THE HEART AND CIRCULATORY SYSTEM 



109 



which supported part of the classical hypothesis. Most recent 

 researches into this problem have confirmed the latter observa- 

 tion. If large quantities of dye are injected, however, the blood 

 appears to show mixing of the streams not only in the ventricle 

 but also in their passage up the conus. Ideally, no more than 

 0-001 ml. of dye (Evans blue is frequently used) should be in- 

 jected. In the frog and toad it has been observed that blood from 

 the right and left auricles maintains its separateness during its 

 passage up the conus although both streams enter the conus 

 simultaneously (fig. 32). This separation is largely because of 

 the presence of the spiral valve in these forms. Furthermore, 

 recordings of changes in the blood pressure throughout the 

 cardiac cycle using modern techniques have confirmed com- 

 pletely that the pulse waves are simultaneous in all three arches 

 in Rana, Bufo, and Xenopus. During their passage up the conus 

 the streams of blood pass into different aortic arches because 



Fig. 31. 



The supposed path of the blood through the heart of the frog 

 according to A. The classical hypothesis, B. The mixing hypothesis 

 c.a. carotid arch; l.a. left auricle; p.c. opening of conus arteriosus 

 to pulmo-cutaneous arch; p.c.a. pulmo-cutaneous arch; r.a. right 

 auricle ; s.a. systemic arch ; s.v. spiral valve; v. ventricle, (from Foxon, 

 1955.) 



