THE HEART 



303 



medulla than the adult ventricles, for the alteration in their 

 relative positions occurs after the nerve-supply has been 

 acquired. 



As the tubular heart grows in size, it becomes bent so that 

 the atrial part passes forwards (i.e. towards the head) dorsal 

 to the ventricular part. About this time, the single atrio- 

 ventricular orifice becomes divided into two by the union of 

 two endocardial cushions which project inwards from its 

 margins. At the same time, septa appear in the atrium and 

 in the ventricle and grow towards the linear partition which 



FIG. 109. Diagram of the primitive tubular Heart of the Embryo. 



A. Sinus venosus. 



B. Atrium. 



C. Ventricle. 



D. Bulbus cordis. 



separates the two atrio-ventricular orifices. Before the inter- 

 atrial septum reaches the partition, it breaks down near its 

 centre to form the foramen ovale, which remains patent till 

 the end of foetal life. The uppermost part of the inter- 

 ventricular septum is the last part to form, and, consequently, 

 is the commonest site for an abnormal communication between 

 the two ventricles. This anomaly, however, is extremely rare. 

 While the interventricular septum is in process of formation, a 

 spiral septum arises which subdivides the truncus arteriosus 

 into the aorta and the pulmonary artery. Owing to the fact 

 that these two arteries are derived by subdivision from a 



