vascular area of the human embryo, that centre (punctum saliens) and the
vessels immediately connected with it, undergo a phaseal metamorphosis,
till such time after birth as they assume their permanent character. In
each stage of metamorphosis, the embryo heart and vessels typify the
normal condition of the organ in one of the lower classes of animals.
The several species of the organ in these classes are parallel to the
various stages of change in the human organ. In its earliest condition,
the human heart presents the form of a simple canal, similar to that of
the lower Invertebrata, the veins being connected with its posterior
end, while from its anterior end a single artery emanates. The canal
next assumes a bent shape, and the vessels of both its ends become
thereby approximated. The canal now being folded upon itself in
heart-shape, next becomes constricted in situations, marking out the
future auricle and ventricle and arterial bulb, which still communicate
with each other. From the artery are given off on either side
symmetrically five branches (branchial arches), which arch laterally
from before, outwards and backwards, and unite in front of the
vertebrae, forming the future descending aorta. In this condition, the
human heart and vessels resemble the Piscean pipe. The next changes
which take place consist in the gradual subdivision, by means of septa,
of the auricle and ventricle respectively into two cavities. On the
separation of the single auricle into two, while the ventricle as yet
remains single, the heart presents that condition which is proper to the
Reptilian class. The interauricular and interventricular septa, by
gradual development from without inwards, at length meet and coalesce,
thereby dividing the two cavities into four--two auricles and two
ventricles--a condition proper to the Avian and Mammalian classes
generally. In the centre of the interauricular septum of the human
heart, an aperture (foramen ovale) is left as being necessary to the
foetal circulation. While the septa are being completed, the arterial
bulb also becomes divided by a partition formed in its interior in such
a manner as to adjust the two resulting arteries, the one in connexion
with the right, the other with the left ventricle. The right ventricular
artery (pulmonary aorta) so formed, has assigned to it the fifth
(posterior) opposite pair of arches, and of these the right one
remaining pervious to the point where it gives off the right pulmonary
branch, becomes obliterated beyond this point to that where it joins the
descending aorta, while the left arch remains pervious during foetal
life, as the ductus arteriosus still communicating with the descending
aorta, and giving off at its middle the left pulmonary branch. The left
ventricular artery (systemic aorta) is formed of the fourth arch of the
left side, while the opposite arch (fourth right) is altogether
obliterated. The third and second arches remain pervious on both sides,
afterwards to become the right and left brachio-cephalic arteries. The
first pair of arches, if not converted into the vertebral arteries, or
the thyroid axes, are altogether metamorphosed. By these changes the
heart and primary arteries assume the character in which they usually
present themselves at birth, and in all probability the primary veins
corresponded in form, number, and distribution with the arterial
vessels, and underwent, at the same time, a similar mode of
metamorphosis. One point in respect to the original symmetrical
character of the primary veins is demonstrable--namely, that in front of
the aortic branches the right and left brachio-cephalic veins, after
joining by a cross branch, descend separately on either side of the
heart, and enter (as two superior venae cavae) the right auricle by
distinct orifices. In some of the lower animals, this double condition
of the superior veins is constant, but in the human species the left
vein below the cross branch (left brachio-cephalic) becomes obliterated,
whilst the right vein (vena cava superior) receives the two
brachio-cephalic veins, and in this condition remains throughout life.
After birth, on the commencement of respiration, the foramen ovale of
the interauricular septum closes, and the ductus arteriosus becomes
impervious. This completes the stages of metamorphosis, and changes the
course of the simple foetal circulation to one of a more complex
order--viz., the systemic-pulmonary characteristic of the normal state
in the adult body.
