THE VASCULAR system is divided for descriptive
purposes into (a) the blood vascular system, which comprises the
heart and bloodvessels for the circulation of the blood; and (b) the lymph
vascular system, consisting of lymph glands and lymphatic vessels, through
which a colorless fluid, the lymph, circulates. It must be noted,
however, that the two systems communicate with each other and are intimately
associated developmentally.
The heart is the central organ of the
blood vascular system, and consists of a hollow muscle; by its contraction the
blood is pumped to all parts of the body through a complicated series of tubes,
termed arteries. The arteries undergo enormous ramification in their
course throughout the body, and end in minute vessels, called arterioles,
which in their turn open into a close-meshed network of microscopic vessels,
termed capillaries. After the blood has passed through the capillaries
it is collected into a series of larger vessels, called veins, by which
it is returned to the heart. The passage of the blood through the heart and
blood-vessels constitutes what is termed the circulation of the blood,
of which the following is an outline.
The human heart is divided by septa into
right and left halves, and each half is further divided into two cavities, an
upper termed the atrium and a lower the ventricle. The heart
therefore consists of four chambers, two, the right atrium and right ventricle,
forming the right half, and two, the left atrium and left ventricle the left
half. The right half of the heart contains venous or impure blood; the left,
arterial or pure blood. The atria are receiving chambers, and the ventricles
distributing ones. From the cavity of the left ventricle the pure blood is
carried into a large artery, the aorta, through the numerous branches of
which it is distributed to all parts of the body, with the exception of the
lungs. In its passage through the capillaries of the body the blood gives up to
the tissues the materials necessary for their growth and nourishment, and at
the same time receives from the tissues the waste products resulting from their
metabolism. In doing so it is changed from arterial into venous blood, which is
collected by the veins and through them returned to the right atrium of the
heart. From this cavity the impure blood passes into the right ventricle, and
is thence conveyed through the pulmonary arteries to the lungs. In the
capillaries of the lungs it again becomes arterialized, and is then carried to
the left atrium by the pulmonary veins. From the left atrium it passes
into the left ventricle, from which the cycle once more begins.
The course of the blood from the left
ventricle through the body generally to the right side of the heart constitutes
the greater or systemic circulation, while its passage from the right
ventricle through the lungs to the left side of the heart is termed the lesser
or pulmonary circulation.
It is necessary, however, to state that
the blood which circulates through the spleen, pancreas, stomach, small
intestine, and the greater part of the large intestine is not returned directly
from these organs to the heart, but is conveyed by the portal vein to
the liver. In the liver this vein divides, like an artery, and ultimately ends
in capillary-like vessels (sinusoids), from which the rootlets of a
series of veins, called the hepatic veins, arise; these carry the blood
into the inferior vena cava, whence it is conveyed to the right atrium. From
this it will be seen that the blood contained in the portal vein passes through
two sets of vessels: (1) the capillaries in the spleen, pancreas, stomach,
etc., and (2) the sinusoids in the liver. The blood in the portal vein carries
certain of the products of digestion: the carbohydrates, which are mostly taken
up by the liver cells and stored as glycogen, and the protein products which
remain in solution and are carried into the general circulation to the various
tissues and organs of the body.
Speaking generally, the arteries may be
said to contain pure and the veins impure blood. This is true of the systemic,
but not of the pulmonary vessels, since it has been seen that the impure blood
is conveyed from the heart to the lungs by the pulmonary arteries, and the pure
blood returned from the lungs to the heart by the pulmonary veins. Arteries,
therefore, must be defined as vessels which convey blood from the heart,
and veins as vessels which return blood to the heart.
Structure of Arteries—The arteries are composed of three coats: an internal or endothelial
coat (tunica intima of Kölliker); a middle or muscular coat (tunica
media); and an external or connective-tissue coat (tunica adventitia).
The two inner coats together are very easily separated from the external, as by
the ordinary operation of tying a ligature around an artery. If a fine string
be tied forcibly upon an artery and then taken off, the external coat will be
found undivided, but the two inner coats are divided in the track of the ligature
and can easily be further dissected from the outer coat.
