HEART 



209 



On the transverse portion of the loop a sulcus now appears, which corresponds 

 to the developing interventricular septum within the ventricular chamber. 



FIG. 262. VIEW FROM BEHIND OP THE HEART OF A HUMAN EMBRYO OF ABOUT FOUR 



WEEKS, MAGNIFIED. (His.) 



The two ducts of Cuvier and the inferior cava are seen opening separately into the sinus, which is 

 ii transversely elongated sac communicating only by a narrow orifice with the right auricle. 



The appearance of this sulcus is the expression of a commencing bilateral 

 expansion of the common ventricle. We can therefore now name the 



FIG. 263. DIAGRAMS TO ILLUSTRATE HOW IN THE DISTENSION OF THE VENTRICLES THE ' LESSER 

 CURVATURE' OF THE HEART-TUBE is REDUCED, AND THE PROXIMAL CHAMBER OF THE AORTIC 

 BULB (B) LOSES ITS MESIAL WALL. (After Keith.) 



a.o., aortic bulb (distal part); au, auricle; B, aortic bulb (proximal part); RV, right ventricle; 

 LV, left ventricle ; P (in b), pulmonary artery. 



lateral dilatations the right and left ventricles. It will be observed that the 



right ventricle is formed from the right portion of the transverse section of the 



VOL. i. p 



