124 GENERAL PHYSIOLOGY 



inorganic bodies, the crystal alone is again in mind. The steam- 

 engine, however, is an inorganic system in which dynamic equi- 

 librium exists very clearly; for by the mediation of heat the 

 systein gives off to the outside as mechanical energy exactly 

 as much energy as is introduced bj' the burning of the coal. 



Finally, irritability has been brought forward as a general 

 characteristic of organisms in contrast to inorganic bodies. In 

 reviewing the history of physiological investigation it was seen 

 that at first very indefinite ideas were associated with the word 

 " irritability," and, in order to guard against misunderstandings, 

 the conception must be definitely formulated. It can be said in 

 general that irritability is the capacity of a body to react to 

 an external influence by some kind of change in its condition, in 

 which the extent of the reaction stands in no definite proportion 

 to the extent of the influence. As a matter of fact, irrita- 

 bility, or excitability, is a property of all living substance, 

 whether the organism responds to the external influence by the 

 production of definite substances, as with secreting gland-cells, 

 or definite forms of energ)', as with muscle-cells, phosphorescent 

 cells, and electric cells, or whether it responds by depression or 

 even standstill of its vital activities. But irritability is not the 

 exclusive property of organisms, for lifeless substances are likewise 

 irritable and respond to external influences by definite changes, 

 e.g., by the production of definite substances or of energj', in which 

 process the extent of the production by no means corresponds 

 always to the extent of the external impulse. The clearest 

 examples of such cases are afforded by exjDlosive substances. By 

 a slight shock nitroglycerine is decomjjosed into water, carbonic 

 acid, oxygen and nitrogen, the process being accompanied by a 

 powerful evolution of energy ; in other words, nitroglycerine 

 responds to an external infiuence by an enormous production of 

 energy and a change of material. Hence irritability is not an 

 absolute sign of diflference between organisms and inorganic 

 bodies, and it is seen that a fundamental contrast between the two 

 is afforded no more by their dynamical than by their structural 

 and genetic relations. We will, therefore, search still further. 



4. Chemical Differences 



It is by a comparison of their chemical relations that a dif- 

 ference is finally found to exist between organisms and inorganic 

 bodies. 



It has been seen that a specific vital element exists in the 

 organism no more than a specific \'ital force. The chemical 

 elements that compose the organism occur without exception in 

 inorganic nature also. Therefore, a fundamental chemical con- 

 trast betwc.'en organic and inorganic substance is not to be 



