PROTOZOA 



US ii rule, move away from the region of greater concentration 

 for some substances, but towards it (provided it be not excessive) 

 for others. (See also pp. 22, 23.) We find, indeed, that there is 

 for substances of the latter category a minimum of concentration, 

 below which no effect is seen, and a maximum beyond which 

 further concentration repels. The easiest way to make such 

 observations is to take up a little strong solution in a capillary 

 tube sealed at the far end, and to introduce its open end into 

 the water, and let the solution diffuse out, so that this end may 

 be regarded as surrounded by zones of continuously decreasing 

 strength. In the process of inflammation (of a Higher Animal) 

 it has been found that the white corpuscles are so attracted by 

 the source of irritation that they creep out of the capillaries, and 

 crowd towards it. 



We cannot imagine a piece of dough exhibiting any of these 

 reactions, or the like of them ; it can only move passively under 

 the action of some one or other of the recognised physical forces, 

 and that only in direct quantitative relation to the work that 

 such forces can effect ; in other words, the dough can have 

 work done on it, l)ut it cannot do work. The Amoeba or leu- 

 cocyte on the contrary does work. It moves under the various 

 circumstances by the transformation of some of its internal 

 energy from the " potential " into the " kinetic " state, the condi- 

 tion corresponding with this being essentially a liberation of heat 

 or work, either by the breaking down of its internal substances, 

 or by the combination of some of them w4th oxygen.^ Such 

 of these changes as involve the excretion of carbonic acid are 

 termed " respiratory." 



This liberation of energy is the " response " to an action (jf 

 itself inadequate to produce it ; and has been compared not 

 inaptly to the discharge of a cannon, where foot-tons of energy 

 are liberated in consequence of the pull of a few inch-grains on 

 the trigger, or to an indefinitely small push which makes electric 

 contact : the energy set free is that which was stored up in the 

 charge. This capacity for liberating energy stored up within, 

 in response to a relatively small impulse from without, is termed 

 " irritability " ; the external impulse is termed the " stimulus." 

 The responsive act has been termed " contractility," because it 

 H(j often means an obvious contraction, but is better termed 



' In i.-lieiuical phrase the process is " exotlieriaic." 



