THE NATURE OF STIMULATION 31 



the entire world, and likewise a great series of constituents, each 

 influencing the others, extend from the medium into the organism. 

 The nature of our sense perception, and consequently the knowl- 

 edge derived therefrom, is such that we are obliged to arbitrarily 

 take into consideration merely fragments from the endless inter- 

 dependence of all things in the world, and so we separate the 

 vital conditions of the organisms from their surrounding factors, 

 as though they were independent. A conscientious theoretical 

 analysis requires that we should never forget that in reality such 

 an isolation does not exist. Only with the recognition of this 

 can we distinguish for practical purposes between internal and 

 external vital conditions. In such a differentiation the internal 

 vital conditions which compose the living system conceived to be 

 isolated, are the organs, the tissues, the cells, the protoplasm and 

 the cell nucleus, and within the protoplasm and the nucleus the 

 arrangement and quantitative relations of certain substances, 

 such as proteins, salts, water and the thousands of special com- 

 ponents with their interactions and continued alterations. On 

 the other hand, the external vital conditions, which act on the 

 periphery, are the conditions of the surrounding medium, as 

 foodstuffs, water, oxygen, static and osmotic pressure, tempera- 

 ture, light, etc. But this distinction has only a practical value 

 for the study of the organism as an independent system. Theo- 

 retically it is as impossible to make a sharp distinction between 

 internal and external vital conditions, as to distinguish between 

 the vital conditions generally and the more remote conditions of 

 the environment. All these conditions form a widely branching 

 system of factors of which one is conditioned by the other reach- 

 ing continually from the interior of the vital system into the 

 surrounding medium, so that on the periphery of the system it 

 cannot always be said whether or not a component still belongs 

 to life. Considering these circumstances we can roughly for the 

 present define the conception of stimulus as follows : 



A stimulus is every change in the vital conditions. 



The most essential point in this definition is the relation of 

 the conception of stimulus to that of vital conditions. These 

 relations, however, call for a brief explanation. Here again the 



