354 HISTORY OF THE HUMAN BODY 



heart into the form of an S-shaped tube, the bending being 

 in such a way that the sinus venosus is dorsal and the conns 

 arteriosus ventral, the atrium anterior and the ventricle pos- 

 terior, a stage represented in adult fishes and in the embryos 

 of higher forms. The atrium increases in width more than the 

 other parts and forms two lateral recesses or broad diverticula, 

 which, from the ventral aspect, appear on either side of the 

 conus arteriosus, suggesting the division into two separate 

 compartments, which is, in point of fact, the next advance. 

 Furthermore, the several parts brought into contact by the 

 flexion become permanently adherent to one another and the 

 heart becomes molded into a more compact organ. 



In the tailed amphibians a new physiological moment is 

 introduced by the reception for the first time of arterial blood, 

 which comes from the skin and lungs through the great pul- 

 mo-cutaneous veins and enters the left side of the atrium 

 (Fig. 102, B). The anatomical response to this consists of 

 the division of the atrium into right and left portions, the 

 former for the reception of impure, and the latter for pure 

 blood.* From now on the sinus venosus plays a subordinate 

 role and consists merely of a vestibule of entrance for the 

 systemic veins, applied to the dorsal side of the right atrium. 

 In birds and mammals it is no longer distinct. The ventricle 

 is still undivided in the urodeles, but in the tailless forms, 

 probably as a further response to the new physiological con- 

 dition, a partial septum appears in this, which suggests a di- 

 vision into right and left ventricles, but contains a large 

 opening through which the two kinds of blood still mingle 

 (Fig. 102, C). In these animals also, the complete differen- 

 tiation of the third and .fourth arterial arches' as aorta and 

 pulmonary artery, respectively, leads to a longitudinal division 

 of the conus arteriosus by means of two longitudinal folds 

 placed opposite to one another which grow from the inner 



* In the urodeles the septum atriorum is not complete, but possesses a 

 few secondary perforations. In the lungless salamanders the left atrium, 

 into which the pulmonary veins would empty under other circumstances, is 

 suppressed. 



