Thorax (1957), 12, 304.

THE INTERVENTRICULAR SEPTUMBY

E. W. T. MORRISFrom the Anatomy Department, St. Thomas's Hospital Medical School, Londoni

(RECEIVED FOR PUBLICATION JULY 26, 1957)

It is difficult to find in the literature a clear andconcise account of the development and form ofthe interventricular septum. Moreover, some ofthe accounts in the clinical literature are at vari-ance with that generally accepted by embryo-logists. For this reason and in view of the recenttechnical advances in the surgery of the heart, itseems opportune to describe the development andanatomy of the interventricular septum and tocorrelate this knowledge as far as possible withthe sites of interventricular septal defects.At an early stage the heart consists of the sinus

venosus, the common atrium, the common ven-tricle, and the bulbus cordis, serially arranged inthat order from the venous to the arterial end(Figs. 1 and 2).The formation of the interventricular septum is

a complicated process involving the partitioningof the common ventricle into a right and left, andthe separation of the distal part of the bulbuscordis into pulmonary and aortic outflow chan-nels in continuity with the ventricles, the proxi-mal portion of the bulbus cordis forming theinfundibulum of the right ventricle.The process begins at about 5 mm. crown-

rump length (35th day) and normally ends about17 mm. crown-rump length (49th day).The septum is formed from the following struc-

tures, which begin their contribution in thisorder: (1) the muscular wall of the commonventricle, (2) the bulbar ridges, and (3) the dor-sal atrioventricular cushion.

(1) The muscular contribution starts as a ridgeon the dorsal wall (Fig. 3) and extends on to theventral wall of the common ventricle. It is thencrescentic in form and its dorsal horn reaches theright end of the dorsal atrioventricular cushion.The ventral horn approaches the ventral atrio-ventricular cushion near its centre (Fig. 4). Whilethis septum is forming the central parts of theatrioventricular cushions fuse (11 mm. C.R.L.).The ventricles now communicate by a foramen,the boundary of which is formed mainly by thefree border of the muscular septum, except

between the tips of its two horns where theboundary is formed by the fused atrioventricularcushion (A, in Fig. 4). This septum does not liein one plane and the main part of its free borderforms a spiral (Figs. 4 and 5).

(2) While the muscular part is forming. changesare taking place in the relative positions of thebulbus cordis and the ventricles. Earlier the hearttube is flexed at the bulboventricular junction sothat the bulbus cordis comes to lie ventrally andto the right of the ventricle (Fig. 2). Their con-tiguous walls form a septum-the bulboven-tricular septum-around the lower free borderof which the two cavities communicate (see Figs.1, 2, and 3). This septum intervenes between thedistal part of the bulbus cordis and the atrio-ventricular opening (Fig. 3). It must disappearbefore the interventricular septum can be com-pleted.The absorption of the bulboventricular sep-

tum takes places whiel>the muscular septum isforming, and, as a result3nd of unequal growthrates in the different parts, the cavity of the upperpart of the bulbus cordis comes to lie astride themiddle and dorsal parts of the upper free borderof the muscular septum (Fig. 4). The next stephas meanwhile begun; it is the division of thedistal part of the bulbus cordis by two ridges, theright and left bulbar ridges, which grow from itswalls and ultimately fuse. These ridges start dis-tally and grow proximally (caudally). One growsdown the right side of the dorsal wall of the bulbto the right end of the ventral component of thefused atrioventricular cushions opposite theattachment of the dorsal horn of the muscularseptum to the dorsal component. In its progressthis ridge grows over and obliterates the ventralpart of the right atrioventricular canal (Fig. 4).The other ridge grows down the left side of theventral wall of the bulbus cordis and approachesand fuses with the muscular septum on its rightside a little distance along its margin from itsventral end (Figs. 4 and 5). The ventral end ofthe muscular septum lies to the left of the left

THE INTERVENTRICULAR SEPTUM

FIG. 1.-Left lateral view of foetal heart. S.V.,sinus venosus. A.T., common atrium.A.V.C., atrioventricular canal. V.,common ventricle. B.C., bulbus cordis.)2~

A.T B.V.S., bulboventricular septum.

A.T.

B.C.

A.V.C.

B.V.S.

FIG. 2.-Ventral view of heart in Fig. 1. Keyi {g > --V. as in Fig. I.

