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The development of the superior caval system in the rat.

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Dcparlriitrtt of Anatowti], ;l’m Y o r k 1‘niversit)i CoIle,qe of Xfdicinc.
The great anterior hlood vessels of mammals m ~ r eamong
the first to be studied both by wason of their great size wiid
of their location nest to tlie lieart. Limoiigthew, tlie superior
\-ma c a m lias ~ e c e i v e da share of this intei*est.
TTliile the dwelopment of the c a d system iii man has been
investigated rather thoroughly, corresponding investigations
on the lower mammals are, for the most part, incomplete.
This study has been attcmpted in tlic hope that a description
of the d e r e l o p m ~ n tof the superior vex1 CH\’B in the r a t may
be helpfnl, hoth in further understanding its development in
man, and in furnishiiig a hasis for a compamtiiTe study of t h r i
clcvclopment of the great anterior wiiis in the hlammalia.
It gix-es me great pleasiwe to bc ahlc to e s p r
deep appreciation f o r the hclp of Dr. E. 11. R1iear.e~sr-ithout
whose aid this woi-k i ~ o u l dnever liave been attempted. 3lanv
thanks a r e also due t o Doctor Shearer for his kiricliiess in
lendiiig the author his collection of rat embryos upon wliic4i
this study is basccl.
The classical n o r k 011 the developmcnt of tlie supcrior veiia
c a w was publiblied in 1850 by Xardiall. In tracing the
growth of the embrponic liurnaii veiioiis system he reported
that the remiiaiits of the left antci-ioi. cardinal w i n in the adult
formed, ontside the ~)cric~ir.diitm,
the left supcrior itilrr.costa1
vciii and that, inside 12ie pericaidiam, it formed the ohliqiw
L O U & A. S C H N E I U K K
auricular vein (since callcd thc oblique vein of Mar~:hall).
The left anterior cardinal vein also helped form the eoroi~ary
sinus. Cameron (’15) reported that there is normally present
in the adult huinan, a small veriule which r u m from the left
superior intercostal vein to the pericardium and then runs in
the vestigial fold of Uarshall for a short distance. This, he
says, is a remiiaiit of tlie extrapericardial part of the left
anterior cardinal.
This study was carried out on fifteen embryos of the albino
rat, Mus iiorregiciis albinus. These were embryos of itccurately timed age, which had originally been obtained from
The X7istar Institute and mounted in series of transverse
Enibryos r c d zn tke studg
Section thiclcnevs
Littpr no.
A g e of embr!jon
1 2 days, 3 1hours
1 2 days, 2 3 hours
1 3 days, 3 hours
13 days, 11hours
14 days, If hours
14 days, 3 hours
14 days, 7 1 hours
1: days,
15 davs, 20 hours
16 days,
(in intc )
1.5 and 20
sections, after fixation in Bonin’s fluid. All had hcen stained
with 3Iallory’s triple coiiiiectivr tissue stain. A summary of
the embryos used in this study will be found in table 1.
Titters had been numbered in the order in which they were
obtained but, for convmience, have been listed in the table in
the order of age. During the course of tlie study three embryos m-ere reconstructed in wax after tlie method o€ Born.
The anatomy of the adult r a t was studied on specimens in
which the arteries hat2 heen injected with a gelatin mass, so
that tlie veins were? for the most part, gorged with blood.
The arrangement of the adult reins has recently becn described by Greeiie ( ’ 3 5 ) . My own observations are completely
in accord with hers. Tlie external jugular vein of the rat is
relatively macli larger tlian in man, draining a large portion
of the head and neck. It joins the subclavian where that vein
crosses the first rib. The junction of the relatively small
internal jugular am1 the subclavian forms the superior vciia
cava (on either side). The two venae carae snperiores enter
the right atrium separately. The intei.na1 mammary axid
vertebral veins of either side drain into the vena cava superior
of the corresponding side. The left superior cava, ncar its
point of entrance to the right atrium, receives the azygos vein.
The azpgos receives all the iiitercostal veins of both sides
with the exception of tlie upper three 01' four right intercostal
veins, which drain into t h e right superior cam, either directly
or by means of a right superior intercostal vein; and the
lower two or tliree right iiitercostals which drain into the
azygor: via the hemiazygos vein.
