IRRITABILITY. 373 



them to the spontaneous decomposition of the decomposable 

 substance, without assuming the intervention of internal 

 stimuli. In any case they are the expression of what we 

 have termed the automatism of the organism (p. 7). With 

 regard to the stimulating effect of an external agent, we 

 may, taking the above view of the intimate cause of spon- 

 taneous movement as a basis, account for it thus, that it 

 precipitates the spontaneous decomposition of the decom- 

 posable substance which the irritable protoplasm contains, 

 and thus determines an evolution of energy which, provided 

 that the anatomical structure of the organ permits, finds its 

 external expression in a movement. This mode of regarding 

 the action of an external stimulus enables us to understand 

 how it is that the energy evolved in consequence of its action 

 is incommensurately greater than the energy of the stimulus. 

 The relation may be illustrated by comparing the force 

 exerted in pulling the trigger of a rifle with the momentum 

 of the travelling bullet. 



In some cases the effect of a stimulus appears to be that 

 it arrests movement. It will be shewn in detail later in the 

 course that spontaneous movement is arrested by stimulation, 

 but it is nevertheless true that stimulation induces move- 

 ment. The immediate effect of stimulation in such a case is 

 to induce a change of form in the protoplasm of the cell or 

 cells, with the further effect that the recovery of irritability is 

 much prolonged, the more so the stronger the stimulus has 

 been. It is for this reason that when an organ exhibiting 

 spontaneous movements is stimulated, its movements will 

 cease for some time. 



There is this general peculiarity to be noted in the relation 

 of motile organs to changes in the tonic conditions, or to the 

 stimulating action of external agents, namely, that the effect 

 induced is not immediately manifested. For instance, if, 

 under a certain combination of external conditions an organ 

 is growing with a certain rapidity, and the external conditions 

 be so changed as to involve a slower or a faster rate of growth, 

 the change in the rate of growth will not coincide in point of 

 time with the change in the external conditions, but the 



