594 A MANUAL OF ANATOMY 



as the ostium interauriculare primum, being situated between the two, prior 

 to complete fusion. The right auriculo- ventricular orifice now pertains to 

 the right auricle, and the left auriculo-ventricular orifice to the left auricle. 



Transtormatlon of the Single Ventricle. — The division of the single ven- 

 tricular chamber into two ventricles, right and left, is effected by the forma- 

 tion of a partition, called the interventricular septum, or septum inferius. 

 .The position of this septum internally coincides with a groove externally, 

 called the interventricular groove, which traverses the ventral and dorsal 

 surfaces of the ventricular division. The interventricular septum appears 

 first in the lower part of the ventricular chamber, and grows in an upward 

 and backward direction. The dorsal part of its upper border blends with the 

 dorsal part of the lower border of the septum intermedium. The ventral 

 part of its upper border falls short of the root of the aortic bulb, so that the 

 interventricular septum is incomplete in this region. The two ventricles 

 therefore communicate with each other at this stage by a somewhat oval 

 aperture, called the ostium interventnculare, which is situated between the 

 root of the aortic bulb and the ventral part of the upper border of the inter- 

 ventricular septum. This aperture, however, becomes closed by the down- 

 ward growth of the membranous aortic septum, which divides the aortic 

 bulb into the ascending aorta and trunk of the pulmonary artery. As the 

 membranous aortic septum grows downwards, it closes the ostium inter- 

 ventriculare by meeting, and fusing with, the ventral part of the upper border 

 of the interventricular septum. The partition between the two ventricles 

 is then complete. The interventricular septum is thus partly muscular and 

 partly membranous. The membranous portion, comparatively small, is 

 formed by the downward growth of the aortic septum, and this part of the 

 interventricular septum is known as the pars membranacea septi. The superior 

 connections of the interventricular septum, as stated, are twofold, as follows: 

 ventrally, it blends with the downward growth of the aortic septum, and, 

 dorsally, it joins the dorsal part of the lower border of the septum inter- 

 medium. 



Cases have been recorded in which the pars membranacea septi has been 

 found perforated, or, in other words, in which the original ostium interven- 

 triculare has not been completely closed by the downward growth of the 

 aortic septum. Wherever such a condition exists, an intercommunication 

 between the venous blood in the right ventricle and the arterial blood in the 

 left ventricle is allowed. 



The ostium interventriculare represents the normal communication which 

 exists between the two ventricles in the hearts of some Reptilia — e.g., Lizards 

 and Chelonia (Tortoises and Turtles). 



Transformation of the Aortic Bulb. — The aortic bulb or bulbus arteriosus is 

 divided into the ascending aorta and the trunk of the pulmonary artery by a 

 partition, which is called the aortic septum. This septum is formed by the 

 meeting and fusion of two longitudinal endocardial ridges which appear on 

 opposite walls of the aortic bulb internally. It is first developed in the upper 

 part of the bulb, and in this situation the formative ridges appear on the 

 ventral and dorsal walls of the bulb. The septum is therefore disposed at 

 first antero-posteriorly. 



As the endocardial ridges grow downwards in a spiral manner, they become 

 disposed laterally, as right and left ridges. This accounts for the ascending 

 aorta and the trunk of the pulmonary artery lying side by side superiorly 

 (aorta to the right, and trunk of the pulmonary artery to the left), whilst 

 inferiorly, close to the heart, the trunk of the pulmonary artery is anterior, 

 and the aorta lies behind it. The aortic membranous septum, as it grows 

 downwards, meets, and fuses with, the ventral part of the upper border 

 of the interventricular septum. In so doing, as stated, it closes the ostium 

 interventriculare, and forms that part of the interventricular septum which 

 is known as the pars membranacea septi. 



Two grooves appear on the exterior of the aortic bulb, which coincide with 

 the attached borders of the aortic septum. As these grooves deepen, the 

 aortic bulb becomes divided into (i) the ascending aorta, and (2) the trunk 

 of the pulmonary artery. 



