112 COMPARATIVE PHYSIOLOGY 



the Stannius experiment on the heart of the frog, where the 

 rhythm originates in the sinus. Several considerations which 

 are borne out by the behaviour of the isolated heart point to 

 the contrary. When the isolated ventricle is perfused with 

 sea water or a suitable saline medium, it ceases to pulsate at 

 once when the pressure falls below about 20 mm. of water, 

 but starts again when the pressure is raised above that level. 

 The increase of pressure beyond this point does not lead 

 to any augmentation of the amplitude ; a certain minimum 

 driving force is necessary to initiate the rhythm. The cessa- 

 tion of ventricular movement on ligation of the auriculo- 

 ventricular junction is due to the fact that the auricles can no 

 longer maintain the requisite critical pressure necessary to set 

 the ventricles in action. If, however, the auricles are separated 

 by a ligature from the efferent branchial vessels they continue 

 to beat synchronously with one another and in unison with 

 the branchial hearts. This co-ordination might reside either 

 in the nature of the contractile tissues, which is unlikely ; or 

 in some reflex mechanism. That a reflex mechanism may 

 co-ordinate the pulsation of one auricle with its fellow is 

 indicated by the fact (Fredericq) that while stimulation of 

 either intact visceral nerve brings the heart to diastolic stand- 

 still, stimulation of the peripheral end of the cut nerve pro- 

 duces inhibition of the auricle on the same side only, whereas 

 stimulation of the central end produces inhibition on the 

 opposite side. Here, then, is a mechanism by which one 

 auricle may reflexly inhibit its fellow, by afferent nerve- con- 

 nexions which recall in one respect the famihar depressor 

 nerve of the mammalian heart. That a reflex mechanism 

 underlies the co-ordination of activity of auricles and branchial 

 hearts is suggested by the effects of direct electrical stimula- 

 tion, when the beating of the heart has been stopped by the 

 faradisation of the inhibitory nerves. If in this condition 

 one branchial heart is stimulated, it contracts, and its contrac- 

 tion is followed first by that of the branchial heart of the 

 opposite gill, and secondly by that of the two auricles, which 

 is the normal sequence of cardiac rhythm. It is also found 

 that if under similar conditions the renal vein is stimulated, 



