358 



COMPARATIVE ANATOMY 



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 localized 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 



^ic -^;c y^' "'^p'^n'.PJ^ '>y^ 



Fig. 298. — Different stages in the differentiation of the parts of the heart, ventral 

 view. A, elasmobranch; B, teleosts; C, amphibia; D, lower reptiles; E, alligator; F, 

 birds and mammals, a, atrium; ao, aorta; b, bulbus arteriosus; c, conus; cd, Cuvierian 

 duct; h, hepatic veins; pa, pulmonary artery; pc, pre- and postcaval veins; pv, pulmonary 

 vein; s, sinus venosus; sa, septum atriorum; v, ventricles. (From Kingsley's "Com- 

 parative Anatomy of Vertebrates.") 



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 the atrium became divided 

 by an incomplete septum into right and left atria while 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 



