Reprinted with permission from
JOURNAL OF PACING AND CLINICAL ELECTROPHYSIOLOGY , Volume 25, No. 3, March 2002
Copyright © 2002 by Futura Publishing Company, Inc., Armonk, NY 10504-0418.
Gross Structure of the Atriums: More Than an
Anatomic Curiosity?
SIEW YEN HO,* ROBERT H. ANDERSON,† and
DAMIÁN SÁNCHEZ-QUINTANA‡
From *Pediatrics, Faculty of Medicine, National Heart & Lung Institute, Imperial College of
Science, Technology and Medicine, and Royal Brompton and Harefield NHS Trust, †Cardiac Unit,
Institute of Child Health, University College, London, and the ‡Department of Anatomy,
Universidad de Extremadura, Departamento de Anatomía Humana, Facultad de Medicina,
Badajoz, Spain
HO, S.Y., ET AL.: Gross Structure of the Atriums: More Than an Anatomic Curiosity? Despite the extensive literature concerning atrial arrhythmias, there are relatively few articles on the anatomy of the atrial
chambers. Since electrophysiological mapping and interventional treatments of atrial arrhythmias involve entering the chambers, this article reviews the gross structures to provide a better understanding of
the atriums, the septum, and the connecting great veins. In addition, based on the human heart, differences between porcine and canine hearts are highlighted. The right and left atriums are characterized by
morphologically distinct appendages. The right atrium contains prominent muscular bundles and an extensive array of pectinate muscles. The distal ramifications of the terminal crest lead to the “flutter” isthmus. By contrast, the left atrium has relatively smooth walls. The atrial septum is limited to the valve of
the oval fossa and its immediate muscular rim. Atrial musculature extends beyond the veno-atrial junctions to the outside of the pulmonary veins. The longest sleeves are around the upper pulmonary veins,
and similar sleeves are seen around the superior caval vein. The structure of the atrium is more than an
anatomic curiosity. It has practical implications for mapping and interventional procedures. (PACE 2002;
25:342-350)
anatomy, arrhythmia, atrial fibrillation, atrial flutter, pulmonary vein
Introduction
The evolution of mapping techniques with interventional treatments for all varieties of atrial arrhythmias has drawn attention to the specific
structure of the atrial chambers. A search through
PubMed, the on-line search system of the National
Library of Medicine, USA, produced 2,695 articles
on atrial arrhythmias in the years 1996 to 2000.
This is to be compared with 903 in the previous 5year period from 1991 to 1995. The figures for
atrial fibrillation are considerably higher, being
4,839 and 2,930, respectively. On both counts, related articles on anatomy represent 7% to 36% of
the published literature, with gross anatomy
Supported in part by the Royal Brompton and Harefield Hospital Charitable Fund together with the Royal Brompton and
Harefield NHS Trust (S.Y.H.) and the British Heart Foundation
together with the Joseph Levy Foundation (R.H.A.). This review is based in part on Anglo-Spanish collaborative research
facilitated by a traveling grant from Acciones Integradas.
Address for reprints: S.Y. Ho, M.D., Paediatrics, National Heart
& Lung Institute
Imperial College of Science, Technology and Medicine, Dovehouse St., London SW3 6LY. Fax: 144-20-7351-8230; e-mail:
yen.ho;vaic.ac.uk
Received July 17, 2001; revised August 6, 2001; accepted August 15, 2001.
342
barely featuring. One interpretation would be that
all is known about gross anatomy, and the atriums
are merely anatomic curiosities. This is far from
the case, especially taking into consideration the
growth in experimental studies. The aim of this article was to provide the arrhythmologist, interventionist, and scientist with a background to the
structure of the right and left atriums and the atrial
septum in humans, highlighting the differences
from the dog and the pig where appropriate.
Location and Relationship to Adjacent
Structures
For uniformity of description, the heart is described as seen in an attitudinally appropriate position in the human for all species. Thus, in the pig
and the dog, spatial relationships are described as
if the animals are standing upright on their hind
legs. The ventral and dorsal aspects are designated
anterior and posterior, respectively, while cranial
and caudal become superior and inferior. The latter adjectives are used in reference to the caval
veins. Reflecting postural differences between the
human and other mammals, the human heart is
more trapezoidal in shape with the apex pointing
to the left. In dogs and pigs, in contrast, the apex
points inferiorly, reminiscent of the “Valentine
heart.”1,2 Consequently, when viewed from the
front, the right heart chambers in the human over-
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STRUCTURE OF ATRIUMS
lap their left heart counterparts, but they are related in a more side-by-side fashion in other mammals. The atrial chambers, nevertheless, are always situated posteriorly in relation to the
ventricular chambers.