VIII.--Such being the phases of metamorphosis of the primary (branchial)
arches which yield the vessels in their normal adult condition, we
obtain in this history an explanation of the signification not only of
such of their anomalies as are on record, but of such also as are
potential in the law of development; a few of them will suffice to
illustrate the meaning of the whole number:--lst, The interventricular
as well as the interauricular septum may be arrested in growth, leaving
an aperture in the centre of each; the former condition is natural to
the human foetus, the latter to the reptilian class, while both would be
abnormal in the human adult. 2nd. The heart may be cleft at its apex in
the situation of the interventricular septum--a condition natural to the
Dugong, A similar cleavage may divide the base of the heart in the
situation of the interauricular septum. 3rd. The partitioning of the
bulbus arteriosus may occur in such a manner as to assign to the two
aortae a relative position, the reverse of that which they normally
occupy--the pulmonary aorta springing from the left ventricle and the
systemic aorta arising from the right, and giving off from its arch the
primary branches in the usual order. [Footnote 1] 4th. As the two aortae
result from a division of the common primary vessel (bulbus arteriosus),
an arrest in the growth of the partition would leave them still as one
vessel, which (supposing the ventricular septum remained also
incomplete) would then arise from a single ventricle. 5th. The ductus
arteriosus may remain pervious, and while co-existing with the proper
aortic arch, two arches would then appear on the left side. 6th. The
systemic normal aortic arch may be obliterated as far up as the
innominate branch, and while the ductus arteriosus remains pervious, and
leading from the pulmonary artery to the descending part of the aortic
arch, this vessel would then present the appearance of a branch
ascending from the left side and giving off the brachio-cephalic
arteries. The right ventricular artery would then, through the medium of
the ductus arteriosus, supply both the lungs and the system. Such a
state of the vessels would require (in order that the circulation of a
mixed blood might be carried on) that the two ventricles freely
communicate. 7th. If the fourth arch of the right side remained pervious
opposite the proper aortic arch, there would exist two aortic arches
placed symmetrically, one on either side of the vertebral column, and,
joining below, would include in their circle the trachea and oesophagus.
8th. If the fifth arch of the right side remained pervious opposite the
open ductus arteriosus, both vessels would present a similar
arrangement, as two symmetrical ducti arteriosi co-existing with
symmetrical aortic arches. 9th. If the vessels appeared co-existing in
the two conditions last mentioned, they would represent four aortic
arches, two on either side of the vertebral column. 10th. If the fourth
right arch, instead of the fourth left (aorta), remained pervious, the
systemic aortic arch would then be turned to the right side of the
vertebral column, and have the trachea and oesophagus on its left. 11th.
When the bulbus arteriosus divides itself into three parts, the two
lateral parts, in becoming connected with the left ventricle, will
represent a double ascending systemic aorta, and having the pulmonary
artery passing between them to the lungs. 12th. When of the two original
superior venae cavae the right one instead of the left suffers
metamorphosis, the vena cava superior will then appear on the left side
of the normal aortic arch. [Footnote 2] Of these malformations, some are
rather frequently met with, others very seldom, and others cannot exist
compatible with life after birth. Those which involve a more or less
imperfect discharge of the blood-aerating functions of the lungs, are in
those degrees more or less fatal, and thus nature aborting as to the
fitness of her creation, cancels it.
[Footnote 1: This physiological truth has, I find, been applied by Dr.
R. Quain to the explanation of a numerous class of malformations
connected with the origins of the great vessels from the heart, and of
their primary branches. See The Lancet, vol. I. 1842.]
