GO JELLY-FJSH, STAR-FISH, AND SEA-UHCUINS. 



a state of increased activity, and also of producing 

 in it a state of greater expectancy. We have, more- 

 over, seen that this effect depends upon the repe- 



' tition of the process of stimulation, and not upon 

 that of the process of contraction. 



Now, effects very similar to these have been 

 found to occur in the case of the excitable plants 

 by Dr. Burdon- Sanderson ; in the case of the frog's 

 heart by Dr. Bowditch ; and in the case of reflex 

 action of the spinal cord by Dr. Stirling. Indeed, 

 the only difference in this respect between these 

 four tissues, so widely separated from one another 

 in the biological scale, consists in the time which 

 may be allowed to elapse between the occurrence 

 of the successive stimuli, in order to produce this 

 so-called summating effect of one stimulus upon 

 its successor: the memory, so to speak, of the 

 heart-tissue for the occurrence of a former stimulus 

 being longer than the memory of the jelly-fish 

 tissue; while the memory of the latter is longer 

 than that of the plant tissue. And I may here 

 add that even in our own organization we may 

 often observe the action of this principle of the 

 summation of stimuli. For instance, we can 



. tolerate for a time the irritation caused by a crumb 

 in the larynx, but very rapidly the sense of irrita- 

 tion accumulates to a point at which it becomes 

 impossible to avoid coughing. And similarly with 

 tickling generally, the convulsive reflex movements 

 to which it gives rise become more and more incon- 

 t roll able the longer the stimulation is continued, 

 until they reach a Tnaxim^um point, where, in 



