82 



COMPARATIVE ELECTRO -PHYSIOLOGY 



NT 



ST 



possible. The actual potential or excitatory level of a tissue 

 depends on its tonic condition and the intensity of the 

 incident stimulus. Now this existing potential of the tissue 

 may be anything within a wide range, s T, when sub-tonic, 

 N when normal, or H T when hyper-tonic or above the 

 ordinary normal degree (fig. 53). Since 

 it is necessary that the incident stimulus 

 should cause the critical level C to 

 be slightly exceeded, if there is to 

 be an excitatory overflow, we can see 

 that the intensity of the stimulus re- 

 quisite to evoke response will be greater 

 in proportion as the tonicity of the tissue 

 itself is low. Thus when the tissue is 

 extremely sub-tonic, a stimulus of or- 

 dinary intensity could never avail to 

 raise the energy of the system above 

 the critical point, and the response 

 must then therefore be positive. Under 

 these circumstances it will only be by 

 the impact of excessively strong sti- 

 mulus, or by the cumulative action of 



a series of moderate stimuli, that the critical point can be 

 reached and passed, and the normal negative response 

 evoked. 



Thus the intensity of the minim ally- effective stimulus in 

 evoking normal response will afford us a measure of the 

 tonicity of the tissue. If the latter be high, then the feeblest 

 stimulus will precipitate outward response, and indeed, if 

 excessive, response will occur on little or no provocation, and 

 such movements we call ' autonomous.' It must be remem- 

 bered, however, that it was by the previous absorption of 

 stimuli that the tissue was brought to this point of unstable 

 equilibrium at which the added impact of an infinitesimal 

 stimulus causes it to bubble over, as it were, into apparently 

 spontaneous activity. 



The predominant expression of the highly tonic tissue 



FIG. 53. Diagrammatic 

 Representation of the 

 Tonic Level 



N, normal ; ST, sub-tonic ; 

 H T, hyper-tonic ; and 

 c, the critical level. 



