io8 COMPARATIVE PHYSIOLOGY OF THE BRAIN 



the ventral cord between two ganglia, for instance 

 between the second and third, we find that in spite of 

 the division all the gills continue to breathe. Any 

 ganglion may be entirely isolated that is, the commis- 

 sure before and behind it may be severed, and the 

 corresponding gill continue to make respiratory 

 motions. This proves that every ganglion is the seat 

 of an automatic periodic activity. But how does it 

 happen that all the gills move simultaneously as long 

 as their ganglia are connected ? The number of the 

 respirations produced is the same even when the 

 abdominal ganglia are isolated. This is probably due 

 to the fact that the number of respirations is de- 

 termined by the temperature and the chemical nature 

 of the blood. The amount of carbon dioxide and 

 certain other substances, especially those formed in 

 the muscles (Zuntz and Geppert), controls the number 

 of respirations. The phase of the movement, on the 

 other hand, is not the same in the various segments 

 where the ganglia are isolated. The gills that are 

 situated anterior to the place of incision may be in- 

 spiring while those behind the incision are expiring. 

 These phenomena lead me to believe that in the nor- 

 mal animal coordination is regulated in the same way 

 that it is regulated in the activity of the heart and in the 

 movements of Medusae. The ganglion that acts first, 

 that is to say, the ganglion that acts quickest, stimulates 

 those connected with it nervously and so determines 

 the correspondence of phase. This view is supported 

 by the fact that no matter how the ganglia may be 



