78 ' ESSENTIALS OF ZOOLOGY 



Near the anterior free end of the spiral valve where it is the 

 widest, there is a pair of small synangial valves which, together 

 with the end of the spiral valve, separate the pylangium from the 

 synangiunL Just below these valves is an aperture which leads into 

 the trunk formed by the union of the two pulmocutaneous arteries. 



The synangial chamber is very short and gives off almost imme- 

 diately two large branches, one to the right and the other to the 

 left. In each of these branches originate the three main trunks or 

 arches of the arterial system. They are formed by two longitudinal 

 septa dividing the vessel into three compartments. All three trunks 

 are therefore enclosed in one large vessel for a short distance before 

 breaking up into three separate vessels. The carotid arch originates 

 from the anterior compartment, the systemic arch from the middle 

 compartment, and the pulmocutaneous arch from the posterior com- 

 partment. Blood enters the anterior and middle compartments from 

 the synangium, but enters the posterior compartment, or pulmocuta- 

 neous arch, from the pylangium. 



The heart beats in a wavelike peristaltic manner. The sinus venosus 

 contracts first, then the auricles (the right auricle preceding the left 

 by a moment), then the ventricle, and finally the conus. 



Venous blood from the right auricle enters the right side of the 

 ventricle, and oxygenated blood from the left auricle enters the left 

 side. Muscular ridges of the ventricular wall tend to hold the blood 

 and reduce mixing. Since the heart's contractions are wavelike, 

 the ventricle immediately forces the blood into the conus through 

 the semilunar valve. Venous blood from the right auricle is closest 

 to the conus, and it passes out first, flowing into the closest open- 

 ing offering the least resistance. This is the opening in the 

 pylangium to the pulmonary arch, leading to the lungs. As the 

 contraction of the ventricle comes to an end, forcing out the re- 

 maining oxygenated blood, the pylangial part of the conus contracts, 

 bringing the spiral valve against its ventral wall. This action, 

 together with that of the synangial valves which are anterior to 

 the common opening of the pulmonary arches, completely shuts off 

 the flow of blood into these arches. The blood therefore passes into the 

 synangium and enters the chambers leading to the systemic arteries 

 or the carotid arteries. Since the carotid arteries offer some resist- 

 ance to blood flow, the blood tends to enter the larger systemic 

 arteries first. As the systemic arteries fill, they offer more resistance 



