CIRCULATORY ORGANS. 



28l 



the intima of all of the blood-vessels is in genetic relations to such lines 

 of cells. It should be remembered that the vascular system is ex- 

 tremely variable, even within the limits of the species. 



THE HEART. 



The heart, as it was left on page 273, was a venous or branchial 

 heart, in that all of the blood which enters it 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. 



FIG. 287. Different stages in the differentiation of the parts of the heart. A, elasmo. 

 branch; B, teleosts; C t 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; pa, pulmonary artery; 

 s, sinus venosus; sa, septum atriorum. 



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

 order to make the complete circuit. Such, in general, is the heart in 

 the cyclostomes and fishes (fig. 287, A, B}. 



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 

 about by the formation of a septum or partition in the atrium, partially 

 or completely dividing the chamber, the pulmonary vein (infra) open- 

 ing into the left half, which thus becomes arterial, while the sinus, 

 with its veins, is connected with the right alone (fig. 287, C). 



Still higher in the scale the partition or septum extends through the 

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

 valves on the right side, mitral on the left) and partially dividing the 

 ventricle (most reptiles fig. 287, D}. In the crocodilia (fig. 287, E) 