The inner coat (tunica intima)
can be separated from the middle by a little maceration, or it may be stripped
off in small pieces; but, on account of its friability, it cannot be separated
as a complete membrane. It is a fine, transparent, colorless structure which is
highly elastic, and, after death, is commonly corrugated into longitudinal
wrinkles. The inner coat consists of: (1) A layer of pavement endothelium, the
cells of which are polygonal, oval, or fusiform, and have very distinct round
or oval nuclei. This endothelium is brought into view most distinctly by
staining with nitrate of silver. (2) A subendothelial layer, consisting of
delicate connective tissue with branched cells lying in the interspaces of the
tissue; in arteries of less than
The middle coat (tunica media)
is distinguished from the inner by its color and by the transverse arrangement
of its fibers. In the smaller arteries it consists principally of plain muscle
fibers in fine bundles, arranged in lamellae and disposed circularly around the
vessel. These lamellae vary in number according to the size of the vessel; the
smallest arteries having only a single layer and those slightly larger three or
four layers. It is to this coat that the thickness of the wall of the artery is
mainly due. In the larger arteries, as the iliac, femoral, and carotid, elastic
fibers unite to form lamellae which alternate with the layers of muscular
fibers; these lamellae are united to one another by elastic fibers which pass
between the muscular bundles, and are connected with the fenestrated membrane
of the inner coat. In the largest arteries, as the aorta and innominate, the
amount of elastic tissue is very considerable; in these vessels a few bundles
of white connective tissue also have been found in the middle coat. The muscle
fiber cells are about 50μ in length and contain well-marked, rod-shaped nuclei,
which are often slightly curved.
The external coat (tunica
adventitia) consists mainly of fine and closely felted bundles of white
connective tissue, but also contains elastic fibers in all but the smallest
arteries. The elastic tissue is much more abundant next the tunica media, and
it is sometimes described as forming here, between the adventitia and media, a
special layer, the tunica elastica externa of Henle. This layer is most
marked in arteries of medium size. In the largest vessels the external coat is
relatively thin; but in small arteries it is of greater proportionate thickness.
In the smaller arteries it consists of a single layer of white connective
tissue and elastic fibers; while in the smallest arteries, just above the
capillaries, the elastic fibers are wanting, and the connective tissue of which
the coat is composed becomes more nearly homogeneous the nearer it approaches
the capillaries, and is gradually reduced to a thin membranous envelope, which
finally disappears.
Some arteries have extremely thin walls in
proportion to their size; this is especially the case in those situated in the
cavity of the cranium and vertebral canal, the difference depending on the
thinness of the external and middle coats.
The arteries, in their distribution
throughout the body, are included in thin fibro-areolar investments, which form
their sheaths. The vessel is loosely connected with its sheath by
delicate areolar tissue; and the sheath usually encloses the accompanying
veins, and sometimes a nerve. Some arteries, as those in the cranium, are not
included in sheaths.
All the larger arteries, like the other
organs of the body, are supplied with bloodvessels. These nutrient vessels,
called the vasa vasorum, arise from a branch of the artery, or
from a neighboring vessel, at some considerable distance from the point at
which they are distributed; they ramify in the loose areolar tissue connecting
the artery with its sheath, and are distributed to the external coat, but do
not, in man, penetrate the other coats; in some of the larger mammals a few
vessels have been traced into the middle coat. Minute veins return the blood
from these vessels; they empty themselves into the vein or veins accompanying
the artery. Lymphatic vessels are also present in the outer coat.
Arteries are also supplied with nerves,
which are derived from the sympathetic, but may pass through the cerebrospinal
nerves. They form intricate plexuses upon the surfaces of the larger trunks,
and run along the smaller arteries as single filaments, or bundles of filaments
which twist around the vessel and unite with each other in a plexiform manner.
The branches derived from these plexuses penetrate the external coat and are
distributed principally to the muscular tissue of the middle coat, and thus
regulate, by causing the contraction and relaxation of this tissue the amount
of blood sent to any part.
The Capillaries.—The smaller arterial branches (excepting those of the cavernous
structure of the sexual organs, of the splenic pulp, and of the placenta)
terminate in net-works of vessels which pervade nearly every tissue of the
body. These vessels, from their minute size, are termed capillaries. They are
interposed between the smallest branches of the arteries and the commencing
veins, constituting a net-work, the branches of which maintain the same
diameter throughout; the meshes of the net-work are more uniform in shape and
size than those formed by the anastomoses of the small arteries and veins.
The diameters of the capillaries
vary in the different tissues of the body, the usual size being about 8μ. The
smallest are those of the brain and the mucous membrane of the intestines; and
the largest those of the skin and the marrow of bone, where they are stated to
be as large as 20μ in diameter. The form of the capillary net varies in the
different tissues, the meshes being generally rounded or elongated.