[Footnote 2: For an analysis of the occasional peculiarities of these
primary veins in the human subject, see an able and original monograph
in the Philosophical Transactions, Part 1., 1850, entitled, "On the
Development of the Great Anterior Veins in Man and Mammalia." By John
Marshall, F.R.C.S., &c. ]
IX.--The portal system of veins passing to the liver, and the hepatic
veins passing from this organ to join the inferior vena cava, exhibit in
respect to the median line of the body an example of a-symmetry, since
appearing on the right side, they have no counterparts on the left. As
the law of symmetry seems to prevail universally in the development of
organized beings, forasmuch as every lateral organ or part has its
counterpart, while every central organ is double or complete, in having
two similar sides, then the portal system, as being an exception to this
law, is as a natural note of interrogation questioning the signification
of that fact, and in the following observations, it appears to me, the
answer may be found. Every artery in the body has its companion vein or
veins. The inferior vena cava passes sidelong with the aorta in the
abdomen. Every branch of the aorta which ramifies upon the abdominal
parietes has its accompanying vein returning either to the vena cava or
the vena azygos, and entering either of these vessels at a point on the
same level as that at which itself arises. The renal vessels also have
this arrangement. But all the other veins of the abdominal viscera,
instead of entering the vena cava opposite their corresponding arteries,
unite into a single trunk (vena portae), which enters the liver. The
special purpose of this destination of the portal system is obvious, but
the function of a part gives no explanation of its form or relative
position, whether singular or otherwise. On viewing the vessels in
presence of the general law of symmetrical development, it occurs to me
that the portal and hepatic veins form one continuous system, which
taken in the totality, represents the companion veins of the arteries of
the abdominal viscera. The liver under this interpretation appears as a
gland developed midway upon these veins, and dismembering them into a
mesh of countless capillary vessels, (a condition necessary for all
processes of secretion,) for the special purpose of decarbonizing the
blood. In this great function the liver is an organ correlative or
compensative to the lungs, whose office is similar. The secretion of the
liver (bile) is fluidform; that of the lungs is aeriform. The bile being
necessary to the digestive process, the liver has a duct to convey that
product of its secretion to the intestines. The trachea is as it were
the duct of the lungs. In the liver, then, the portal and hepatic veins
being continuous as veins, the two systems, notwithstanding their
apparent distinctness, caused by the intervention of the hepatic
lobules, may be regarded as the veins corresponding with the arteries of
the coeliac axis, and the two mesenteric. The hepatic artery and the
hepatic veins evidently do not pair in the sense of afferent and
efferent, with respect to the liver, both these vessels having
destinations as different as those of the bronchial artery and the
pulmonary veins in the lungs. The bronchial artery is attended by its
vein proper, while the vein which corresponds to the hepatic artery
joins either the hepatic or portal veins traversing the liver, and in
this position escapes notice.[Footnote]
[Footnote: In instancing these facts, as serving under comparison to
explain how the hepatic vessels constitute no radical exception to the
law of symmetry which presides over the development and distribution of
the vascular system as a whole, I am led to inquire in what respect (if
in any) the liver as an organ forms an exception to this general law
either in shape, in function, or in relative position. While seeing that
every central organ is single and symmetrical by the union of two
absolutely similar sides, and that each lateral pair of organs is double
by the disunion of sides so similar to each other in all respects that
the description of either side serves for the other opposite, it has
long since seemed to me a reasonable inference that, since the liver on
the right has no counterpart as a liver on the left, and that, since the
spleen on the left has no counterpart as a spleen on the right, so these
two organs (the liver and spleen) must themselves correspond to each
other, and as such, express their respective significations. Under the
belief that every exception (even though it be normal) to a general law
or rule, is, like the anomaly itself, alone explicable according to such
law, and expressing a fact not more singular or isolated from other
parallel facts than is one form from another, or from all others
constituting the graduated scale of being, I would, according to the
light of this evidence alone, have no hesitation in stating that the
liver and spleen, as opposites, represent corresponding organs, even
though they appeared at first view more dissimilar than they really are.
In support of this analogy of both organs, which is here, so far as I am
aware, originally enunciated for anatomical science, I record the
following observations:--1st. Between the opposite parts of the same
organic entity (between the opposite leaves of the same plant, for
example), nature manifests no such absolute difference in any case as
exists between the leaf of a plant and of a book. 2ndly. When between
two opposite parts of the same organic form there appears any
differential character, this is simply the result of a modification or
metamorphosis of one of the two perfectly similar originals or
archetypes, but never carried out to such an extreme degree as to
annihilate all trace of their analogy. 3rdly. The liver and the spleen
are opposite parts; and as such, they are associated by arteries which
arise by a single trunk (coeliac axis) from the aorta, and branch right
and left, like indices pointing to the relationship between both these
organs, in the same manner as the two emulgent arteries point to the
opposite renal organs. 4thly. The liver is divided into two lobes, right
and left; the left is less than the right; that quantity which is
wanting to the left lobe is equal to the quantity of a spleen; and if in
idea we add the spleen to the left lobe of the liver, both lobes of this
organ become quantitatively equal, and the whole liver symmetrical;
hence, as the liver plus the spleen represents the whole structural
quantity, so the liver minus the spleen signifies that the two organs
now dissevered still relate to each other as parts of the same whole.
