Metazoa Rana. 289 



Starting from the truncus artericsus the blood is pumped by 

 the ventricle into a number of vessels springing from the 

 truncus. These vessels are six in number, three on either 

 side : nearest the origin of the truncus, a pair of vessels we 

 may know as pulmo-cutaneous ; next two aortse ; and lastly 

 the two common carotids. We have already seen that the 

 heart consists of three chambers, a ventricle and two auricles. 

 The sinus venosus opens into the right auricle, while it has 

 been noted that one vessel, to be known now as the pul- 

 monary vein, opens into the left auricle. The auricles in 

 turn open into the ventricles, and the auriculo- ventricular 

 openings are guarded by valves which prevent regurgitation. 

 Further it will be noted that the truncus arteriosus springs 

 more from the right than the left side of the ventricle. 

 When the ventricle contracts, therefore, the blood occupying 

 the right side of the ventricle will be the first to enter the 

 truncus. Now we have already learnt that the sinus venosus 

 receives all the impure blood from the systemic veins ; while 

 the pulmonary vein, on the other hand, contains pure blood 

 from the respiratory organ, the lung. Under these circum- 

 stances, when the auricles contract and pour their contents 

 into the ventricle, the ventricle will contain pure blood on 

 its left side, impure blood on its right side, and mixed blood 

 in the middle. Therefore the ventricular contraction will 

 drive into the truncus first of all impure blood, which will find 

 its way into the first of the branches of the truncus, namely, 

 the pulmo-cutaneous arteries, by which vessels the blood 

 is conveyed to the lung and skin (the two great respiratory 

 organs in the frog) -to be purified. The blood immediately 

 following will pass into the two aortae, and the last and 

 purest blood will enter the carotids. This process is greatly 

 assisted by a special valve in the interior of the truncus, 

 which closes the entrance to the pulmo-cutaneous arteries 

 after they have been filled. By the aortas (which afterwards 

 unite) the blood (mixed) is carried to all the chief organs of 

 the trunk, to the limbs, stomach, intestine, &c. supplying 



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