30 COMPARATIVE PHYSIOLOGY OF THE BRAIN 



beat altogether. During the pause, the end b suc- 

 ceeds in sending out a wave of contraction which 

 reaches a before it has had time to send out a wave 

 of its own. One sees occasionally at the time of a 

 reversal that at first both ends send out contraction- 

 waves which may meet in the middle of the heart. 

 At the next heart-beat, the end which is about to stop 

 delays the sending out of the wave a little more, and 

 at the next heart-beat the wave starting from the 

 other end can pass over the whole heart without being 

 blocked. 



Hence the coordination of movements in Medusae 

 (or in the heart) is not due to a hypothetical centre 

 of coordination situated in the ganglion-cells, but to 

 the fact that the element which is first active acts 

 as a stimulus upon its next element, and so on. 



3. It may be shown that even more specialised 

 forms of coordination do not depend upon the pre- 

 sence or interference of ganglia. When the back of 

 a frog is touched with acetic acid, the frog wipes off 

 the acid with its foot. If one leg is tied, it uses the 

 other for this purpose. The turtle acts in a similar 

 manner when acetic acid is applied to the back of its 

 shell. It cannot reach the stimulated spot, but the 

 legs move dorsally under the shell as far as possible 

 towards it. Physiology has contented itself in regard 

 to these phenomena by pointing to the complicated 

 nature and impenetrable structural secrets of the 

 central nervous system. Yet the same reactions oc- 

 cur in a Hydromedusa, in which case the term 