5thly. The liver, as being three-fourths of the whole, possesses the
duct which emanates at the centre of all glandular bodies. The spleen,
as being one-fourth of the whole, is devoid of the duct. The liver
having the duct, is functional as a gland, while the spleen having no
duct, cannot serve any such function. If, in thus indicating the
function which the spleen does not possess, there appears no proof
positive of the function which it does, perhaps the truth is, that as
being the ductless portion of the whole original hepatic quantity, it
exists as a thing degenerate and functionless, for it seems that the
animal economy suffers no loss of function when deprived of it. 6thly.
In early foetal life, the left lobe of the liver touches the spleen on
the left side; but in the process of abdominal development, the two
organs become separated from each other right and left. 7thly. In
animals devoid of the spleen, the liver appears of a symmetrical shape,
both its lobes being equal; for that quantity which in other animals has
become splenic, is in the former still hepatic. 8thly. In cases of
transposition of both organs, it is the right lobe of the liver--that
nearest the spleen, now on the right side--which is the smaller of the
two lobes, proving that whichever lobe be in this condition, the spleen,
as being opposite to it, represents the minus hepatic quantity. From
these, among other facts, I infer that the spleen is the representative
of the liver on the left side, and that as such, its signification being
manifest, there exists no exception to the law of animal symmetry. "Tam
miram uniformitatem in planetarum systemate, necessario fatendum est
intelligentia et concilio fuisse effectam. Idemque dici possit de
uniformitate illa quae est in corporibus animalium. Habent videlicet
animalia pleraque omnia, bina latera, dextrum et sinistrum, forma
consimili: et in lateribus illis, a posteriore quidem corporis sui
parte, pedes binos; ab anteriori autem parte, binos armos, vel pedes,
vel alas, humeris affixos: interque humeros collum, in spinam excurrens,
cui affixum est caput; in eoque capite binas aures, binos oculos, nasum,
os et linguam; similiter posita omnia, in omnibus fere animalibus."
--Newton, Optices, sive de reflex, &c. p. 411.]
X.--The heart, though being itself the recipient, the prime mover, and
the dispenser of the blood, does not depend either for its growth,
vitality, or stimulus to action, upon the blood under these uses, but
upon the blood circulating through vessels which are derived from its
main systemic artery, and disposed in capillary ramifications through
its substance, in the manner of the nutrient vessels of all other organs.
The two coronary arteries of the heart arise from the systemic aorta
immediately outside the semilunar valves, situated in the root of this
vessel, and in passing right and left along the auriculo-ventricular
furrows, they send off some branches for the supply of the organ itself,
and others by which both vessels anastomose freely around its base and
apex. The vasa cordis form an anastomotic circulation altogether
isolated from the vessels of the other thoracic organs, and also from
those distributed to the thoracic parietes. The coronary arteries are
accompanied by veins which open by distinct orifices (foramina Thebesii)
into the right auricle. Like the heart itself, its main vessels do not
depend for their support upon the blood conveyed by them, but upon that
circulated by the small arteries (vasa vasorum) derived either from the
vessel upon which they are distributed, or from some others in the
neighbourhood. These little arteries are attended by veins of a
corresponding size (venules) which enter the venae comites, thus
carrying out the general order of vascular distribution to the minutest
particular. Besides the larger nerves which accompany the main vessels,
there are delicate filaments of the cerebro-spinal and sympathetic
system distributed to their coats, for the purpose, as it is supposed,
of governing their "contractile movements." The vasa vasorum form an
anastomosis as well upon the inner surface of the sheath as upon the
artery contained in this part; and hence in the operation for tying the
vessel, the rule should be to disturb its connexions as little as
possible, otherwise its vitality, which depends upon these minute
branches, will, by their rupture, be destroyed in the situation of the
ligature, where it is most needed.