9_ l A . T ~~~~~~~~~~~~~~~~~~~~~~~.T

(\ q ~~~~~~~~~~B.C.V~~~~~~~~~~~~~~~~~~~~~~~~~~~~BVS

bulbar ridge. When the interventricular septumis completed the part of the original interven-tricular foramen, which is bounded by the ventralhorn of the muscular septum and. the fused atrio- "- Vventricular cushions between the horn tips, also s Ilies to the left of the completed interventricular'*.iseptum and so in the outflow channel of the left 'ventricle, and the ridge formed by the ventralhorn is subsequently absorbed leaving a smoothwall. The interatrial septum joins the atrial aspectof the fused atrioventricular cushions about theircentre. As a result the fused atrioventricularcushions between the attachment of the inter- FIG. 3.-Model of posterior partatrial septum and the dorsal horn of the muscu- of the bulbus cordis and

common ventricle. M.S.,lar septum intervene between the outflow part of commencing muscular inter-the left ventricle and the right atrium (Fig. 9). ventricular septum. D.C.This becomes reorientated and finally lies in the /; ventriCularsepu.dv C.andoVcar doralauhind.ventaplane of the interventricular septum. In the final , sbendocardialcushions. Re-form of the heart it becomes the atrioventricular mn sbaig.part of the pars membranacea septi (Fig. 7C).At this stage in the formation of the interven-

tricular septum the distal bulbus cordis is dividedby a septum with a lower free border which forms V.C.the upper boundary of a deficiency in the inter- A-V_ Cventricular septum, the lower boundary of whichis formed by the upper free border of the muscu- D.C.lar part (Fig. 6). Through this septal defect thetwo ventricles communicate. V.

(3) This aperture is closed and the septum thuscompleted by tissue derived from the dorsal atrio- M.S.ventricular cushion in the neighbourhood of theattachment of the right bulbar ridge and the

305

E. W. T. MORRIS

FIG. 4.-Same view as in Fig. 3 at a later stage. A, fused endo-cardial cushions. D.M.S.. dorsal part of the muscularinterventricular septum. V.M.S., ventral part of themuscular interventricular septum. R.B.R., right bulbarridge growing down the wall of the B.C. and finallyobliterating the ventral part of the right atrioventricularcanal. L.B.R., left bulbar ridge (cut edge).

,L.B.R.

V.M.S.

D.M.S.

FIG. 5.-The anterior part of the same model as Fig. 4. Key as inFig. 4.

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THE INTER VENTRICULAR SEPTUM

FIG. 6.-Model of a foetal heart at a later stagethan Figs. 4 and 5. The bulk of the wallof the future right ventricle has beenremoved. P.T., pulmonary trunk. A.O.,aorta. Y, site of origin of tissue fromdorsal endocardial cushion which com-

pletes the interventricular septum. Othersymbols as in Fig. 4.

A needle passed through the sep-tum in the commissure betweenthe septal cusps of the aorticvalve emerges into the right ven-tricle at the commissure betweenthe corresponding cusps in thepulmonary valve (Figs. 7 and 8).

dorsal end of the muscular septum (Fig. 6). Thistissue grows along and becomes fused with theedges of the aperture from dorsal to ventral.

THE ANATONIICAL SITUATION OF THE EMBRYOLOGI-CAL COMPONENTS IN THE FINAL FORM OF THE

INTERVENTRICULAR SEPTUMTHE MUSCULAR PART.-This forms the bulk of

the septum.THE BULBAR SEPTUM.-This forms the division

between the outflow channels and is continuousbelow with the anterior part of the muscular sep-tum. As seen from the left ventricle it forms thepart of the interventricular septum below the com-missure of the two septal (coronary) cusps of theaortic semilunar valve and the adjacent parts ofthese cusps. This aspect of this part of the sep-tum contains no ventricular muscle. From theright ventricular aspect this part is seen to formthe upper anterior part of the interventricularseptum which lies below the corresponding com-missure in the pulmonary valve. On this side,the septum is muscular up to the semilunar valve.

A. PART FORMED FROM DORSAL- ATRIOVENTRICULAR C U S H I ON.-

This forms the interventricularpart of the pars membranacea

A.V.C.(R.) septi. It lies anterior to the atrio-ventricular part of the pars mem-

-M.S. branacea septi and the whole pars

membranacea septi lies below andbehind the bulbar septum andabove the middle of the upperborder of the muscular septum(Figs. 7 and 8). As seen fromthe left ventricle, the pars mem-branacea septi lies below the com-missure between right septal and

non-septal cusps of the aortic valve and extendsforwards below the adjacent half of the rightseptal cusp (Fig. 7).The anterior part of the attached border of the

septal cusp of the mitral valve crosses the parsmembranacea septi between the atrioventricularand interventricular parts (Figs. 7 and 8).

OTHER FEATURES OF THE RIGHT VENTRICULARSURFACE OF THE INTERVENTRICULAR SEPTUMCRISTA SUPRAVENTRICULARIS.-From the point

of view of classifying septal defects the impor-tant structure is the crista supraventricularis. Thisis a smooth ridge of myocardium in the anteriorwall of the right ventricle lying parallel and closeto the attachment of the large anterior cusp of thetricuspid valve. It enlarges as it travels towardsthe left and ends by fusing with the interventricu-lar septum between the part formed from the bul-bar ridges and the interventricular part of the parsmembranacea septi.CONUS MUSCLE.-Arising from the septum just

below the attachment of the crista supraventricu-

A.O.

