STIMULUS, STIMULATION, IRRITABILITY. 59t 



Steam and in the internal structure of the engine ; there was wanting simply the 

 external impulse, by means of which the valve wis opened, to call forth the working 

 of the engine, the movement of which however essentially depends on the con- 

 struction and putting together of its .parts. It is obvious that neither the mere 

 movement of the hand in opening the valve, nor the mere tension of the steam 

 thereby set in action, constitutes the cause of the working of a steam engine,^ but 

 that this is rather to be sought to a great extent in the internal structure of the latter; 

 and in this sense we have also to regard the internal structure of the plant, which is 

 set in motion by opportune external changes, as the essential cause of phenomena of 

 irritability. 



Perhaps we obtain the clearest expression for the internal condition of an irritable 

 organ by saying, that its parts are in a condition of unstable equilibrium, with the 

 addition, however, that every disturbance of this unstable equilibrium will become 

 sooner or later again compensated, whereby the irritable condition again reappears ; 

 since the peculiar characteristic of irritable organs consists in that, in consequence of a 

 stimulus, a stimulation is in fact called forth — i. e. a new condition, in which the same 

 stimulus can no longer be effective — ^but that after some time the stimulation ceases, 

 and the organ again returns into its original state, whence it can then again be driven 

 to the same stimulation by the same stimulus. The pecuUarity of irritable organs lies 

 less in the fact that their parts can be set in motion in virtue of the unstable 

 equilibrium, than in the fact that they subsequently again resume their irritable con- 

 dition—their unstable equilibrium — spontaneously. 



Let us dwell a little longer upon these matters. We may look upon a house of • 

 cards, as built by a child, as a very well-known example of unstable equilibrium : a 

 slight push suffices to make the whole of the artificial structure tumble down. 

 Here also we have a strikingly large effect consequent on a small cause, as is 

 usually the case with phenomena of irritability; but the fallen card-house does not 

 rebuild itself again on its own account, and is thereby distinguished from an 

 irritable organ. Something very similar occurs with crystals. The same chemical 

 compound, the same salt may often crystallise in two different forms, but so 

 that the one crystalline form arises only under very definite and narrowly cir- 

 cumscribed external conditions, and corresponds to an unstable equilibrium of the 

 molecules ; while the other crystalline form of the same salt is stable. The one 

 unstable arrangement of the molecules can therefore be transformed by insignificant 

 external alterations into this second stable form, and this occurs in a manner which 

 calls to mind veiry strikingly the phenomena of irritability of organisms. Among the 

 numerous examples known I will only dwell upon one, because I have myself had 

 the opportunity of becoming more closely acquainted with it. It was first, however, 

 described in Gmelin's ' Handbuch der C hemic, ' B. I., 1843, p. 95. 'Potassium nitrate 

 (saltpetre) usually crystallises in prisms of the arragonite-form. If, however, a drop of 

 the potassium nitrate dissolved in water is allowed to evaporate on a glass plate, and 

 the crystals observed under the microscope as they form, it is noticed that in addition 

 to crystals of the arragonite form, obtuse rhombohedrons of the calcspar form 

 also are formed at the edges of the drop. On further growth, as the two kinds of 

 crystals come into proximity, the rhombohedra become rounded off and dissolve, 

 because they are more easily soluble, while the prisms of the arragonite-form go on 



