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 chordae 

 tendinese which extend from the edges of the valves to the opposite 

 wall of the ventricle, and are kept taut during systole by short muscles 

 (columnas carnea) at the base. Other valves, more simple in character, 

 occur around the opening from the sinus into the atrium and, in some 

 vertebrates, where the hepatic veins empty into the sinus. 