The most characteristic anatomic feature of
both atrial chambers is the appendage (or auricle),
which is the pouch-like extension of the body of
the atrium, itself possessing also a venous component (venous sinus) and vestibule. The septum
separates the chambers. In the human, the appendages have distinctive shapes, allowing morphological differentiation between right and left
(Fig. 1). The right appendage is large and triangular with a broad base, whereas the left appendage
is a tube with a narrow junction, or mouth, to the
rest of the atrium. In canine and porcine hearts the
right appendages are more snout-like in shape
(Fig. 1). Extending anteriorly and superiorly, the
tip of the right appendage points leftward to lie
over the root of the aorta. The rest of the pectinated appendage forms the entire anterior atrial
wall. It is a mistake to consider only the tip as representing the appendage. The left appendage, located more superiorly than the right appendage,
points toward the aortic root. It is often found
overlying the main stem of the left coronary artery
with its tip reaching the root of the pulmonary
trunk. The left appendage in the dog resembles
that in the human heart, like a crooked little finger
with crenellations. In the pig, however, the left appendage is spade-shaped but retains the distinctive narrow junction.2–5
In the human heart, the superior and inferior
caval veins enter the right atrium at an obtuse angle, or nearly in alignment, with the superior vein
anterior to the inferior vein. The right upper pulmonary vein passes behind the superior cavoatrial
junction, while the lower pulmonary vein passes
behind the intercaval area (Fig. 2). In contrast, in
the pig and dog, the caval veins enter at a distinct
angle to one another. Within this angle run the
right pulmonary artery and upper pulmonary vein
(Fig. 2). In the dog heart, the portion of the right
lower pulmonary vein closest to the atrium
courses inferiorly, nearly parallel to the supradiaphragmatic segment of the inferior caval vein.
Demarcating the extensive and pectinate appendage from the smooth-walled venous component of the right atrium is the terminal groove
(“sulcus terminalis”), marking the site internally
of the terminal crest. This furrow is more pronounced in canine and porcine hearts than in the
human heart (Fig. 1). Indeed, the groove in the human heart usually is filled with the fatty tissues
that overlie the sinus node.6
Figure 1. The right (RAA) and left (LAA) atrial appendages in the human, dog, and pig viewed
from the right and left aspects. The RAA has a broad base marked by the terminal groove (broken
lines), whereas the LAA has a narrow junction with the atrium. Note the spatial relationships
between the RAA and the aorta (Ao) and that between the LAA and the pulmonary trunk (PT).
The angle between the superior (SCV) and inferior (ICV) caval veins is more marked in dog and
pig. The azygos vein (arrow) in the pig passes between the LAA and the left pulmonary veins. V
5 pulmonary veins.
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HO, ET AL.
Figure 2. Endocasts prepared from hearts of the three species and viewed from the anterior, right,
and left aspects. Note the extensive rough imprint made by the pectinate muscles of the right
appendage. By contrast, the left atrial wall is smoother. The great cardiac vein (arrowhead) courses
in the left atrioventricular junction and continues into the coronary sinus. The sinus is joined by
the azygos vein (arrow) in the pig.
The spatial relationships between the atrial
appendages and the aortic root cause the anterior
wall of both atriums to hug the whole of the noncoronary aortic sinus and part of the right coronary aortic sinus and the ascending aorta with the
transverse pericardial sinus interposing between
the atrial and arterial structures (Fig. 3). It is
within this anterior atrial wall that the interatrial
muscular bundle, Bachmann’s bundle, courses
subepicardially from the site of termination of the
terminal crest in front of the superior caval vein.
