20 COMPARA TIVE PHYSIOLOG Y OF THE BRAIN 



motions are due to changes in the physical character 

 of the colloidal material in the protoplasm. These 

 changes may consist in changes in the state of matter 

 or in the absorption of water by these colloids, or in 

 secondary changes derived from those before men- 

 tioned. We know that the physical qualities of the 

 colloids are influenced greatly by the nature and 

 osmotic pressure of the ions in the surrounding solu- 

 tion. For that labile equilibrium of the colloids 

 which is required for spontaneous rhythmical contrac- 

 tions, the Na, Ca, and K ions must be present in 

 definite proportions in the tissues. This proportion 

 must be different for the centre and the margin of a 

 Hydromedusa. While for the margin the proportion 

 in which these three ions exist in the sea-water is 

 adequate, for the centre of a Hydromedusa more 

 Na ions and less Ca ions are required. Hence, if we 

 put a centre without the margin into normal sea-water 

 it does not beat, but it will beat when put into a pure 

 NaCl or NaBr solution of the same osmotic pressure 

 as sea-water. In the pure NaCl solution Na ions of 

 the solution will enter into the tissues and take the 

 place of some of the Ca ions. This will give the col- 

 loids of the muscles those qualities which allow rhyth- 

 mical contractions. If too many Na ions enter the 

 tissues of the centre it will lose its irritability. The 

 latter will, in this case, be restored again by adding a 

 trace of CaCl 2 to the solution. It thus happens that 

 the problem of spontaneous activity is no longer a 

 question of the presence or absence of the ganglion- 



