THE BLOOD VASCULAR SYSTEM 



The blood vascular system is very much like that of the 

 shark, but with additional adaptations to land life. The 

 lungs have not assumed much importance, and in conse- 

 quence the heart is of the fish type with the atrium 

 doubled by a perforate septum, The heart of Necturus 

 has some similarity to that of Ceratodus, a lung fish, in 

 which there is a partial separation of the atrium. In 

 Necturus the blood that goes from the heart to the gills 

 does not return again, but goes directly to the systemic 

 vessels by way of the efferent arteries. The blood from the 

 lungs enters the left side of the heart through a single 

 vessel, since the pulmonary veins join near the heart. 

 The septum between the atria is complete but it is fenes- 

 trated so that it is not a perfect membrane. The truncus 

 is not divided by a ridge or flap as in the frog but is a 

 straight tube. The oonus arteriosus is supplied with 

 pocket valves and the bulbus is enlarged to serve as an 

 equalizer to keep the pressure constant. The changes in 

 the arterial system consist of the reduction of the aortic 

 arches from five to three, and the development of the 

 pulmonary artery from the sixth. Thus the sixth arch 

 forms the pulmonary artery, the fourth the systemic arch 

 and the third with a part of the anterior arches, forms 

 the vessels leading to the head. The changes in the veins 

 are more radical. A new vein the postcava forms a connec- 

 tion between the right subcardinal and the sinus venosus, 

 thus taking some of the blood from the kidneys. The 

 lateral veins have joined to form a single, ventral ab- 

 dominal, changing the anterior connection so that instead 



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