The left atrium lacks a terminal crest (“crista terminalis”), so there is no corresponding groove between the appendage and the pulmonary venous
component. The pulmonary venous component is
shaped like a pillow with the pulmonary veins entering the four corners of its posterior aspect in human hearts (Fig. 4). The left veins enter the atrium
more superiorly than the right veins. Variation in
the number of pulmonary veins in the human
heart is not uncommon.7 Sometimes two veins of
one or both sides unite prior to entering the
atrium. In others, an additional vein is found,
more frequently on the right side. Five or six pulmonary venous orifices are described for the ca344
nine heart, although some veins become confluent
just before entering the left atrium. In the authors’
limited experience, it is more common to find the
right upper veins joining to enter as one vein with
the orifice located slightly superiorly to the common left upper venous orifice (Fig. 4). The right
and left lower veins draining the diaphragmatic
lobes enter a large common orifice that is located
inferior to the left upper venous orifice. However,
the porcine heart has only two pulmonary venous
orifices. These are positioned side by side in the
left atrium, receiving the two groups of veins from
each lung. In all species, the pulmonary trunk
passes posteriorly from its infundibular origin
with the superior wall of the left atrium being related to the bifurcating pulmonary arteries (Fig. 4).
The posterior wall of the left atrium is related
to the trachea and its bifurcation. Externally, its
inferior wall is related to a systemic venous structure, the coronary sinus. This receives the middle
and great cardiac veins in the human and dog, but
also receives the so-called azygos vein in the pig.
Pigs lack a right azygos vein. Their left azygos, or
unpaired, vein drains the intercostal veins from
the thorax and, in some cases, the lumbar veins
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STRUCTURE OF ATRIUMS
Figure 3. The cardiac base is dissected by removing most of the atriums to show the location of
the transverse sinus (dotted lines) and its relationship to the aorta (Ao) and atrial walls. The
dissections in the dog and pig hearts also display the course of the coronary sinus (CS) relative
to the inferior wall of the LA. Note the aorta is more deeply wedged between the left and right
atrioventricular junctions in the human heart compared to dog and pig. LA 5 left atrium; PT 5
pulmonary trunk; RA 5 right atrium.
Figure 4. Endocasts of hearts from the human (A and B) and dog (C and D) viewed posteriorly (A
and C) and inferiorly (B and D). The pulmonary veins in the human heart enter the left atrium at
the four “corners” but are arranged differently in dog. The inferior views show the course of the
great cardiac vein (arrowheads) and the remnant of the oblique vein of the LA (arrows). Persistence
of the left superior caval vein would take the course indicated by the broken line. View E is a
human heart specimen viewed from the left. The ligament of Marshall (arrow) passes from between
the ostium of the LAA and the orifice of the LU superiorly to descend inferiorly into the CS. The
latter is also joined by the great cardiac vein (arrowhead). Ao 5 aorta; CS 5 coronary sinus; ICV
5 inferior caval vein; LA 5 left atrium; LAA 5 left atrial appendage; LU 5 left upper pulmonary
vein; PT 5 pulmonary trunk; V 5 pulmonary vein.
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HO, ET AL.
and the abdominal inferior caval vein.8,9 Rather
confusingly, the left azygos vein is also known as
the oblique vein of the left atrium which, eponymously, is the vein of Marshall in man, although
the territories drained are different. When persisting in humans and dogs, the oblique vein is the
remnant of the left superior caval vein.10 Normally
in the human, and in most dogs, the oblique vein
is ligamentous in its superior course. Nevertheless, the course taken by the vein in all three
species is obliquely from superior to inferior along
the lateral atrial wall, extending between the appendage and the left pulmonary veins (Figs. 2 and
4).
