THE VASCULAR SYSTEM 



353 



muscular layer, transforming it into a pulsating engine to 

 promote the flow of blood. Similar pulsating vessels occur in 

 all animals furnished with a closed circulation, usually a single 

 one, but in some cases several in number. Thus, in arthropods 

 and molluscs there is a single median heart, located dorsally 

 upon the main blood channel in that region, but in annelids 

 several of the lateral commissures are enlarged and function 

 as hearts. In vertebrates the hypertrophied region which 



forms the heart is located 

 along the median ventral 

 blood-vessel formed by the 

 union of the two vitelline 

 veins, and involves its pos- 

 terior portion. This brings 

 it topographically very far 

 anterior, just back of the 

 gill region, and this primary 

 position is, indeed, that per- 

 manently retained in fishes 

 and amphibians; but in rep- 

 tiles, birds, and mammals it 

 suffers a considerable change 

 of location in a posterior di- 

 rection and comes to lie in 

 FIG. 101. Four stages in the de- 

 velopment of the amniote heart. a thoracic cavity, formed by 



the ribs and sternum, with 

 some participation of parts of the shoulder-girdle. 



In its first stage, as shown by Amphioxus and in early em- 

 bryos, the heart is still a straight tube, formed posteriorly 

 by the joining of the hepatic veins, and, in true vertebrates, 

 the two ducts of Cuvier and the two vitelline veins also, the 

 last being embryonic and transitory. The chamber into which 

 these vessels empty soon differentiates off from the rest as 

 the sinus venosus, and, in like manner, by the formation of 

 transversely placed constrictions, there are added successively 

 an atrium, a ventricle, and a conns arteriosus, the latter being 

 continued into the median artery that supplies the gills. 



The next advance is seen in a flexion of the axis of the 



