272 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
of the heart, due to its alternating enlargement (diastole) and contrac- 
tion (systole). 
While the mesocardia are present the cardiac tube is a straight 
canal, lying in the pericardial sac and connected with its walls in front 
and behind. With their disappearance the tube increases in length 
more rapidly than the pericardium, the result beng the flexure of the 
tube on itself, something like the letter «©, the flexures being largely 
in the vertical plane. At the middle point of the flexure the tube re- 
mains small, forming the atrio-ventricular canal, but in front of 
and behind this the walls become thickened and the lumen enlarged. 
The posterior and dorsal of the chambers thus formed becomes the 
atrium (auricle), the ventral and anterior the ventricle of the heart. 
The atrium is bounded posteriorly by a constriction, behind which 
the tube expands into another chamber, the sinus venosus, which 
extends back to the posterior wall of the pericardium and receives the 
ducts of Cuvier and the omphalomesenteric veins. The ventricle, also, 
does not reach the anterior wall of the pericardium, but the anterior 
part of the heart tube forms a smaller trunk, the truncus arteriosus, 
while from the pericardium to the mandibular arteries is an arterial 
vessel, the ventral aorta. 
Muscles, as stated above, are developed in the wall of the heart, 
but to an unequal extent in the different parts, being scanty in the 
sinus venosus, and most abundant in the ventricle. Folds or valves of 
the endocardium appear in places at an early date and are so arranged 
that they permit the blood to flow forward but prevent any backflow. 
In the base of the truncus these valves take the form of pockets on the 
walls, there being several (3-5) rows with several valves in a row in the 
elasmobranchs (fig. 287, A) and ganoids. This valvular part of the 
truncus is called the conus arteriosus. In other vertebrates the conus 
is reduced to a single row of valves. 
Valves also occur in the atrio-ventricular canal (fig. 279) but here 
the pocket-like condition is impossible. The folds extend from the 
canal into the ventricle and are prevented from folding back into the 
atrium, under the heavy ventricular pressure, by ligaments—chorde 
tendineze—which extend from the edges of the valves to the opposite 
wall of the ventricle, and are kept taut during systole by short muscles 
(columne carnea) atthebase. Othervalves, moresimplein character, 
occur around the opening from the sinus into the atrium and, in some 
vertebrates, where the hepatic veins empty into the sinus, 