XI.--The branches of the systemic aorta form frequent anastomoses with
each other in all parts of the body. This anastomosis occurs chiefly
amongst the branches of the main arteries proper to either side. Those
branches of the opposite vessels which join at the median line are
generally of very small size. There are but few instances in which a
large blood vessel crosses the central line from its own side to the
other. Anastomosis at the median line between opposite vessels happens
either by a fusion of their sides lying parallel, as for example (and
the only one) that of the two vertebral arteries on the basilar process
of the occipital bone; or else by a direct end-to-end union, of which
the lateral pair of cerebral arteries, forming the circle of Willis, and
the two labial arteries, forming the coronary, are examples. The
branches of the main arteries of one side form numerous anastomoses in
the muscles and in the cellular and adipose tissue generally. Other
special branches derived from the parent vessel above and below the
several joints ramify and anastomose so very freely over the surfaces of
these parts, and seem to pass in reference to them out of their direct
course, that to effect this mode of distribution appears to be no less
immediate a design than to support the structures of which the joints
are composed.
XII.--The innominate artery. When this vessel is tied, the free direct
circulation through the principal arteries of the right arm, and the
right side of the neck, head, and brain, becomes arrested; and the
degree of strength of the recurrent circulation depends solely upon the
amount of anastomosing points between the following arteries of the
opposite sides. The small terminal branches of the two occipital, the
two auricular, the two superficial temporal, and the two frontal,
inosculate with each other upon the sides, and over the vertex of the
head; the two vertebral, and the branches of the internal carotid, at
the base and over the surface of the brain; the two facial with each
other, and with the frontal above and mental below, at the median line
of the face; the two internal maxillary by their palatine, pharyngeal,
meningeal, and various other branches upon the surface of the parts to
which they are distributed; and lastly, the two superior thyroid
arteries inosculate around the larynx and in the thyroid body. By these
anastomoses, it will be seen that the circulation is restored to the
branches of the common carotid almost solely. In regard to the
subclavian artery, the circulation would be carried on through the
anastomosing branches of the two inferior thyroid in the thyroid body;
of the two vertebral, in the cranium and upon the cervical vertebrae; of
the two internal mammary, with each other behind the sternum, and with
the thoracic branches of the axillary and the superior intercostal
laterally; lastly, through the anastomosis of the ascending cervical
with the descending branch of the occipital, and with the small lateral
offsets of the vertebral.
XIII.--The common carotid arteries, Of these two vessels, the left one
arising, in general, from the arch of the aorta, is longer than the
right one by the measure of the innominate artery from which the right
arises. When either of the common carotids is tied, the circulation will
be maintained through the anastomosing branches of the opposite vessels
as above specified. When the vertebral or the inferior thyroid branch
arises from the middle of the common carotid, this vessel will have an
additional source of supply if the ligature be applied to it below the
origin of such branch. In the absence of the innominate artery, the
right as well as the left carotid will be found to spring directly from
the aortic arch.
XIV.--The subclavian arteries. When a ligature is applied to the inner
third of this vessel within its primary branches, the collateral
circulation is carried on by the anastomoses of the arteries above
mentioned; but if the vertebral or the inferior thyroid arises either
from the aorta or the common carotid, the sources of arterial supply in
respect to the arm will, of course, be less numerous. When the outer
portion of the subclavian is tied between the scalenus and the clavicle,
while the branches arise from its inner part in their usual position and
number, the collateral circulation in reference to the arm is maintained
by the following anastomosing branches:--viz., those of the
superficialis colli, and the supra and posterior scapular, with those of
the acromial thoracic; the subscapular, and the anterior and posterior
circumflex around the shoulder-joint, and over the dorsal surface of
the scapula; and those of the internal mammary and superior intercostal,
with those of the thoracic arteries arising from the axillary. Whatever
be the variety as to their mode or place of origin, the branches
emanating from the subclavian artery are constant as to their
destination. The length of the inner portion of the right subclavian
will vary according to the place at which it arises, whether from the
innominate artery, from the ascending, or from the descending part of
the aortic arch.
XV.--The axillary artery. As this vessel gives off throughout its whole
length, numerous branches which inosculate principally with the
scapular, mammary, and superior intercostal branches of the subclavian,
it will be evident that, in tying it above its own branches, the
anastomotic circulation will with much greater freedom be maintained in
respect to the arm, than if the ligature be applied below those
branches. Hence, therefore, when the axillary artery is affected with
aneurism, thereby rendering it unsafe to apply a ligature to this
vessel, it becomes not only pathologically, but anatomically, the more
prudent measure to tie the subclavian immediately above the clavicle.