In adults, the coronary sinus passes approximately 0.5–1.3 cm proximal to the plane corresponding internally to the mitral valvar orifice
(Fig. 5). Reportedly, the orifice of the coronary sinus in patients with atrioventricular nodal reentrant tachycardia is larger than in control patients.11,12 The sinus in these patients is typically
funnel-like in contrast to its normal tubular
shape.11
Internal Aspects
Atrial chambers are muscular sacks perforated by the orifices of the great veins within the
venous component, the orifices of the tricuspid
and mitral valves surrounded by their vestibules
at the atrioventricular junction and, in the fetus,
the oval fossa at the septum. The topography of
right and left atriums differ in that the left atrium
is much smoother (Fig. 2). The large appendage
dominates the right atrium with its wall lined internally by pectinate muscles that extend like the
teeth of a comb from the terminal crest to reach anteriorly as far as the smooth vestibule surrounding
the tricuspid valvar orifice. In the anterior wall,
the larger pectinate muscles are arranged nearly in
parallel fashion, with thin branches in between,
leaving areas of thin atrial wall (Fig. 6). Superiorly, at the tip of the appendage, the pectinate
muscles lose their parallel arrangement. The terminal crest sweeps like a twisted `C,’ originating
from the septal wall, passing anterior to the orifice
of the superior caval vein, descending posteriorly
and laterally, and then turning anteriorly to skirt
the right side of the orifice of the inferior caval
vein (Fig. 6). Close to its origin, the terminal crest
is joined by a prominent bundle, the sagittal bundle, or “septum spurium” that extends anterolaterally toward the tip of the appendage. The distal
ramifications of the terminal crest, as it turns toward the orifice of the coronary sinus, vary from
heart to heart. The prevailing pattern, of abundant
crossovers and interlacing muscle bundles, suggests nonuniform anisotropy in the “flutter isthmus.”13,14 In addition, the posterior portion of the
isthmus tends to be thin-walled and fibrotic,
while the mid-portion is trabeculated, and the anterior portion smooth (Fig. 6A and D).13 The posterior two portions often form a pouch, described
as the sub-Eustachian pouch but, attitudinally, anterior to the Eustachian valve.15 It is properly
termed the sub-Thebesian pouch. Bounded by the
tricuspid orifice on one side, and the terminal
Figure 5. Long-axis sections through two hearts approximating to the two chamber
echocardiographic parasternal planes. The section in A is to the left of the atrial septum while
that shown in B is to the right. Section A shows the CS in the inferior pyramidal space and its
distance from the attachment of the mitral valve (arrowhead). The inferior pyramidal space in B
has been dissected to reveal the RCA and its tributary to the atrioventricular node (arrowhead).
Ao 5 aorta; CS 5 coronary sinus; ICV 5 inferior caval vein; LA 5 left atrium; LPV 5 left pulmonary
veins; MCV 5 middle cardiac vein; OF 5 oval fossa; PT 5 pulmonary trunk; RCA 5 right coronary
artery; TC 5 terminal crest.
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Figure 6. A and B are views of the internal aspects of human right and left atriums respectively
while C shows the internal aspect of the right atrium in dog. A and C are displayed in simulated
right anterior oblique projection to show the course of the TC (broken line) relative to the septum,
the parietal wall (reflected) and orifices of the superior caval vein (*) and the inferior caval vein.
The pectinate muscles in the lateral wall are ridges separated by the thin atrial wall. In probe
patency of the OF, the anterosuperior margin (open arrow in A) is nonadherent, corresponding
to the crescent in B (open arrow). The TC branches distally toward the “flutter” isthmus ($ ). Note
the more prominent tubercle of L and ER in dog shown in C. A and C show the location of the
atrioventricular conduction system marked with an open circle at the apex of Koch’s triangle.
Panel D is a four chamber section through a human heart taken through the level corresponding
to the apex of the triangle of Koch. The infolded atrial wall (dotted line) enclosing epicardial
tissues is the posterior rim of the OF (double arrows). The posterior, middle, and anterior portions
of the flutter isthmus are indicated by 1, 2, and 3, respectively. Note the ramifications of the TC
(broken line) leading to the flutter isthmus. By contrast, the “septal” isthmus ([) is the smooth
vestibular part of the atrial wall overlying the ventricular wall. AM 5 aortic mound; Ao 5 aorta;
CS 5 coronary sinus; ER 5 Eustachian ridge; ICV 5 inferior caval vein; L 5 Lower; LA 5 left
atrium; LAA 5 left atrial appendage; MV 5 mitral valve; OF 5 oval fossa; PT 5 pulmonary trunk;
RLPV 5 right lower pulmonary vein; RUPV 5 right upper pulmonary vein; T 5 tip of appendage ;
TC 5 terminal crest; TV 5 tricuspid valve.
crest and Eustachian ridge on the other, the major
circuit for common flutter must pass across the
pectinate muscles on the lateral wall to be “funneled” into the flutter isthmus, and thence toward
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the triangle of Koch.16 The so-called flutter isthmus forms the inferior border of the right atrium
when viewed in the simulated right anterior
oblique projection (Fig. 6A).15 In the same projec-
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HO, ET AL.
tion, the “septal isthmus” is situated more superiorly. Although dubbed “septal,” this isthmus comprises the smooth vestibular atrial wall between
the orifice of the coronary sinus and the hinge of
the tricuspid valve.13 It overlies epicardial fat and
ventricular myocardium (Figs. 5B and 6D).
