SUO HANDBOOK OF PHYSIOLOGY. 



which result from its completion, do not run side by side, but are 

 twisted round each other. 



As the septum grows down toward the ventricles, it meets and coa- 

 lesces with the upwardly growing ventricular septum, and thus from 

 the right and left ventricles, which are now completely separate, arise 

 respectively the pulmonary artery and aorta, which are also quite dis- 

 tinct. The auriculo-ventricular and semi-lunar valves are formed by the 

 folds of the endocardium. 



At its first appearance, as we have seen, the heart is placed just 

 beneath the head of the foetus, and is very large relatively to the whole 

 body; but with the growth of the neck it becomes further and further 

 removed from the head, and is lodged in the cavity of the thorax. 



Up to a certain period the auricular is larger than the ventricular divi- 

 sion of the heart; but this relation is gradually reversed as development 

 proceeds. Moreover, all through foetal life, the walls of the right ven- 

 tricle are of very much the same thickness as those of the left, which 

 may probably be explained by the fact that in the foetus the right ven- 

 tricle has to propel the blood from the pulmonary artery into the aorta, 

 and thence into the placenta, while in the adult it only drives the blood 

 through the lungs. 



Arteries. The primitive aorta arises from the bulbus arteriosus and 

 divides into two branches which arch backward, one on each side of the 

 foregut and unite again behind it, and in front of the notochord into a 

 single vessel. 



This gives off the two omphalo-mesenteric arteries, which distribute 

 branches all over the yolk-sac ; this area vasculosa in the chick attaining 

 a large development, and being limited all round by a vessel known as 

 the sinus terminalis. 



The blood is collected by the venous channels, and returned through 

 the omphalo-mesenteric veins to the heart. 



Behind this pair of primitive aortic arches, four more pairs make 

 their appearance sucessively, so that there are five pairs in all, each one 

 running along one of the visceral arches. 



These five are never all to be seen at once in the embryo of higher 

 animals, for the two anterior pairs gradually disappear, while the pos- 

 terior ones are making their appearance, so that at length only three 

 remain. 



In fishes, however, they all persist throughout life as the branchial 

 arteries supplying the gills, while in amphibia three pairs persist through- 

 out life. 



In reptiles, birds, and mammals, further transformations occur. 



In reptiles the fourth pair remains throughout life as the permanent 

 right and left aorta; in birds the right one remains as the permanent 



