6S IRRITABILITY 



In all these examples, the living processes occur with equal 

 constancy and unaltered rapidity, provided a stimulus is not 

 operative. Here, however, the gradual alterations, the result of 

 development, must not be overlooked. An excellent example of 

 this is seen in the eggs of sea urchin, where the development is 

 readily perceptible. In all these instances, however, the condi- 

 tion is immediately changed by the influence of the stimulus. 

 The previous state of constancy in the vital process is disturbed. 

 The rapidity of its course is changed, being either increased or 

 decreased, and the specific vital manifestations concerned are, 

 therefore, augmented or diminished. We will now study the 

 vital process with the methods -of chemical investigation and con- 

 sider the problem from the standpoint of metabolism. It may 

 be noted here, that other methods, such as the transformation of 

 energy or changes of form of the living system, would serve 

 equally well as indicators for this purpose. In every instance 

 there is a uniformity of the processes; the difference, however, 

 is in the nature of the indicators and the terms used. The meth- 

 ods and the terms used in chemical investigation and description 

 reach proportionately much deeper than those employed when 

 the transformation, energy or the variations of form of the 

 organisms are studied, and permit of the finest differentiation of 

 the processes. The atomistic terminology is, for this reason, 

 preeminently fitted for the description of vital processes. When 

 we study the vital process metabolically, we can, as shown in the 

 above-mentioned instance, divide the processes into a metabolism 

 of stimulation in contradistinction to a metabolism of rest. 



The comprehension of the metabolism of rest demands a closer 

 consideration. On closer observation we must say that this 

 much-used conception is merely an abstraction nowhere realized 

 in a strict sense. In truth, there is nowhere in nature a metab- 

 olism of rest. No cell exists which in a mathematical sense 

 remains for even two successive moments under absolutely the 

 same external conditions. If we imagine a single living cell of 

 the simplest kind living in a fluid nutritive medium, and if we 

 suppose its body and surroundings so magnified that the single 

 molecules and atoms were respectively of the size of cannon and 



