246 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



pulmonary aorta, and these are attached together to the fibrous 

 pericardium by the arterial mesocardium (Fig. 203). 



(2) The sinus venosus, which becomes part of the right auricle, 

 and its vessels are still attached by the venous mesocardium. 

 The heart has become so doubled on itself that the venous meso- 

 cardium comes almost in contact with the arterial inesocardium, 

 the narrow space which separates them being the transverse sinus 

 (Fig. 203). By comparing Figs. 3 93, 202 and 203 it will be 

 seen that the transverse sinus is formed by the breaking down of 

 the dorsal mesocardium of the heart. 



The venous mesocardium becomes much more extensive by the 

 ingrowth of the pulmonary veins. These grow in from the lungs, 

 and pierce the pericardium to reach the left auricle. They reach 

 the auricle through the mesentery or venous mesocardium of the 

 sinus venosus (Fig. 202). The ingrowth of the left pulmonary 

 veins causes a prolongation of the venous mesocardium to the 

 left side; when the heart is removed the venous mesocardium is 

 seen to be F-shaped in section. The oblique sinus lies in the 

 concavity of the venous mesocardium (Fig. 203). 



Ectopia Cordis. — Occasionally children are born with their 

 hearts exposed on the surface of the chest. In extreme cases, 

 only the dorsal wall of the pericardium is present, and it is flush 

 and continuous with the skin of the chest. In such cases there 

 has been either a failure on the part of the right and left soinato- 

 pleures to unite in the ventral line, or, more probably, they did 

 unite, but an early cystic condition led to a rupture of the peri- 

 cardium and overlying layer of the somatopleure. In such cases 

 the sternum is partially absent, or if present it is cleft, the right 

 and left halves being widely parted. The pathology of the con- 

 dition is probably similar to that of ectopia vesicae. 



The Dorsal Aortae. — In the 3rd week there are still two 

 dorsal aortae (Fig. 205). The blood passes from the two ventral 

 aortic stems through the aortic visceral arches to end in a dorsal 

 aorta on each side (Fig. 202). These run backwards side by side, 

 and at first end in the yolk sac, but with the formation of the 

 allantois they pass out on the body stalk to the membranes and 

 placenta. The umbilical arteries are thus the direct continuations 

 of the dorsal aortae (Young and Robinson) ; the middle sacral artery 

 is an inter-segmental vessel of later formation (Fig. 221, p. 273). 