The rounded form of mesh is most
common, and prevails where there is a dense network, as in the lungs, in most
glands and mucous membranes, and in the cutis; the meshes are not of an
absolutely circular outline, but more or less angular, sometimes nearly
quadrangular, or polygonal, or more often irregular.
Elongated meshes are observed in
the muscles and nerves, the meshes resembling parallelograms in form, the long
axis of the mesh running parallel with the long axis of the nerve or muscle.
Sometimes the capillaries have a looped arrangement; a single vessel
projecting from the common net-work and returning after forming one or more
loops, as in the papillae of the tongue and skin.
The number of the capillaries and the size
of the meshes determine the degree of vascularity of a part. The closest
network and the smallest interspaces are found in the lungs and in the choroid
coat of the eye. In these situations the interspaces are smaller than the
capillary vessels themselves. In the intertubular plexus of the kidney, in the
conjunctiva, and in the cutis, the interspaces are from three to four times as
large as the capillaries which form them; and in the brain from eight to ten
times as large as the capillaries in their long diameters, and from four to six
times as large in their transverse diameters. In the adventitia of arteries the
width of the meshes is ten times that of the capillary vessels. As a general
rule, the more active the function of the organ, the closer is its capillary
net and the larger its supply of blood; the meshes of the network are very
narrow in all growing parts, in the glands, and in the mucous membranes, wider
in bones and ligaments which are comparatively inactive; bloodvessels are
nearly altogether absent in tendons, in which very little organic change occurs
after their formation. In the liver the capillaries take a more or less radial
course toward the intralobular vein, and their walls are incomplete, so that
the blood comes into direct contact with the liver cells. These vessels in the
liver are not true capillaries but “sinusoids;” they are developed by the
growth of columns of liver cells into the blood spaces of the embryonic organ.
Structure.—The wall of a capillary
consists of a fine transparent endothelial layer, composed of cells joined edge
to edge by an interstitial cement substance, and continuous with the
endothelial cells which line the arteries and veins. When stained with nitrate
of silver the edges which bound the epithelial cells are brought into view.
These cells are of large size and of an irregular polygonal or lanceolate
shape, each containing an oval nucleus which may be displayed by carmine or
hematoxylin. Between their edges, at various points of their meeting, roundish
dark spots are sometimes seen, which have been described as stomata, though
they are closed by intercellular substance. They have been believed to be the
situations through which the colorless corpuscles of the blood, when migrating
from the bloodvessels, emerge; but this view, though probable, is not
universally accepted.
In many situations a delicate sheath or
envelope of branched nucleated connective tissue cells is found around the
simple capillary tube, particularly in the larger ones; and in other places,
especially in the glands, the capillaries are invested with retiform connective
tissue.
Sinusoids.—In certain organs, viz., the heart, the liver, the suprarenal and
parathyroid glands, the glomus caroticum and glomus coccygeum, the smallest
bloodvessels present various differences from true capillaries. They are wider,
with an irregular lumen, and have no connective tissue covering, their
endothelial cells being in direct contact with the cells of the organ.
Moreover, they are either arterial or venous and not intermediate as are the
true capillaries. These vessels have been called sinusoids by
Bloodvessels first make their appearance in several
scattered vascular areas which are developed simultaneously between the
entoderm and the mesoderm of the yolk-sac, i. e., outside the body of
the embryo. Here a new type of cell, the angioblast or vasoformative
cell, is differentiated from the mesoderm. These cells as they divide form
small, dense syncytial masses which soon join with similar masses by means of
fine processes to form plexuses. These plexuses increase both by
division and growth of its cells and by the addition of new angioblasts which
differentiate from the mesoderm. Within these solid plexuses and also within
the isolated masses of angioblasts vacuoles appear through liquefaction of the
central part of the syncytium into plasma. The lumen of the bloodvessels thus
formed is probably intracellular. The flattened cells at the periphery form the
endothelium. The nucleated red blood corpuscles develop either from small
masses of the original angioblast left attached to the inner wall of the lumen
or directly from the flat endothelial cells. In either case the syncytial mass
thus formed projects from and is attached to the wall of the vessel. Such a
mass is known as a blood island and hemoglobin gradually accumulates within it.
Later the cells on the surface round up, giving the mass a mulberry-like
appearance. Then the red blood cells break loose and are carried away in the
plasma. Such free blood cells continue to divide. The term blood island
was originally used for the syncytial masses of angioblasts found in the area
vasculosa, but it is probably best to limit the term to the masses within the
lumen from which the red blood cells arise as Sabin 88 has done. Blood islands have been seen in the area
vasculosa in the omphalomesenteric vein and arteries, and in the dorsal aorta.