307

E. W. T. MORRIS

_-._.~; \.

.e'SEr " \~

i o-ni

-e

laris are a group of papillary muscles to which areattached the chordae tendinae of the adjacentparts of the anterior and septal cusps. One ofthese is usually larger than the rest, and this isthen called the conus muscle.

CONDUCTING SYSTEMATRIOVENTRICULAR BUNDLE.-Arising from the

atrioventricular node, the bundle passes into and

through the trigonum fibrosum dexter and thenceinto the lower border of the atrioventricular sep-tum, which lies just anterior to the trigone. Itpasses from the atrium into the ventricle at thejunction of the atrioventricular septum and inter-ventricular part of the pars membranacea septiand runs in the latter at its fusion with the muscu-lar septum or on either side of the upper borderof the muscular septum.

308

I

8It.. 0)utflos channiel ofI righIt seetricle. A. other end of t-Ot

(Fig. 7) seeni emerging betwseen thie septal cusps of' thle

ptIlranairtrsive. Thle dotted iennarks thc line of U'!lstIotof the bulbar ridiges. and the site of type (ao dlet-ects as seen

trom right side. B. septal cusp oif the tricUspid valvse itsanterior edge hias beenl retracted to show the site otf theinterventricular part of the patrs memntranacea septi. Cthe cut Lirface otf the cristai suprasentricularis alt itsjunction swith the ii erventlirtilar setptum. C).otiUS

rI SCle

Right Branch.-The right branch is a directcontinuation of the bundle and curves down-wards towards the apex, eit-her subendocardiallyor buried in the muscle. In this part of its course

it runs dorsal to the conus muscle and in some

hearts can be traced into the moderator band.

Left Branch.-The left branch arises from theatrioventricular bundle at varying sites from thepoint where it emerges from the trigonum to its

end. It usually forms a flat subendocardial bandof parallel fibres of paler colour than the truemyocardial tissue and running in a different direc-tion. The fibres fan out and two bands can some-

times be seen, one going to the neighbourhood ofeach papillary muscle.

DEVELOPMENTThe conducting tissue can be recognized as

differentiating from the cardiac musculature

*:N

-C.

- D.

-B.

E. W. T. MORRIS

* .....^ .., Or '.

Cv }wkr Ei.\;

NNw f ,<d;;..:S ...,4 Wi*; .:

FIG. 9.-10 mm. pig embryo. Transverse section. A.T., atria. V.V., ventricles. I.A.S., interatrial septum.M.S., muscular part of the interventricular septum. X, region of the fused atrioventricular endocardialcushion which forms the atrioventricular part of the pars membranacea septi.

during the sixth week (C.R.L. 7-9 mm.) andrapidly extends, then the atrioventricular node isrecognizable in the posterior wall of the atriumand the bundle extends along the dorsal wall ofthe atrioventricular canal under the dorsal atrio-ventricular cushion into the upper border of thedorsal horn of the muscular septum, where itdivides into a right and left branch. It is there-fore in situ before the final stage of the septum

'is complete and is then related to the posteriorand inferior borders of the foramen which stillexists.

TYPES OF UNCOMPLICATED DEFECTSThese may occur in the following sites:(a) In the septum separating the aortic and pul-

monary outflow, i.e., in that part of the septumformed from the bulbar ridges; as seen from the

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THE INTERVENTRICULAR SEPTUM

right ventricle they occur above the crista supra-ventricularis and from both aspects they are situ-ated.below the commissure of the septal cusps ofthe semilunar valves. Their site is therefore con-sistent with the bulbar ridges not fusing.

(b) In the region of the interventricular part ofthe pars membranacea septi and adjacent muscu-lar septum: in general these defects as seen fromthe right ventricle lie between the attachment ofthe septal cusp of the tricuspid valve and the septalattachment of the crista supraventricularis.The crista supraventricularis therefore separates

type (a) from type (b), the former lying above andthe latter below and behind it (Fig. 8).

(c) In the muscular part of the septum: theseusually lie near the apex and are sometimes mul-tiple, but may occur in the upper part of theseptum posteriorly, under the posterior part ofthe septal cusp of the tricuspid valve. In thispurely muscular defect the atrioventricular bundleand branches will probably lie anterior to andabove the defect.

ORIGIN OF THE DEFECTS.-By analogy with theknown aetiology of congenital defects of otherparts of the body it seems likely that septal de-fects may result either from failure of fusion ofthe several parts contributing to the septum, orfrom a breakdown of the tissue once formed. Itseems reasonable to assume that type (c) is formedby the latter method. Of types (a) and (b), it isimpossible to say more than that they are in sitescompatible with a failure of fusion.

