208 



VASCULAE SYSTEM 



shall speak of the ascending limb as the bulb, and of the remainder of the tube 

 as the truncus arteriosus. As the heart-tube increases in length, and the peri- 

 cardium expands in a backward direction, the sinus venosus is drawn into the 

 cavity out of the septum transversum, necessarily of course carrying with it a 

 covering of the connective tissue of the septum. The ducts of Cuvier undergo a 

 similar apparent anterior displacement, and now run in lateral folds, bounding 

 the aperture between the pericardium and the remainder of the coelom, and named 

 the lateral mesocardia (fig. 293, p. 239). 



As the ventricular loop increases in size, it is more and more displaced in a 

 backward direction, so that the auricle and its annex, the sinus venosus, which 

 now consists of a transverse portion and a larger right and a smaller left horn, 

 come to lie on the dorsal aspect and ultimately in front of the ventricular 

 segment (fig. 262). The root of the bulb is thus brought into close relationship 



right 

 auricle 



left auricle 



left ventricle 



.FiG. 261. RECONSTRUCTION OF THE HEART OF A HUMAN EMBRYO OF 6'8 MM. (After Piper.) 

 The pericardium has been represented as opened to show the ventral aspect of the heart. 



with the auricular canal. As a further result of these changes, and consequent 

 backward expansion of the pericardium, the ducts of Cuvier take a gradually 

 increasing antero-posterior inclination on their way to reach the sinus venosus. 



Meantime the primitive auricle has thrown out ear-like dilatations on each side, 

 and the descending limb and transverse portion of the ventricular loop have become 

 dilated to form the primitive common ventricle. The ascending limb remains at 

 first more tubular ; but it also soon dilates, and, as a result of the increasing distension 

 of the whole distal part of the loop, the cleft between its two limbs, representing 

 a centre, itself stationary, round which growth is proceeding, becomes relatively 

 shorter, until it disappears at the base of the heart, forming there a fold (bulbo- 

 auricular fold) of the heart-wall, where the wall of the bulb passes directly 

 into the wall of the auricular canal (fig. 263). The expansion of the loop 

 is further accompanied by a rotation round the stationary point, of the 

 dilating ascending limb and truncus arteriosus towards the front (fig. 261). 



