THE HEAET. 571 



fuse with the lower border of the spina vestibuli to complete 

 the interauricular septum, and from which also the auriculo- 

 ventricular valves are derived. 



The ventricles. The ventricular cavity becomes partially 

 divided towards the close of the fourth week by a fold, the 

 septum inferius (Fig. 244, sd), which arises from its dorsal and 

 posterior wall, and the position of which is indicated externally 

 by a slight groove on the surface of the heart. The completion 

 of the interventricular septum is a somewhat complicated pro- 

 cess, and will be described after the truncus arteriosus has been 

 dealt with. 



The ventricular wall consists at first of an outer muscular 

 tube, and an inner and much smaller endothelial tube, the two 

 tubes being separated by a considerable quantity of gelatinous 

 connective tissue (c/. Fig. 215). The thickening of the ventricular 

 wall is effected, in the first instance, by the outgrowth of bands 

 from the muscular tube into the gelatinous tissue : these bands 

 interlace and unite with one another to form a spongework 

 of muscular trabecule. The gelatinous tissue now becomes 

 greatly reduced in amount, so that the endothelial and muscular 

 walls are brought much closer together, and the endothelium 

 becomes moulded to the surface of the muscular wall, covering 

 the trabeculse, and lining the depressions of the spongework. 

 The wall of the ventricle is now in much the same condition as 

 it remains in throughout life in the frog. In the later stages of 

 development, the outer, compact muscular wall thickens very 

 considerably, and the spongework becomes less conspicuous, 

 forming ultimately the columnse carneae. 



The walls of the two ventricles are of equal thickness through- 

 out almost the whole of fcetal life, as the resistance to be over- 

 come by the two is approximately equal until the time of birth. 



The truncus arteriosus. In the truncus arteriosus the most 

 important change is the formation of the aortic septum, by 

 which the single tube becomes divided into two, lying side by 

 side, which become the systemic and pulmonary trunks re- 

 spectively ; or, in the adult, the ascending aorta and the pul- 

 monary artery. 



This division of the truncus arteriosus is effected by two 



