314 THE POPULAR SCIENCE MONTHLY. 



citable tissues. If a single stimulation is supplied to a paralyzed jelly- 

 fish, a short period, called the period of latency, will elapse, and then 

 the jelly-fish will give a single weak contraction. If, as soon as the 

 tissue has relaxed, the stimulation is again repeated, the period of 

 latency will be somewhat shorter, and will be followed by a somewhat 

 stronger contraction. Similarly, if the stimulation is repeated a third 

 time, the period of latency will be still shorter, and the ensuing con- 

 traction still stronger. And so on up to nine or ten times, when the 

 period of latency will be reduced to its minimwn, while the force of 

 the contraction will be raised to its maximum. So that in the jelly- 

 fish the effect of a series of excitations supplied at short intervals from 

 one another, is that of both arousing the tissue into a state of increased 

 activity, and also of producing in it a state of greater expectancy. 

 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. Sterling. Indeed, the only diflference in this respect 

 between these four tissues, so widely separated from one another in 

 the biological scale, consists in the tim,e 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 

 m,emory, so to speak, of the heart-tissue, for the occurrence of a for- 

 mer 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 our throats ; but very rapidly the sense of irritation accumulates to 

 a point at which it becomes impossible to avoid coughing. And simi- 

 larly with tickling generally, the convulsive reflex movements to which 

 it gives rise become more and more in controllable the longer the 

 stimulation is continued, until they reach a maximum point, where, in 

 persons susceptible of this kind of stimulation, the muscular action 

 passes completely beyond the power of the will. Lastly, I may further 

 observe, what I do. not think has ever been observed before, that even 

 in the domain of psychology the action of this principle admits of be- 

 ing clearly traced. Who, for instance, has not felt it in the case of the 

 ludicrous ? We can endure for a short time, without giving any visible 

 response, the psychological stimulation which is supplied by a comical 

 spectacle ; but if the latter continues sufficiently long in a sufficiently 

 ludicrous manner, our appropriate emotion very rapidly runs up to a 

 point at which it becomes incontrollable, and we burst into an ex- 

 plosion of ill-timed laughter. But in this case of psychological tick- 

 ling, as in the previous case of physiological tickling, some persons are 

 much more susceptible than others. Nevertheless, there can be no 

 doubt that, from the excitable tissues of a plant, through those of a 



