Thorax: first published as 10.1136/thx.12.4.304 on 1 December 1957. Downloaded from Thorax (1957), 12, 304. THE INTERVENTRICULAR SEPTUM BY E. W. T. MORRIS From 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 and between the tips of its two horns where the concise account of the development and form of boundary is formed by the fused atrioventricular the interventricular septum. Moreover, some of cushion (A, in Fig. 4). This septum does not lie the accounts in the clinical literature are at vari- in one plane and the main part of its free border ance with that generally accepted by embryo- forms a spiral (Figs. 4 and 5). logists. For this reason and in view of the recent (2) While the muscular part is forming. changes technical advances in the surgery of the heart, it are taking place in the relative positions of the seems opportune to describe the development and bulbus cordis and the ventricles. Earlier the heart anatomy of the interventricular septum and to tube is flexed at the bulboventricular junction so correlate this knowledge as far as possible with that the bulbus cordis comes to lie ventrally and the sites of interventricular septal defects. to the right of the ventricle (Fig. 2). Their con- At an early stage the heart consists of the sinus tiguous walls form a septum-the bulboven- venosus, the common atrium, the common ven- tricular septum-around the lower free border tricle, and the bulbus cordis, serially arranged in of which the two cavities communicate (see Figs.copyright. that order from the venous to the arterial end 1, 2, and 3). This septum intervenes between the (Figs. 1 and 2). distal part of the bulbus cordis and the atrio- The formation of the interventricular septum is ventricular opening (Fig. 3). It must disappear a complicated process involving the partitioning before the interventricular septum can be com- of the common ventricle into a right and left, and pleted. the separation of the distal part of the bulbus The absorption of the bulboventricular sep- http://thorax.bmj.com/ cordis into pulmonary and aortic outflow chan- tum takes places whiel>the muscular septum is nels in continuity with the ventricles, the proxi- forming, and, as a result3nd of unequal growth mal portion of the bulbus cordis forming the rates in the different parts, the cavity of the upper infundibulum of the right ventricle. part of the bulbus cordis comes to lie astride the The process begins at about 5 mm. crown- middle and dorsal parts of the upper free border rump length (35th day) and normally ends about of the muscular septum (Fig. 4). The next step 17 mm. crown-rump length (49th day). has meanwhile begun; it is the division of the The septum is formed from the following struc- distal part of the bulbus cordis by two ridges, the on September 24, 2021 by guest. Protected tures, which begin their contribution in this right and left bulbar ridges, which grow from its order: (1) the muscular wall of the common walls and ultimately fuse. These ridges start dis- ventricle, (2) the bulbar ridges, and (3) the dor- tally and grow proximally (caudally). One grows sal atrioventricular cushion. down the right side of the dorsal wall of the bulb (1) The muscular contribution starts as a ridge to the right end of the ventral component of the on the dorsal wall (Fig. 3) and extends on to the fused atrioventricular cushions opposite the ventral wall of the common ventricle. It is then attachment of the dorsal horn of the muscular crescentic in form and its dorsal horn reaches the septum to the dorsal component. In its progress right end of the dorsal atrioventricular cushion. this ridge grows over and obliterates the ventral The ventral horn approaches the ventral atrio- part of the right atrioventricular canal (Fig. 4). ventricular cushion near its centre (Fig. 4). While The other ridge grows down the left side of the this septum is forming the central parts of the ventral wall of the bulbus cordis and approaches atrioventricular cushions fuse (11 mm. C.R.L.). and fuses with the muscular septum on its right The ventricles now communicate by a foramen, side a little distance along its margin from its the boundary of which is formed mainly by the ventral end (Figs. 4 and 5). The ventral end of free border of the muscular septum, except the muscular septum lies to the left of the left Thorax: first published as 10.1136/thx.12.4.304 on 1 December 1957. Downloaded from THE INTER VENTRICULAR SEPTUM 305 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. A.T)2~ B.V.S., bulboventricular septum. A.T. B.C. A.V.C. B.V.S. FIG. 2.