146 IRRITABILITY 



by the stimulus at the point of stimulation, which acts upon the 

 neighboring molecules? The conduction of excitation is a prop- 

 erty of all living substance, and we may presume that this 

 occurs in all living systems in the same manner. If one examines 

 the forms of energy which are produced in a living substance by 

 the breaking down of the molecules, w r e find that chiefly three 

 forms of energy may be taken in consideration in the problem 

 of conductivity : heat, electricity and osmotic energy. Light 

 cannot be looked upon as a form of energy which is produced 

 by all living substance, and the other forms of energy, as the 

 chemical energy and surface tension, remain local. At a first 

 glance one is inclined to assume that heat is the form of energy 

 which is liberated by the breaking down of the stimulated mole- 

 cule and which spreads to the neighboring molecules and brings 

 about their decomposition. For we know that heat facilitates 

 dissociation, and the analogy between living substance and explo- 

 sive material is very close. In both instances the decomposition, 

 which extends over a great mass of molecules, is accomplished by 

 the heat produced in the breaking down of a few molecules. In 

 fact, the conduction of excitation of a nerve can in many respects 

 be compared with the burning of a fuse. 1 Nevertheless, it must 

 not be forgotten that this analogy, which on first glance seems so 

 apt, upon closer observation presents serious difficulties. It can 

 be experimentally shown that an increase in the temperature in 

 the living substance follows stimulation, but it is also known that 

 in momentary excitation following a single stimulus, as in the 

 muscle after the application of an induction shock, the heat pro- 

 duction is extremely small. This difficulty becomes particularly 

 apparent if we endeavor to gain an approximate idea of the 

 numerical proportions of the irritable, that is the disintegrating 

 molecules to the remaining mass of a living system. The 

 water content above all represents an enormous proportion. 

 If we calculate this to be for the nerve, for instance, roughly 

 about 75 per cent., which is a low estimate, only 25 per cent. 



1 Compare Pfluger: "Ueber die physiologische Verbrennung in den lebendigen Or- 

 ganismen." In Pfliigers Archiv. Bd. 10, 1875. Further: L. Hermann: "Handbuch der 

 Physiologic, Bd. II, Allgemeine Nervenphysiologie," 1879. 



