The normal and abnormal inter-atrial septum.

This paper is an elementary study of the normal and abnormal inter-atrial septum and an attempt to clarify the nomenclature and classification of the various kinds of septa on embryological grounds. During foetal life, the left ventricle receives almost no blood from the lungs, and must therefore get it from the vene cave via the common atrium, if it is to perform any work to help in its proper development. Herein lies the reason for the apparent complexity in the development of the interatrial septum, because the septum must be so built that it will at all times in fcetal life allow blood to flow from right to left. Obviously a simple septum cannot do this, and the only solution is the natural one-the evolution of a one-way valvular partition, which allows the free flow of blood from right to left throughout foetal life but can readily be closed by the rise of left atrial pressure that takes place at birth. The simplest form of valve has two components, one fixed and the other moving. In the interatrial septum, the fixed component (or seating) is the thick septum secundum, and the moving component is the thin septum primum. In order that the valve shall open from right to left (and close in the reverse direction), the moving part must be on the left side of the fixed part, and so the septum primum forms to the left of the septum secundum. Fig. 1 traces the evolution of the normal inter-atrial septum by means of cardboard models and sectional diagrams. Its understanding makes easier the study of the ostium primum and ostium secundum septal defects illustrated by models and photographs of actual specimens in Fig. 2. Apart from ostium primum and secundum defects, which are readily explained, there is another type of defect, more difficult to interpret, which may be associated with abnormal pulmonary venous drainage. An example is shown in Fig. 3, and the possible embryology is discussed.