The differentiation of angïoblasts
from the mesoderm occurs not only in the area vasculosa but within the embryo
and probably most of the larger bloodvessels are developed in situ in
this manner. This process of the differentiation of angioblasts from the
mesoderm probably ceases in different regions of the embryo at different
periods and after its cessation new vessels are formed by sprouts from vessels
already laid down in the form of capillary plexuses.
The first rudiment of the heart
appears as a pair of tubular vessels which are developed in the splanchnopleure
of the pericardial area. These are named the primitive aortae, and a
direct continuity is soon established between them and the vessels of the
yolk-sac. Each receives anteriorly a vein—the vitelline vein—from the
yolk-sac, and is prolonged backward on the lateral aspect of the notochord
under the name of the dorsal aorta. The dorsal aortae give branches to
the yolk-sac, and are continued backward through the body-stalk as the
umbilical arteries to the villi of the chorion.
Eternod describes the circulation in an
embryo which he estimated to be about thirteen days old. The rudiment of the
heart is situated immediately below the fore-gut and consists of a short stem.
It gives off two vessels, the primitive aortae, which run backward, one on
either side of the notochord, and then pass into the body-stalk along which
they are carried to the chorion. From the chorionic villi the blood is returned
by a pair of umbilical veins which unite in the body-stalk to form a single
vessel and subsequently encircle the mouth of the yolk-sac and open into the
heart. At the junction of the yolk-sac and body-stalk each vein is joined by a
branch from the vascular plexus of the yolk-sac. From his observations it seems
that, in the human embryo, the chorionic circulation is established before that
on the yolk-sac.
Further Development of the Arteries.—Recent observations show that practically none of the
main vessels of the adult arise as such in the embryo. In the site of each
vessel a capillary network forms, and by the enlargement of definite paths in
this the larger arteries and veins are developed. The branches of the main
arteries are not always simple modifications of the vessels of the capillary
network, but may arise as new outgrowths from the enlarged stem.
It has been seen (page 506) that each
primitive aorta consists of a ventral and a dorsal part which are continuous
through the first aortic arch. The dorsal aortae at first run backward
separately on either side of the notochord, but about the third week they fuse
from about the level of the fourth thoracic to that of the fourth lumbar
segment to form a single trunk, the descending aorta. The first aortic arches
run through the mandibular arches, and behind them five additional pairs are
developed within the visceral arches; so that, in all, six pairs of aortic
arches are formed. The first and second arches pass between the ventral and
dorsal aortae, while the others arise at first by a common trunk from the
truncus arteriosus, but end separately in the dorsal aortae. As the neck
elongates, the ventral aortae are drawn out, and the third and fourth arches
arise directly from these vessels.
In fishes these arches persist and give
off branches to the gills, in which the blood is oxygenated. In mammals some of
them remain as permanent structures while others disappear or become
obliterated
The Anterior Ventral Aortae.—These persist on both sides. The right forms (a)
the innominate artery, (b) the right common and external carotid
arteries. The left gives rise to (a) the short portion of the aortic
arch, which reaches from the origin of the innominate artery to that of the
left common carotid artery; (b) the left common and external carotid
arteries.
The Aortic Arches.—The first and second arches disappear early, but the dorsal end of the
second gives origin to the stapedial artery a vessel which atrophies in man but
persists in some mammals. It passes through the ring of the stapes and divides
into supraorbital, infraorbital, and mandibular branches which follow the three
divisions of the trigeminal nerve. The infraorbital and mandibular arise from a
common stem, the terminal part of which anastomoses with the external carotid.
On the obliteration of the stapedial artery this anastomosis enlarges and forms
the internal maxillary artery, and the branches of the stapedial artery are now
branches of this vessel. The common stem of the infraorbital and mandibular
branches passes between the two roots of the auriculotemporal nerve and becomes
the middle meningeal artery; the original supraorbital branch of the stapedial
is represented by the orbital twigs of the middle meningeal. The third aortic
arch constitutes the commencement of the internal carotid artery, and is
therefore named the carotid arch. The fourth right arch forms the right
subclavian as far as the origin of its internal mammary branch; while the
fourth left arch constitutes the arch of the aorta between the origin of the
left carotid artery and the termination of the ductus arteriosus. The fifth
arch disappears on both sides. The sixth right arch disappears; the sixth left
arch gives off the pulmonary arteries and forms the ductus arteriosus; this
duct remains pervious during the whole of fetal life, but is obliterated a few
days after birth. His showed that in the early embryo the right and left arches
each gives a branch to the lungs, but that later both pulmonary arteries take
origin from the left arch.