The earliest stage reconstrnctecl was a litter of embryos of
1 2 days and 23 hours (fig. 1). a t this stage is represented
the cardinal venous plan which is characteristic of all mammalian embryos. Blood from tlie liead is returned t o tlie heart
by means of the large anterior cardinal veins. R'unning anteriorly tliroagh the body, carrying blood from the more caudaI
regions, are the paired posterior cardinal veins, each of which
joins tlie corresponding anterior cardinal t o form the common
cardinal veins or ducts of Cuvier, which cnipty into the sinus
venosns of the heart. The posterior cardinals receive a series
of segmental veins which accompany the segmental braiiclics
of the dorsal aorta and tlle spinal nerves. Some of tlie cervical segmentals join the anterior cardinal and the rest join the
posterior cardinal. I t will be seen in later stages that t h e w
is a gradual anterior shifting of tlie segmental veins, so that
eventually not only all the cervical hut w e n the upper thoracic veins come to drain into tlie anterior cardinals. At this
stage tlie drainage from the anterior limb bud (future snhclavian vein) is into the posterior cardinal just caudal to the
entrance of the latter into tlie common cardinal vein. This
also shifts anteriorly in development. This shifting process is
ao.dors., dorsal aorta
v.eard.ant., anterior cardinal vein or dext., left or right
posterior cardinal vein, inferior vena eava
v.cav.sup.sin. c drxt., left o right superior vena eavn
v.jug.cxt., external jugular vcin IY, fourth cervical segmental
v.subrl.sin., lcft suhclavian vein
Fig. 1 Rcconstructioil of the anterior veinF of an embryo of 1 2 days, 23 hours,
L extrolaternl
d a e to the caudal migration of tlic heart, which, of course,
draws with it tlic ducts of Cnvier.
An earlier stage (an embryo of 12 days and 11hours) m7as
studied. In this series the anterior cardinals a r e shorter for
the heart is located more cephalad. It is interesting to note
that, a t this stage, all except the first two cervical segmental
veins enter the posterior cardinal, only these two joining the
anterior cardinal. The entrance of the subclavian anlage is
also more caudal than in the later stage, entering the postcrior cardinal well away from the beginning of the common
I n embryos of 13 days and 11hours the heart has migrated
cauclally and there has been a corresponding increase in thc
length of the anterior cardinal veins. At this stage, the riglit
posterior cardinal veil1 is, for the first time, appreciablv
smaller than the left. The segmentals received by the anterior cardiiials ni-e still from the cervical regions and the
thoracic segmentals drain into the postcardinals. These latter
a r e the veins which will eventually hecoine the intercostal
veins and which, in the adult, join the azygos vein.
In an embryo of 14 days, 1; hours (fig. 2 ) , the rihs are, for
the first time, clearly TTisible. The rat, in contrast to most
mammals, has thirteen ribs and, therefore, twelve intercostal
spaces, each drained by a n intercostal vein and supplied by
a n intercostal artery ~ l 1 i c htakes origin from the aorta. The
highest segmental vein in the thoracic series (which will later
become the first intercostal) joins the posterior cardinal immediately behind the entrance of the postcardinal into thc duct
of Cujrier. At this stage the subclavian veins have come t o
drain into the anterior cardinals. Tn one embryo the first
intercostal vein of the left side has ali*eadycome to drain into
the anterior carcliiial vein. This is the oiily intercostal w i n
to drain into the anterior cardirial vein in this series. Both
posterior cardinals have now greatly diminished in size and
importance, due to the development of the inferior c a d system. This has been descrihrd by Butler ( '27). The dimjnution of tbe posterior cardinals is not, however, equal on the
two sides. On the left side, though relatively smaller than in
earlier stages, it is still easily traceable throughout the length
of the thoracic region, lying to the left of, and slightly posterior to, the aorta and receiving the left thoracic segmentals.
Those latter may now be regarded as the intercostal veins,
and the left posterior cardinal as the definite a d a g e of the
azygos vein. The posterior cardinal on the right side has
become extremely small and difficult to trace except at its
anterior end where it joins the duct of Cuvier. It is, in fact,
Fig. 9 Reconstruction of the antcrior veins of an embryo of 14 days, 1& hours,
dorsal view.
impossible to trace it at all in most cases, and it is difficult
t o understand what hecomes of the hlood in the right intercostals, since at this stage there seems to be n o connection
between them and the left posterior cardinal o r azygos rein.