In the human, the Eustachian valve varies in
development from large to virtually absent, from
muscular to membrane-like, and occasionally
lace-like in a Chiari network. Similarly, the
Thebesian valve, guarding the orifice of the coronary sinus, shows a wide variation in morphology.
These venous valves tend to be poorly formed or
absent in the porcine and canine hearts. The Eustachian ridge, separating the orifices of the inferior caval vein from the coronary sinus, and the tubercle of Lower, which is the area of the intercaval
wall, tend to be less prominent in humans than in
other mammals (Fig. 6). Owing to the sharper angulation between the caval veins in porcine and
canine hearts, the tubercle of Lower and the Eustachian ridge are pronounced protrusions that almost form a `V’ with the apex pointing anterosuperiorly. The oval fossa, a disc-like depression, is
more inferiorly and posteriorly situated than in
the human. The valve of the fossa is surrounded
by a muscular rim on the right atrial aspect, albeit
that the rim is not well defined in every heart. In
all species, smaller depressions, which are the orifices of Thebesian veins, can be seen in all parts of
the atrial wall. It is not uncommon to find one of
these large enough to lodge the tip of a catheter.
The internal aspect of the right atrium also
provides the landmarks to the triangle of Koch, an
established guide to the location of the atrioventricular conduction tissues.17,18 In attitudinal orientation, the Eustachian ridge containing the tendon of Todaro forms the posterior border,19 while
the hinge line of the septal leaflet of the tricuspid
valve forms the anterior border.20 The atrioventricular node and bundle lie in the angle enclosed
by these two borders (Fig. 6A and C). At the base
of the triangle lies the orifice of the coronary sinus. The dimensions of Koch’s triangle vary considerably, as shown in normal postmortem hearts
and in patients with nodal reentry tachycardia.20–22
The left atrium is relatively featureless internally. Pectinate muscles are confined mainly
within the atrial appendage. They give endocasts
a coral-like appearance (Fig. 2). The constriction
at the mouth of the appendage sometimes produces a pronounced shelf between the appendage
and the left upper pulmonary veins. Whether
tubular-shaped as in human and dog or spadeshaped as in pig, the left appendage is like a culde-sac with its narrow ostium. The venous component and the vestibule to the mitral valve are
348
smooth-walled (Fig. 2). The septal aspect is usually marked by shallow and irregular pits on the
valve of the oval fossa. A crescent marks the free
edge of the valve. It is through this margin that a
probe or catheter can be pushed obliquely and anterosuperiorly along the fossal surface on the right
side to enter the left atrium (Fig. 6). In the human,
the site of the crescent is just behind the anterior
wall of the left atrium, since the plane of the atrial
septum itself runs obliquely. The anterior wall can
be thin close to this point, increasing the risk of
exiting the heart during attempted septal puncture.23
The Atrial Septum
If a septum is defined as a wall that separates
adjacent cardiac chambers so that a cut made in it
will go from one chamber to the other without
traversing epicardial tissues or exiting the heart,
then the true extent of the atrial septum is confined to the floor of the oval fossa and its anteroinferior muscular rim as viewed from the right
atrial aspect (Fig. 6A and C).7 On the left atrial aspect, the featureless topography does not allow
such demarcation. Besides, the flap valve overlaps
the oval rim quite considerably. The septum is
less extensive than the blade-shaped structure described by Sweeney and Rosenquist,24 whose observations were based on transilluminating the
wall. The major portion of the rim around the
fossa is an infolding of the muscular atrial wall
that is filled with epicardial fat (Fig. 6D). Superiorly and posteriorly, this is the interatrial groove,
also known as Waterston’s or Sondergaard’s
groove. Dissections into this groove permit the left
atrium to be entered without transgressing into the
right atrium. Anteriorly and inferiorly, the rim
and its continuation into the atrial vestibules overlie the muscle masses of the ventricles with the
fat-filled inferior pyramidal space intervening
(Fig. 5).22 The right atrial aspect gives an impression of a larger septal component, mainly due to
the expanse of the atrial wall that is described as
the aortic mound. This is the anterior wall of the
right atrium just behind the aortic root (Fig. 6A
and C).