THE DISTRIBUTION of the systematic arteries is like a
highly ramified tree, the common trunk of which, formed by the aorta, commences
at the left ventricle, while the smallest ramifications extend to the peripheral
parts of the body and the contained organs. Arteries are found in all parts of
the body, except in the hairs, nails, epidermis, cartilages, and cornea; the
larger trunks usually occupy the most protected situations, running, in the
limbs, along the flexor surface, where they are less exposed to injury.
There is considerable variation in the
mode of division of the arteries: occasionally a short trunk subdivides into
several branches at the same point, as may be observed in the celiac artery and
the thyrocervical trunk: the vessel may give off several branches in
succession, and still continue as the main trunk, as is seen in the arteries of
the limbs; or the division may be dichotomous, as, for instance, when the aorta
divides into the two common iliacs.
A branch of an artery is smaller than the
trunk from which it arises; but if an artery divides into two branches, the
combined sectional area of the two vessels is, in nearly every instance,
somewhat greater than that of the trunk; and the combined sectional area of all
the arterial branches greatly exceeds that of the aorta; so that the arteries
collectively may be regarded as a cone, the apex of which corresponds to the
aorta, and the base to the capillary system.
The arteries, in their distribution,
communicate with one another, forming what are called anastomoses, and
these communications are very free between the large as well as between the
smaller branches. The anastomosis between trunks of equal size is found where
great activity of the circulation is requisite, as in the brain; here the two
vertebral arteries unite to form the basilar, and the two anterior cerebral
arteries are connected by a short communicating trunk; it is also found in the
abdomen, where the intestinal arteries have very ample anastomoses between
their larger branches. In the limbs the anastomoses are most numerous and of
largest size around the joints, the branches of an artery above uniting with
branches from the vessels below. These anastomoses are of considerable interest
to the surgeon, as it is by their enlargement that a collateral circulation
is established after the application of a ligature to an artery. The smaller
branches of arteries anastomose more frequently than the larger; and between
the smallest twigs these anastomoses become so numerous as to constitute a
close network that pervades nearly every tissue of the body.
Throughout the body generally the larger
arterial branches pursue a fairly straight course, but in certain situations
they are tortuous. Thus the external maxillary artery in its course over the
face, and the arteries of the lips, are extremely tortuous to accommodate
themselves to the movements of the parts. The uterine arteries are also
tortuous, to accommodate themselves to the increase of size which the uterus
undergoes during pregnancy.
The pulmonary artery conveys the
venous blood from the right ventricle of the heart to the lungs. It is a short,
wide vessel, about
Relations.—The whole of this vessel is contained within the pericardium. It is
enclosed with the ascending aorta in a single tube of the visceral layer of the
serous pericardium, which is continued upward upon them from the base of the
heart. The fibrous layer of the pericardium is gradually lost upon the external
coats of the two branches of the artery. In front, the pulmonary artery
is separated from the anterior end of the second left intercostal space by the
pleura and left lung, in addition to the pericardium; it rests at first upon
the ascending aorta, and higher up lies in front of the left atrium on a plane
posterior to the ascending aorta. On either side of its origin is the
auricula of the corresponding atrium and a coronary artery, the left coronary
artery passing, in the first part of its course, behind the vessel. The
superficial part of the cardiac plexus lies above its bifurcation, between it
and the arch of the aorta.
The right branch of the pulmonary
artery (ramus dexter a. pulmonalis), longer and larger than the
left, runs horizontally to the right, behind the ascending aorta and superior
vena cava and in front of the right bronchus, to the root of the right lung,
where it divides into two branches. The lower and larger of these goes to the
middle and lower lobes of the lung; the upper and smaller is distributed to the
upper lobe.
The left branch of the pulmonary artery
(ramus sinister a. pulmonalis), shorter and somewhat smaller than the
right, passes horizontally in front of the descending aorta and left bronchus
to the root of the left lung, where it divides into two branches, one for each
lobe of the lung.
Above, it is connected to the
concavity of the aortic arch by the ligamentum arteriosum, on the left of which
is the left recurrent nerve, and on the right the superficial part of the
cardiac plexus. Below, it is joined to the upper left pulmonary vein by
the ligament of the left vena cava.
The terminal branches of the pulmonary
arteries will be described with the anatomy of the lungs.