Probably this matter can only bc completely cleared up by
study of injected specimens which unfortunately were not
available. At this stage, the external jugular has appeared
as a small vessel joining the anterior cardinal at a point some
distance anterior to the junction of the subclavian with the
anterior cardinal.
An embryo of 14 days and 11hours (fig. 3 ) is an important
stage in the development of the azygos system, since it shows
clearly the appearance of a venous plexus dorsal to the aorta
in the lower thoracic region, through wliich tlie right intercostal reins come eventually to drain into the axygos. This
plexus apparently represents all that there is in the rat of
Fig.3 Reeonstruetion of the veins of an embryo of 14 days, 11 bows, dorsolateral view.
L O U I S d. S C H N E I U E R
the siipracartliiial veins which a r e characteristic of most
mammals. The low.vcst iiitei*costals of both sides drain into
paired cliannels derived from this plexus, both of which chaiiriels end iii tlie l e f t posterior cardinal vein, wliicli we may iiow
call the azygos. The terminologr of the more anterior portions of the caval system may now also bc adapted to the
adult form. I t is appai-ent that the anterior cardinal on
either side, aiitcrior (ceplialad) to the point at \vhich it is
joined by tlie subclavian, represents the interiial jugular of
the adult. Caudal to this juiiction it may now be called the
superior cai-a. On the left side the superior vrna c a w is
represented by the anterior cardinal vein from its junction
with the siibclavian to the point at which the azygos (former
posterior cardinal) joiiis it, and from hcre to its eiitrarice into
the heart by the former duct of C'uvier. On tlie riglit side
also the siiperior r e n a cava is derived from the anterior
cartlinal and duct of Cluvier, but the distinction between these
two original segments is not so clear, due to the disappearunce
of the right posterior cardinal.
In an emhryo of 16 days, the veins have rcachcd practically
the adult condition. The azygos vein receives the lieniiazpgos
vein in its lower portion, while the anterior portion receives
t h e fourth l o the tenth intercostals of 130th sides. This anterior portion of the azygos is derived directly from tlie left
posterior cardiiial vein. The caudal portion, together with
the liemiazygos seems to be a dcrivative of the dorsal aortic
o r supracardinal plexus. The two embryos of this litter
show tliat the azygos and hemiazygos a r e of approximately
the same size. The hcniiazygos ascends to the right and the
azygos to the left of the vertebral column and each receives
the last few intercostals of the ipsolateral side. In embryo
T'IIa tlre azygos received the hemiazygos in the eleventh space
and each rcceived the elcveiitli aiid twelfth intercostals from
its own sidc. In embryo V I I b the liemiaq-gos entered in the
teiith space and rach drained the last three intercostal spaces
of its owii side. The dissection of several adults also revealed
the junction of the azygos aiid hrmiazygos to be in the tenth
or eleventh intercostal space. This eiiibrj-o (VIIh) is of
further interest in that it shows the manner in mhich the external jugular vein is shifted from its original point of drainage into the anterior earclinal (iiiteimd jugular) t o its adult
condition of joining thc subclavian (fig. 4, 1)j . On the right
side of this erribryo, the external jugular joins tlie mhclavian,
v. subcl
v cdv. sup sin,
v azygos
Fig. 4 1)ingrammatir schema of dcvelopiuent, ventral view. A , about 13 days;
B, about 14 days; C, about 1421 days; D, about 7 6 days.