Muscular Bands and Sleeves
As the internal topography of the right atrium
suggests, its walls are made up of muscular bundles
of varying prominence. These include the terminal
crest, Eustachian ridge, the rim of the oval fossa, the
pectinate muscles, and the tricuspid vestibule. On
the epicardial aspect, the intercaval bundle is an array of obliquely arranged myofibers that sweep
from the posterior interatrial groove across the intercaval area to merge with the pectinate muscles in
the lateral wall.25,26 The anterior right atrial wall is
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STRUCTURE OF ATRIUMS
mainly formed by myofibers that run parallel to the
atrioventricular groove (Fig. 7). A band of these
fibers can be traced from the superior cavoatrial
junction leftward to become the superficial fibers of
the left atrium. Best known as Bachmann’s bundle,
this band crosses the anterior interatrial groove. Although not ensheathed by fibrous tissue, and of
varying widths and thicknesses in different hearts,
and without distinct margins, the parallel arrangement of its fibers almost certainly confer upon it the
status of the superhighway for interatrial conduction.27 This is notwithstanding the fact that there
are other interatrial bundles crossing the superior
or posterior parts of the interatrial groove, and still
others that connect the wall of the coronary sinus to
the left or right atriums,7,28,29 or the ligament of
Marshall to the left atrium.30
Being mainly smooth, the left atrial wall gives
the impression of muscular homogeneity. Detailed dissections through its full thickness, however, reveal it to be composed of overlapping
broad bands of myofibers. These run in different
directions, but are not insulated by fibrous
sheaths.7,28 Generally, the superficial myofibers
run parallel to the atrioventricular junction, while
the deeper fibers run obliquely or perpendicularly
to the junction (Fig. 7). The superior wall, however, is composed mainly of perpendicular or
oblique fibers of the septopulmonary bundle.
The recording of electrical activity in the thoracic veins,31,32 and the ablation technique developed by Haïssaguerre et al.33 for treating paroxys-
mal atrial fibrillation, have focused attention on
muscular sleeves in these veins.34 Encircling the
veins to varying extents, the sleeves are continuations of atrial musculature along the epicardial aspect of the venous wall. They tend to be thicker and
more complete near the cavoatrial junction, but taper and fragment as they move further away from
the junction.26,35,36 The upper pulmonary veins
usually have longer sleeves than the lower pulmonary veins, which are often devoid of musculature. Measured from the veno-atrial junction, the
sleeves in fixed human specimens ranged from 0.2
to 1.7 cm in one study28 and extended to 2.5 cm in
another. 36 It is no coincidence that ectopic focuses
are most commonly found in the superior veins
since these veins have the longest sleeves. The
study by Saito and colleagues35 found no significant
difference in lengths of sleeves between patients
with atrial fibrillation and those without atrial arrhythmias. Although cells looking like nodal cells
have been described in the rat heart,37 these have
not been found in human or dog hearts.35,36
Muscular sleeves are seldom well developed
around the inferior caval vein. By contrast, the superior caval vein usually has a discernible cuff of
atrial muscle extending some distance from the
cavoatrial junction. In dogs and pigs, the superior
sleeves are longer, passing proximal to the pericardial reflection in the dog.
Conclusions
The complex structure of the atrial chambers
merits an anatomic revisit, especially in the light
Figure 7. The architecture of the myofibers in the subepicardium of a human heart shown from
the anterior (A) and posteroinferior (B) aspects. Bachmann’s bundle is the band of parallel
myofibers between the broken lines that cross the interatrial groove (open arrow). View B shows
the intercaval myofibers in the RA and the left atrial fibers extending to the upper pulmonary
veins. CS 5 coronary sinus; ICV 5 inferior caval vein; LA 5 left atrium; LL and LU 5 left lower
and upper pulmonary veins respectively; RA 5 right atrium; RL and RU 5 right upper and lower
pulmonary veins respectively; SCV 5 superior caval vein; TS 5 transverse sinus.
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HO, ET AL.
of newer modalities for investigation and treatment of atrial arrhythmias. The atriums are sacks
full of holes, with the geometric arrangement dictating the paths of conduction of impulses, superimposed with regional and local variations in
gross morphology, not to mention fine structure.
The practical implications of the gross differences
between species are obvious. Atrial structure
should not remain merely an anatomical curiosity!
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