THE LOWER VERTEBRATA. 



143 



the ventricle becomes filled with deoxygenated blood from 

 the right auricle and sinus venosus, while the left side 

 receives oxygenated blood from the left auricle (into which 

 the pulmonary veins open). The spongy nature of the ven- 

 tricle retards the mixing of these two qualities of blood. 

 The truncus ar- 

 teriosus 



comes 



..CAAOT/O 



69 _ HEART OF FBOQ| 

 (After Howe-0 



OPEN . 



off from the 



right side of 



the ventricle, so 



when the ven- 



tricle contracts, 



the deoxygen- 



ated blood of 



the right side is 



necessarily the 



first to be 



forced along the 



truncus ; after- 



wards comes 



somewhat mixed 



blood, and 



finally the oxy- 



genated blood of the left side. On laying open the truncus 



(fig. 69) it will be found that each of the divisions that 



branch off from it right and left is already internally 



divided into three passages, and that the separation of 



the pulmo-cutaneous passages begins even before the 



division into right and left branches is visible externally. 



The truncus consists of two portions a conns arteriosus 



or pylangium, nearer the ventricle, and a bulbus arteriosus 



or synangium farther from it. At each end of the former 



there is a series of three valves, and along its wall runs 



a longitudino spiral valve. The apertures leading to the 



pulmo-cutaneous passages are just at the junction of the 



pylangium and synangium, and so placed in relation to 



the valves that when blood is passing to the systemic and 



carotid passages it tends to force the valves over and so 



close the pulmo-cutaneous apertures. 



when a liquid is being forced along under pressure 