-Ventral view of heart in Fig. 1. Key i {gV. >as--in Fig. I. 9_ l A . T ~~~~~~~~~~~~~~~~~~~~~~~.T V~~~~~~~~~~~~~~~~~~~~~~~~~~~~BVS ( bulbar ridge. When the interventricular septum copyright. is completed the part of the original interven- tricular foramen, which is bounded by the ventral horn of the muscular septum and. the fused atrio- "- V ventricular cushions between the horn tips, also s I \q lies to the left of the completed interventricular'*.i ~~~~~~~~~~B.C. septum and so in the outflow channel of the left ' http://thorax.bmj.com/ ventricle, and the ridge formed by the ventral horn is subsequently absorbed leaving a smooth wall. The interatrial septum joins the atrial aspect of the fused atrioventricular cushions about their centre. As a result the fused atrioventricular cushions between the attachment of the inter- FIG. 3.-Model ofposterior part atrial septum and the dorsal horn of the muscu- of the bulbus cordis and lar common ventricle. M.S., septum intervene between the outflow part of commencing muscular inter- the left ventricle and the right atrium (Fig. 9). ventricular septum. D.C. on September 24, 2021 by guest. Protected This becomes reorientated and finally lies in the /; andoVcarventriCularsepu.dvdoralauhind.ventaC. plane 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 divided by a septum with a lower free border which forms V.C. the upper boundary of a deficiency in the inter- ventricular septum, the lower boundary of which A-V_ C is formed by the upper free border of the muscu- D.C. lar part (Fig. 6). Through this septal defect the two ventricles communicate. V. (3) This aperture is closed and the septum thus completed by tissue derived from the dorsal atrio- M.S. ventricular cushion in the neighbourhood of the attachment of the right bulbar ridge and the Thorax: first published as 10.1136/thx.12.4.304 on 1 December 1957. Downloaded from 306 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 muscular interventricular septum. V.M.S., ventral part of the muscular interventricular septum. R.B.R., right bulbar ridge growing down the wall of the B.C. and finally obliterating the ventral part of the right atrioventricular canal. L.B.R., left bulbar ridge (cut edge). copyright. http://thorax.bmj.com/ , L.B.R. V.M.S. on September 24, 2021 by guest. Protected D.M.S. FIG. 5.-The anterior part of the same model as Fig. 4. Key as in Fig. 4. Thorax: first published as 10.1136/thx.12.4.304 on 1 December 1957. Downloaded from THE INTER VENTRICULAR SEPTUM 307 A.O. FIG. 6.-Model of a foetal heart at a later stage than Figs. 4 and 5. The bulk of the wall of the future right ventricle has been removed. P.T., pulmonary trunk. A.O., aorta. Y, site of origin of tissue from dorsal endocardial cushion which com- pletes the interventricular septum. Other symbols as in Fig. 4. A needle passed through the sep- tum in the commissure between the septal cusps of the aortic valve emerges into the right ven- tricle at the commissure between the corresponding cusps in the pulmonary valve (Figs. 7 and 8). A. PART FORMED FROM DORSAL - ATRIOVENTRICULAR C U S H I O N.- This forms the interventricular part 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 and copyright. behind the bulbar septum and above the middle of the upper border of the muscular septum (Figs. 7 and 8). As seen from the left ventricle, the pars mem- branacea septi lies below the com- http://thorax.bmj.com/ missure between right septal and dorsal end of the muscular septum (Fig. 6). This non-septal cusps of the aortic valve and extends tissue grows along and becomes fused with the forwards below the adjacent half of the right edges of the aperture from dorsal to ventral. septal cusp (Fig. 7). The anterior part of the attached border of the THE ANATONIICAL SITUATION OF THE EMBRYOLOGI- septal cusp of the mitral valve crosses the pars CAL COMPONENTS IN THE FINAL FORM OF THE membranacea septi between the atrioventricular INTERVENTRICULAR SEPTUM and interventricular parts (Figs.