Finally, there arises the important question as to the finer mechanism of conductivity. The progression of excitation from cross section to cross section in a living system is brought about by the decomposition of the molecules in one region acting as a stimulus and producing a disintegration of the molecules in another region, etc. We have already seen that the intensity is dependent upon the amount of energy produced by the disintegration of the molecules following the stimulus, that is, upon the amount liberated in a definite space in a definite time. The question which now arises is this: What form of energy is produced by the stimulus at the point of stimulation, which acts upon the neighboring molecules? The conduction of excitation is a property 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, we 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 molecule 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 explosive 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.[113] 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 production 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. of dry substances remain. Even of this 25 per cent. by far the largest part is apportioned to connective tissue, for which 15 per cent. is certainly not too high a figure. Neither can the remaining 10 per cent.