164 IRRITABILITY 



can only be achieved by indirect means, and we must content our- 

 selves with the ascertainment of a smaller number of its points 

 from which by interpolation its form can be constructed. Indeed 

 in this connection a certain number of results have already been 

 gained quite sufficient to experimentally confirm the correctness 

 of these types of curves, primarily obtained by purely theoretical 

 deductions. That irritability very gradually reaches its maximal 

 height has been already shown, as previously mentioned by 

 Bowditch 1 in his investigations on the influence of rhythmical 

 induction shocks on the apex of the heart of the frog. He found 

 that in order to produce response, the weaker the stimuli the 

 longer must be the intervals between them. It follows from this, 

 that after a discharge the irritability in response to strong stimuli 

 reappears more rapidly than for weak, i.e., that they only grad- 

 ually regain their maximum. The exact periods of time for the 

 course of the return of irritability for the heart have unfortu- 

 nately not been so far ascertained. On the other hand, the inves- 

 tigations of Ishikawa 2 furnish the material for the construction of 

 the restitution curve for the centers of the spinal cord of the frog. 

 Ishikawa did not employ the threshold of stimulation as an indi- 

 cator for the course of restitution, but used instead the duration 

 of the reflex time following on a stimulus of a certain strength. 

 The reflex time is greatly prolonged after an excitation of ex- 

 tended duration and only regains its normal value in the same 

 degree as restitution takes place. By a great number of pains- 

 taking experiments Ishikawa ascertained the duration of the 

 reflex time at intervals of thirty seconds to one minute, and ob- 

 tained figures which show that restitution does actually take 

 place, at first rapidly and then with constantly decreasing 

 speed. The detailed study of the course of self-regulation of the 

 individual forms of living substance will doubtless be more ex- 

 actly determined in the near future. But even at the pres- 

 ent we are fully justified in describing the form of restitution 

 curve as a logarithmic in type. Therefore, a relative refractory 



1 Bowditch, 1. c. 



2 Hidetsurumaru Ishikawa: "Ueber die scheinbare Bahnung." Zeitschrift f. allgem. 

 Physiologic Bd. XI, 1910. 



