulation is interrupted,
and in the same manner, as under the same circumstances, in the larvæ
of frogs and lizards; the disturbance displaying itself not merely by
a cessation of the process, but also by retrograde movements of the
currents, or by oscillatory motions of the blood-globules.
In proportion as the wings are developed, the circulation in the
laminæ diminishes, and ultimately ceases, preparatory to the
detachment of the laminæ themselves. At the same time, however, it
presents itself under a new form in the wings. In these the excurrent
or arterial stream takes its course along the inner margin of the
wing, and the recurrent or venous returning along the outer; whilst,
occasionally, other transverse currents take their course through the
net-work of the wing from its inner to its outer margin. As the wings
are further developed, the circulation in them, like that in the
caudal laminæ, gradually becomes weaker and ultimately ceases[395].
The next observations were made on the transparent larva of a
neuropterous insect (probably a _Semblis_ or _Sialis_), in which
the pulsations of the dorsal vessel were distinctly seen at its
posterior extremity, from which they were propagated towards the
anterior; these two divisions of that vessel appearing to bear
to each other the relation of a _heart_ and _aorta_. There were
no traces of other vessels, though regular and rapid currents of
blood-globules, exterior to the tracheæ, proceeded from the head
towards the posterior extremity of the body, where each of these
currents entered the heart, which again propelled its contents with
accelerated velocity through the anterior part of the dorsal vessel
towards the head. The lateral currents also were accelerated upon
each contraction of the heart, proving that they must communicate
with the dorsal vessel at the anterior part of the body, though the
opacity of the head rendered it impossible to ascertain the mode of
_anastomosis_. An excurrent and returning current were also traced
to each of the legs[396]. But the phenomena of the circulation was
most distinctly visible in the larva of _Ephemera vulgata_, even
more distinctly than it is possible to trace it in the larvæ of
frogs and newts. In this animal the circulation, with the help of
the microscope, is at once visible in the three last segments of
the body; and with a little attention is discoverable not only in
the three terminal _caudulæ_, and in the upper joints of the legs,
but also in the head, and particularly the roots of the antennæ. In
the posterior part of the body there are on each side two currents
of blood, not bounded by parietes, situate on each side of the
intestinal canal, the inner one being the most considerable. The
external one communicates with the internal by several intermediate
branches; from this probably the streams are detached, which in the
form of loops are seen at the upper joints of the legs, though it
is not possible precisely to ascertain this, nor even whether these
lateral currents continue distinct in the thorax, which probably
they do. At the ninth abdominal segment these currents which flow
posteriorly from the head, change their direction, and are inflected
so as to enter the pulsating heart, from which the current again
flows towards the head. Before they enter the heart they give off
three streams, one for each of the three _caudulæ_. The currents
in these _caudulæ_ present the phenomena of the circulation with
peculiar distinctness, and are particularly remarkable from the
circumstance, that the excurrent and recurrent streams, though
closely approximated without any visible separation, flow without
disturbing each other. The excurrent stream is accelerated in
correspondence with the pulsations of the heart; the recurrent on
the contrary being always somewhat more sluggish, and the first to
stagnate and cease when the strength of the animal is impaired. In
the anterior part of the head currents can be discovered, forming
loops like those of the legs, at the roots of the antennæ; each
current proceeding from the cranial surface, and in returning taking
its course towards the region of the larynx[397].
M. Carus has likewise observed currents of blood in the larvæ of
water-beetles (_Hydrophilus_ and _Dytiscus_)[398]; but at present he
appears to have detected it in no terrestrial larva. Whether this is
occasioned by their opacity, or it exists only in the ovum, as he seems
to suspect[399], must be left for determination to future observers;
it is scarcely probable, however, that the larvæ of _Dytisci_ and
_Hydrophili_ should differ from other _Coleoptera_ in their circulation.
The endeavours of M. Carus to discover any proofs of a circulation
in insects in their last state, except in the wings at their first
development, were without success[400]. He observes that the fact
of the currents of fluids in larvæ not being defined by vascular
parietes, enables us to comprehend the rapidity and facility with
which the traces of the circulation are lost in the perfect insect.
On the other hand, that the existence of a circulation at one period,
and its cessation at another, elucidate many circumstances connected
with the physiology of these animals: for instance, the contrast
between the rapid growth and transformations of the larvæ, and the
stationary existence of the imago, &c. Lastly he remarks, that the
phenomena of this circulation do not throw any light on the obscure
subject of the mode of nutrition in perfect insects; which therefore
must still be supposed to be effected according to the idea of
Cuvier, without the intervention of vessels[401].
Whatever be the functions of the dorsal vessel, this seems the most
proper place to state to you what further is known respecting it.
Its construction is nearly alike in insects in all their states,
except that in the imago it is shorter and narrower. Reaumur has
affirmed, and before him Malpighi made a similar observation, that
in chrysalises newly disclosed from the larva, and yet transparent,
the motion of the included fluid is the reverse of what it has been
in that state, it being propelled from the head to the tail, which
he found to be the case also in the imago[402]. If this be true,
and there is no reason to doubt his accuracy, when they are more
advanced, it resumes its old course, as Lyonet observed, from the
tail to the head[403]. But probably it is not always uniformly in
the same direction, since Malpighi states that a very slight cause
will change its course, and that the pulsations differ in quickness
in different portions of the heart[404]. If its course were really
always the same, and in one direction, without any reflux, it would
seem to follow that the fluid must be absorbed at one end, and, if
there was no outlet, transpire at the other, which would be a kind
of circulation. In _Syrphus Pyrastri_ and other aphidivorous flies,
this dorsal vessel, instead of the usual form which it had in the
larva, assumes a very peculiar appearance. If, taking one of these
flies by the head and wings and holding it up to the light, you
survey under a lens the base of the lower part of its abdomen, you
will see through its transparent skin, which exactly forms such a
window as physicians have sometimes wished for in order to view the
interior of their patients, a flask-shaped vessel having its long end
directed towards the trunk, in which there is a manifest pulsation
and transmission of some fluid. This vessel extends in length from
the junction of the trunk with the abdomen to about the termination
of the second segment. The included fluid does not run in the dorsal
vessel in a regular course, but is propelled at intervals by drops,
as if from a syringe, first from the wide end towards the trunk,
and then in the contrary direction, forming a very interesting and
agreeable spectacle. One circumstance led Reaumur to conjecture that
the neck of this vessel, which he at first regarded as simple, is
in fact composed of two or more approximated tubes, and that the
blood is conveyed forward by the outward ones, and backward by the
intermediate one[405]: he even thinks that he saw a kind of secondary
heart, at the extremity next the trunk, for the purpose of causing
the reflux. This illustrious author observed the above remarkable
structure not only in the _Syrphi_, but in many of their affinities,
and thinks that it is also widely diffused amongst the _Muscidæ_[406].