104 DEVELOPMENT OP THE CIRCULATION. 



believes the venous circulation to be performed entirely un- 

 der the influence of the respiration that during the expan- 

 sion of the chest a vacuum is produced in the heart, which 

 the blood rushes in to supply but, unfortunately for this 

 theory, the foetal circulation goes on when there is no respi- 

 ration. Others, with more liberality, and no doubt with 

 greater truth, allow each part of the vascular tissue its ap- 

 propriate office, and suppose each to be equally essential in 

 its place to perfect regularity and harmony in this function. 



The Development of the circulation begins with the veins, as 

 has been traced in the Portal system to the vena cava, at 

 the upper part of which is the heart. 



"The heart," says Meckel, "appears at first like a half 

 ring lying loose the portion first seen is the left ventricle, 

 immediately after, the aorta shows itself, appearing as a 

 considerable dilatation. A little later, the upper extremity 

 of the vein dilates, then contracts before the venous trunk, 

 and thus produces the auricles. 



"All the parts which are finally double are still single 

 at this period. The auricle first becomes double ; an imper- 

 fect septum descends from its circumference and floats in 

 its cavity, so that the two parts communicate by a very 

 broad opening, the inter auricular canal, called afterwards 

 the foramen ovale, and still later, the fossa ovalis. 



"The doubling of the ventricle does not take place in the 

 same manner, but is produced by the prolongation of the 

 primitive portion at its upper part. The right ventricle 

 appears first as a small tubercle which gradually extends 

 itself towards the summit of the heart and communicates 

 with the left ventricle. This communication takes place at 

 the upper part of the two ventricles, because at first the 

 left cavity only prolongs itself. Hence the aorta arises 

 at first from both ventricles. 



"The pulmonary artery is the last to detach itself so as 

 to constitute a distinct trunk, but it was indicated before 

 along the aorta. In fact the aorta, which at first arises 

 solely from the heart, divides at some distance from this 

 organ into two branches at least, and as the aorta is blended 



