108 VERTEBRATE RESPIRATION 



from one of the ejf event branchial vessels. A subsidiary coronary 

 circulation is also derived in many cases directly from the 

 dorsal aorta. 



(d) The heart and aortic arches of Amphibia 



At first sight the presence of a single undivided ventricle in the 

 heart of most Amphibia seems to provide an ideal intermediate 

 stage between the single circulation of a fish and the circulation 

 of reptiles where the ventricle is partially divided. Suggestions 

 about the way in which different types of blood can retain their 

 separateness within the undivided ventricle have been discussed 

 many times and have renewed their interest with the application 

 of more modern techniques to this problem. The classical des- 

 criptions of the fate of the blood from the two auricles were based 

 upon the absence of any mixing of the blood in the ventricle, to- 

 gether with the passage of the different bloods consecutively into 

 the conus which arises from the right side of the ventricle. Initi- 

 ally blood from the right auricle was considered to pass up the 

 conus (here often called truncus) and into the pulmo-cutaneous 

 arch (6th aortic arch) because of its low peripheral resistance. 

 Later the remainder of the right auricle blood, together with 

 some oxygenated blood from the left auricle, would pass up the 

 truncus to be distributed in the systemic (4th aortic arch), and 

 finally pure oxygenated blood would be forced into the carotid 

 arch (3rd aortic arch). Since Brlicke and Sabatier proposed 

 this mechanism, several workers have doubted various parts of 

 it. For instance, injection of dyes into the venous return to the 

 heart gave evidence for a random distribution of the left and right 

 auricular bloods into the three arches, and pressure measure- 

 ments in the arches themselves suggested a simultaneity of the 

 pulse pressures. X-ray-opaque material has recently been used 

 to follow the distribution of blood into the aortic arches, and 

 Foxon observed a random distribution following its injection 

 into the pulmonary vein. These results led to some scepticism 

 concerning the classical hypothesis and a greater appreciaton of 

 the mixing hypothesis (fig. 31). Foxon observed, however, that 

 the two sets of blood did not mix completely in the ventricle, 



