EMERGENCY RESPIRATION 51 



In a few cases (Clarias, Saccobranchus, Hypopomus, and 

 Symbranchus, according to Carter and Beadle) the air-breathing 

 organs are in parallel with the gills, receiving blood from the 

 afferent branchial arteries and returning it to the dorsal aorta. 

 This is from the point of view of respiratory efficiency the ideal 

 arrangement, provided blood can be shunted from the gills to 

 the air-breathing organs and vice versa. 



In most other cases, including those in which the gas bladder 

 is the air-breathing organ, the blood supply comes from the 

 dorsal aorta and is returned through a vein to the heart. 

 This means a great sacrifice in a respiratory efficiency which 

 had probably been built up through a very long line of 

 ancestors. 



The blood coming to the "lung" is not venous, but is the 

 same mixture which is supplied to all other organs, and the 

 arterialized blood leaving it is mixed in the heart or before 

 with the venous blood returning from the rest of the body. 

 No organ is supplied with real arterial blood. 



In the Dipnoi the first steps are taken to reestablish by 

 complicated vascular arrangements the respiratory efficiency 

 of the circulation. There are separate pulmonary veins taking 

 the arterialized blood through the incompletely divided sinus 

 venosus and heart to the two first branchial arches, while a 

 considerable portion of the purely venous blood passes through 

 the third and fourth arches and goes to the lungs through a 

 special pulmonary artery. This scheme is further elaborated 

 in Amphibia and Reptilia and finally leads again to complete 

 respiratory efficiency in the warm-blooded animals. 



