THE HEART 



297 



or venous heart, since it represents the extremes of development 

 in the cyclostomes and fishes (the dipnoi excepted). In these ani- 

 mals all of the blood which enters the heart is venous blood and is 

 all pumped directly to the gills to lose its carbon dioxide and to take 

 up oxygen, before being distributed to the various parts of the body. 

 In its course through the body it passes but once through the heart 

 in order to make the complete circuit. The term is not strictly 

 correct for the embryonic amniotes, for in these only oxygenated 

 blood passes through the heart when it is in this stage. 



When, however, lungs are formed (dipnoi and amphibia) to share 

 in the respiratory processes, the heart begins to divide into arterial or 

 systemic, and venous or respiratory halves. This division is brought 



Fig. 317. — Different stages in the differentiation of the parts of the heart. A, 

 eiasmobranch; 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; A, 

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

 s, sinus venosus; sa, septum atriorum; v, ventricles. 



about by the fomation of a septum or partition in the atrium, par- 

 tially or completely dividing the chamber, the pulmonary vein 

 (P-3i5) opening into the left half, which thus becomes arterial, 

 while the sinus, with its veins, is connected with the right alone 

 (fig. 317, C). 



Still higher in the scale the partition or septum extends through 

 the atrio-ventricular canal, dividing its valves into two groups (tri- 

 *cuspid valves on the right side, mitral on the left) and partially divid- 

 ing the ventricle (most reptiles fig. 317, Z)). In the crocodilia (fig. 

 317, £) the division of the ventricle is completed by the extension of 

 the septum to the anterior end, but there is an opening (foramen 

 Pannizse) between the two sides of the aortic trunk, so that some 



