Aids to embryology / by J.S. Baxter.

  • James Sinclair Baxter
Date:
1948
Catalogue details

Licence: In copyright

Credit: Aids to embryology / by J.S. Baxter. Source: Wellcome Collection.

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    the canal into right and left halves. In this manner the primitive atrium is separated into two chambers, but even before it has been completed the cranial part of the septum primum breaks down, and a new communication, the foramen secundum, puts the right and left atria into communication once more. This happens towards the end of the seventh week. Next, another septum grows caudally from the atrial wall. This is the septum secundum and it lies to the right of the septum primum, in the space (inter- septovalvular) between it and the left venous valve. Growth caudally takes place until the free concave border of the septum secundum overlaps the cranial border of the septum primum. The cleft between the two free edges of these septa is now known as the foramen ovale, and the two flaps of tissue are ob- viously arranged so that oxygenated blood reaching the right atrium from the inferior vena cava during foetal life readily passes through it to the left atrium. After birth the pulmonary circulation is established, pressure rises in the left atrium and the two septa are pressed together and eventually adhere. The annulus ovalis of the adult heart represents, then, the free edge of the septum secundum while the septum primum forms the membranous fossa ovalis. The left venous valve eventually fuses with the right aspect of the interatrial septum. The upper part of the right venous valve, together with the septum spurium, is converted into the crista terminalis while the lower part of it is represented in the adult heart by the valves of the inferior vena cava and of the coronary sinus. Further Development of the Atria.—Reference was made earlier to changes which resulted in the transfer of much of the venous return to the right horn of the sinus venosus, and as a consequence, how the left horn of the sinus became diminished in size. It
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