THE VASCULAR SYSTEM 275 



anastomosis between the precardinals or jugulars is converted into the 

 left innominate vein which brings venous blood from the left arm and 

 left side of the head across to the right jugular vein. The left common 

 cardinal consequently degenerates but persists in part as the coronary vein. 



Evolution of the Heart. The chief changes which the heart has under- 

 gone in phylogenesis may be briefly summarized as follows. The verte- 

 brate heart is a differentiated portion of a median ventral blood-vessel. 

 The contractile function which originally extended throughout the length 

 of this vessel became locaUzed and concentrated in the subpharyngeal 

 region. Primarily the heart had neither valves nor chambers but con- 

 sisted of a two-layered tube with a muscular wall and an endothelial 

 lining. The first subdivision of the heart was into a receiving chamber 

 or atrium and an anterior propulsive division, the ventricle. Later were 

 added a posterior sinus venosus and an anterior conus. Atrioventricular 

 and semilunar valves in turn made their appearance, thus ensuring a 

 one-way flow of blood. With the elongation of the heart in confined space, 

 a sigmoid flexure was formed and the atrium consequently came to lie 

 dorsal to the ventricle. Fishes added a muscular bulbus anterior to the 

 conus. In the Dipnoi and Amphibia the connexion of the sinus venosus 

 was shifted to the right atrium while aerated blood from the lungs entered 

 the left atrium. In the Dipnoi and Amphibia, while the atrium became 

 divided by an incomplete septum into right and left atria, the ventricle 

 remained undivided, so that some mixing of aerated and impure blood 

 occurs. The increased flexure of the heart brings the atria in Amphibia 

 anterior to the ventricle. In the crocodilian reptiles the complete division 

 of the heart into arterial and venous halves is effected, but the beneficial 

 effects of this separation are partly neutralized by the mixing of the two 

 kinds of blood in the dorsal aorta. In mammals the sinus venosus becomes 

 merged with the walls of the right auricle. 



Evolution of the Aortic Arches. The device of oxygenating blood in 

 pharyngeal gills is peculiar to chordates. Nevertheless aortic arches 

 connecting ventral and dorsal aortae in the pharyngeal region occur in 

 annelids. It is a matter of opinion whether this point of resemblance 

 between anneUds and chordates has a phylogenetic significance or is 

 simply a case of convergence. In chordates the number of aortic arches 

 is correlated with the number of visceral arches. Amphioxus has the 

 largest number, nineteen pairs, of primary visceral arches among chor- 

 dates. The aortic arches are correspondingly numerous. The largest 

 number of aortic arches in vertebrates occur in some species of cyclostomes, 

 fifteen pairs in Bdellostoma stouti. Although some elasmobranchs 

 have more, six pairs are usually the maximum number of aortic 

 arches in gnathostomes. Of these the first two pairs, belonging to the 

 mandibular and hyoid arches, partly lose their respiratory function and 



