CIRCULATORY ORGANS. 281 
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. 
pany 
A B Cc E Y F 
Qa a | [sea | afta sa aja 
¢ s Ss XS L wv PC 7 
OE Yo? WM HOLE “RE 
Fic. 287.—Different stages in the differentiation of the parts of the heart. A, elasmo. 
branch; B, teleosts; C, amphibia; D, lower reptiles; Z, 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; 
5, 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) 