FREQUENCY OF DIFFERENT TYPES OF DEFECTIn a series of 156 cases of ventricular septal

defects reported by Warden, DeWall, Cohen,Varco, and Lillehei (1957) and Kirklin, Harsh-barger, Donald, and Edwards (1957) six (3.8%)were of type (a) and 145 (91.7%) were of type (b).

Class (b) is thus the most common defect inthese series, and this is what might be expectedfrom a consideration of the stages in the develop-ment of the septum. This class of defect is com-patible with a failure of the final stage in theformation of the septum, that is, the closure ofthe temporary interventricular foramen by agrowth of tissue from the dorsal atrioventricularcushion, and it is in the final stages of a complexembryological process that developmental errorsmost commonly occur. This leads to a furtherreflection, namely, that when there are associatedcongenital defects of the heart the ones most com-

2C

monly found are those with which a ventriculatseptal defect is combined to make up the clini-cal entity of Fallot's tetralogy. In 120 cases ofventricular septal defect in the series of Wardenet al., 45 were associated with other structuralabnormalities of the heart. In 33 (73.3%) of thesecases the associated defects completed the com-plex of Fallot's tetralogy. In this condition thedivision of the outflow channel has been com-pleted by the fusion of the bulbar ridges. Thestage of development of the interventricular sep-tum which is at fault in this condition is the finalstage and the type of interventricular defect associ-ated with it is therefore class (b). It seems reason-able to suggest that the final closure of the tem-porary interventricular foramen by tissue derivedfrom the dorsal atrioventricular cushion repre-sents a critical stage in the development of theheart. Its failure leads not only to the commonesttype of uncomplicated interventricular septal de-fect, but also contributes to the most commonlyoccurring complex of defects (Fallot's tetralogy).In this condition the other defects arise at a laterstage in development than the septal defect whichaccompanies them.

SUMMARYThe steps in the development of the inter-

ventricular septum have been described. Inter-ventricular defects have been simply classified,and the relevant anatomical features of theventricles have been described.The possible relationship between the types of

defect described and the steps in the developmentof the interventricular septum have been indicated.

Finally, other congenital defects of the heartfrequently associated with interventricular septaldefects are briefly discussed. A short bibliographyis given which should enable any aspect to befollowed up in more detail.

I wish to express my gratitude to Professor D. V.Davies for help and advice, and to Miss Dew for thedrawings in Figs. 1 to 6 and to Mr. A. H. Woodingfor the photographs of Figs. 7 to 9.

BIBLIOGRAPHYEMBRYOLOGYArey, L. B. (1954). Developmental Anatomy, 6th ed. Saunders,

Philadelphia.Frazer, J. E. (1953). Manual of Embryology, 3rd ed. Bailliere,

Tindall and Cox, London.Hamilton, W. J., Boyd, J. D., and Mossman, H. W. (1952). Human

Embryology, 2nd ed. Heffer, Cambridge.Kramer, T. C. (1942). Amer. J. Anat., 71, 343.Odgers, P. N. B. (1938). J. Anat. (Lond.), 72, 247.Tandler, J. (1912). Manual of Human Embryology, Vol. 2, ed.

Keibel, F., and Mall, G. P. Lippincott, Philadelphia.

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E. W. T. MORRIS

ANATOMYJohnston, T. B., and Whillis, J. (1954). Gray's Anatomy, 31st ed.

Longmans, Green, London.Walmsley, T. (1929). In Quain's Elemtzents of Anatomtiy, 11th ed.,

Vol. IV, Pt. Ill. Longmans, Green. London.

CONDUCTING SYSTEMField. E. J. (1951). Brit. Heart J., 13, 129.Muil, A. R. (1954). J. Anat. (Lond.), 88, 381.Walls, E. W. (1945). Ibid., 79, 45.-- (1947). Ibid., 81, 93.Widran, J., and Ltv, M. (9151). Circdaltion, 4, 863.

SEPTAL DEFECTSAbbott, M. E. (1936). Atlas of Congeniital Car-diac Disease. Ameri-

can Heart Association, New York.Becu, L. M., Fontana, R. S., DuShane, J. W., Kirklin, J. W., BurchelJ.

H. B., and Edwards, J. E. (1956). Circulation, 14, 349.Kirklin, J. W., Harshbarger, H. G., Donald, D. E., and Edwards,

J. E. (1957). J. thorac. Surg., 33, 45.Rokitansky, C. von (1875). Die Defecte der Scheidewainzde des

Herzens. Braumuller, Vienna.Spitzer, A. (1923). Virchows Arch. path. Anat., 243, 81.Warden, H. E., DeWall, R. A., Cohen, M., Varco, R. L., and Lillehei,

C. W. (1957). J. thoroc. Surg., 33, 21.

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