The
Aorta
|
The aorta is the main trunk of a series of
vessels which convey the oxygenated blood to the tissues of the body for their
nutrition. It commences at the upper part of the left ventricle, where it is
about
the Ascending Aorta (Aorta Ascendens) — The ascending aorta is about
Relations.—The ascending aorta is covered at its commencement by the trunk of the
pulmonary artery and the right auricula, and, higher up, is separated from the
sternum by the pericardium, the right pleura, the anterior margin of the right
lung, some loose areolar tissue, and the remains of the thymus; posteriorly,
it rests upon the left atrium and right pulmonary artery. On the right side,
it is in relation with the superior vena cava and right atrium, the former
lying partly behind it; on the left side, with the pulmonary artery.
Branches.—The
only branches of the ascending aorta are the two coronary arteries which supply
the heart; they arise near the commencement of the aorta immediately above the
attached margins of the semilunar valves.
The Coronary Arteries.—The Right Coronary Artery (a. coronaria [cordis] dextra)
arises from the right anterior aortic sinus. It passes at first between
the conus arteriosus and the right auricula and then runs in the right portion
of the coronary sulcus, coursing at first from the left to right and then on
the diaphragmatic surface of the heart from right to left as far as the
posterior longitudinal sulcus, down which it is continued to the apex of the
heart as the posterior descending branch. It gives off a large marginal
branch which follows the acute margin of the heart and supplies branches to
both surfaces of the right ventricle. It also gives twigs to the right atrium
and to the part of the left ventricle which adjoins the posterior longitudinal
sulcus.
The Left Coronary Artery (a.
coronaria [cordis] sinistra), larger than the right, arises
from the left anterior aortic sinus and divides into an anterior descending and
a circumflex branch. The anterior descending branch passes at first
behind the pulmonary artery and then comes forward between that vessel and the
left auricula to reach the anterior longitudinal sulcus, along which it
descends to the incisura apicis cordis; it gives branches to both ventricles.
The circumflex branch follows the left part of the coronary sulcus,
running first to the left and then to the right, reaching nearly as far as the
posterior longitudinal sulcus; it gives branches to the left atrium and
ventricle. There is a free anastomosis between the minute branches of the two
coronary arteries in the substance of the heart.
Peculiarities.—These vessels occasionally arise by a common trunk, or their number may
be increased to three, the additional branch being of small size. More rarely,
there are two additional branches.
The Arch of the Aorta (Arcus Aortae) — The arch of the aorta begins at the level of
the upper border of the second sternocostal articulation of the right side, and
runs at first upward, backward, and to the left in front of the trachea; it is
then directed backward on the left side of the trachea and finally passes
downward on the left side of the body of the fourth thoracic vertebra, at the
lower border of which it becomes continuous with the descending aorta. It thus
forms two curvatures: one with its convexity upward, the other with its
convexity forward and to the left. Its upper border is usually about
Relations.—The arch of the aorta is covered anteriorly by the pleurae and
anterior margins of the lungs, and by the remains of the thymus. As the vessel
runs backward its left side is in contact with the left lung and pleura.
Passing downward on the left side of this part of the arch are four nerves; in
order from before backward these are, the left phrenic, the lower of the
superior cardiac branches of the left vagus, the superior cardiac branch of the
left sympathetic, and the trunk of the left vagus. As the last nerve crosses
the arch it gives off its recurrent branch, which hooks around below the vessel
and then passes upward on its right side. The highest left intercostal vein
runs obliquely upward and forward on the left side of the arch, between the
phrenic and vagus nerves. On the right are the deep part of the cardiac
plexus, the left recurrent nerve, the esophagus, and the thoracic duct; the
trachea lies behind and to the right of the vessel. Above are the
innominate, left common carotid, and left subclavian arteries, which arise from
the convexity of the arch and are crossed close to their origins by the left
innominate vein. Below are the bifurcation of the pulmonary artery, the
left bronchus, the ligamentum arteriosum, the superficial part of the cardiac
plexus, and the left recurrent nerve. As already stated, the ligamentum
arteriosum connects the commencement of the left pulmonary artery to the aortic
arch.
Between the origin of the left subclavian
artery and the attachment of the ductus arteriosus the lumen of the fetal aorta
is considerably narrowed, forming what is termed the aortic isthmus,
while immediately beyond the ductus arteriosus the vessel presents a fusiform
dilation which His has named the aortic spindle—the point of junction of
the two parts being marked in the concavity of the arch by an indentation or
angle. These conditions persist, to some extent, in the adult, where His found
that the average diameter of the spindle exceeded that of the isthmus by