but, on the left side, the manner of this change is explained
by the presence of a venous loop by which the external jugular communicates with both internal jugular aiid subclavian
veins. Apparently the terminal part of the external jugular
of the adult is a new formation, which causes its original
communication with the internal jugular to drop out,
The development of the great anterior veins in the rat follows a much simpler course than the corresponding development in man. Butler ('27) has shown that this is also true
in the development of the inferior veiia cava due to the absence of the snprwcardinal veins. He reports that, while there
are no definite supracardinal veins in the rat, there is, at one
time, a definite supracardinal veiioiis plexus. This absence
of the siipracardinal as a pervious venous cliaiinel results in
a different method of formation for the azygos vein than is
shown in man. I n humans, the azygos develops mostly from
the supracardinal vein and only its terminal part develops
from the postcardinal. I n the rat only a very small portion
of the azygos vein comes from the snpracardinal plexus and
most of it is a direct derivative of the left posterior cardinal
vein. The development of the superior rena cava is much
simpler than in man because the adult rat presents a macli
less advanced type of venous system with a double superior
vena cava the normal condition. The development of the
superior vena cava in maxi may be described as follo~vs:
The veins of the head and neck are derivcd from the anterior cardinals, which are, at first, paired and symmetrical but
are comparatively short and receive blood from the cervical
region through segmental branches which belong only to the
most cranial of the cervical segments. The other segmental
cervicals (including the siibelavian reins) open at first into
the posterior cardinals. Later the anterior cardinal veins become elongated and the segmental eervicals, including the
subclavian, open into them. Next an ariastornosing vessel is
formed obliquely across from a point on the left cardinal
opposite the entrance of the subclavian vein to a point nearer
the heart on the right cardinal. That part of the left cardinal
cranial to the entrance o i the subclavian becomes the left
internal jugular. The anastomosis forms the left innominate
vein. That part of the left cardinal between the eiitraiice of
the subclavian vein and the duct of Cuvier, the duct itself
and part of the left horn of the sinus venosi~sform the coronary sinus. On the right side the more distal portion of the
anterior cardinal forms the internal jugular, that part ~ h i c h
runs betwecii the entrance of the subclavian and the anastomosing vein becomes the right innoininate and the common
stem formed by the latter and the left inn om in at^ becomes
the superior veiia cava wliieli opens into the right atrium.
The cxteriial jugular appears on each side later than the anterior cardinal as an iiidepencleiit vessel which lies parallel to
the internal jugular vein and opens into it near the entrance
of the subclavian. This opening later shifts t o the subclai-ian
rein itself, wliere it is found in the adult. Tlie vestigial fold
and tlie oblique vein of Xarshall are all that usually remain
in the adult of the left superior vena cam. The right superior
cava, within the pericardiiim, passes in front of the right
pulmoiiary vessels and the left has a similar relation (it is
hound t o the pulmonary vessels by a fold of serous pericardium), T h e n the left superior vena cava atrophies, the
pericardial reflection remains in front of the left pulmonary
vessels. This is the vestigial fold of Xarshall. The intrapericardial portion of the left superior vena cava becomes the
oblique vein which turns around the left auricle to end in the
left horn of the sinus venosus (coronary sinus). The extrapericardial part of the left duct of Ciivier joins the superior
intercostal vein.
It will be noticed that development in the rat follows very
nearly the same pattern. The adult rat presents an anterior
venous system very much like that of a human embryo of
about 4 months. The superior veiia c a w of each side is developed from the terminal segment of tlir ant,erior cardinal
vein and from the duct oS Cuvier. The azygos vein arises
principally from the left posterior cardinal, but its caudal
portion, together with the heniiazygos, is derived from a new
plexus comparable to the supracardinal veins of other mammals. Tlie segmental veins of the thoracic series (which are
destined to become the intercostal veins of the adult) empty
at first into the postcardinal and then shift their position
c,ephalad so that the tliree most anterior vcins of the series
open into the anterior cardinal vein. The subclavian vein
also shows this cephalic migration for its origin is also from
the postcardinal, clianging later t o tlic anterior cardinal vein.
In the final metamorphosis to the taclult coiiditiori the first
three (usually) iiitcrcostal veins unite to l'orrn the superior
intevcostal vein uhich, 011 the left side, einpties into the azygos
vein a i d , o n tlic i*i$it, into the superior vena c w a . The
superior intercostal w i n of either or both sides ma^ he lacking so that, even on the left side, the two or tliwe highest
intercostals map drain directly into the vena Cam superior.
E. G. 1927 The iclativr role played by the embryoiiic b e i n s in ihe
devclopnicnt of the mammalinn vena c a m posterior. d n i . J. Anat.,
vol. 39, p. 267.
C4NERON. J. 1915 A specimen showing complete remains of the left supcrior
vena, c a w ; m-ith a description of a venulc Fvliich represents the 'lost'
cxtrapericsrdiac portion of t h e left duct of Cu1ic.r. J. Anat. and
Physiol , vol. 49, p. 140.
GBEENC,E. C . 1933 Anatomy of the rat. Trans. 91n. Philos. Sue, n.s., vol. 27.
>rARSHALL,, J. 1850 On t h e development of the great antcrior ?&is in inan and
rnarmmalia, etc. Philos. Trans. Roy. Soc. of London.
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development, superior, rat, system, cava
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