DIFFERENT TYPES OF RESPONSE 117 



is maintained in a state of equilibrium by the two opposed 

 chemical processes of building up or assimilation, and break- 

 down or dissimilation. Stimulus causes a down or dissimi- 

 latory change, which is again compensated, during recovery, 

 by the building up, or assimilative change. In the case of 

 uniform responses, the two processes exactly balance each 

 other. But on occasions when the down change is the greater 

 of the two, the potential energy of the system falls below 

 par, for the building-up process cannot then sufficiently 

 repair the chemical depreciation caused by the downward 

 change. Hence occurs diminution of response, or fatigue, 

 which is supposed to be further accentuated by the produc- 

 tion and accumulation of deleterious ' fatigue-stuffs.' The 

 disappearance of fatigue, after a period of rest, is explained 

 by the renovating action of the blood supply, which is also 

 regarded as the means of carrying away the fatigue-stuffs. 



A serious objection to these explanations, however, lies in 

 the fact that even excised and bloodless muscles exhibit 

 recovery from fatigue, after a period of rest. In isolated 

 vegetable tissues, again, where there is no active circulation 

 of renovating material, the same effect, and its removal after 

 a period of rest, are observed. 



Thus the difficulties met with in explaining fatigue accord- 

 ing to a purely chemical theory are great enough. But still 

 greater are those which we encounter when we come to deal 

 with the staircase effect — typically shown in cardiac muscle 

 — in which successive responses to uniform stimuli exhibit a 

 gradual enhancement of amplitude. Here the result obtained 

 is in direct opposition to the theory described ; for in this 

 particular case, we have to assume that the same stimulus 

 which is usually supposed to cause a chemical break-down 

 becomes efficient to produce an effect exactly the reverse. 

 It is true that the heart, usually speaking, is charged with 

 blood ; but this particular staircase increment of response, 

 under uniform stimulation, is observed even in the initial 

 twitches of bloodless muscle (fig. 64), and here there can be 

 no question of a supply of renovating blood. 



