88 IRRITABILITY 



lation upon the complex system. In the beginning, the process 

 of excitation was studied exclusively on the muscle and nervous 

 system. A physical factor served as indicator, such as muscle 

 contraction or production of electricity. These showed, besides 

 the direct and primary effect of stimulation, the secondary pro- 

 cess of conductivity. Even graphic registration is merely an 

 expression of the phenomena composed of a great mass of indi- 

 vidual elements. The visible course of the phenomena, as shown, 

 for instance, by the latent period by the ascent and descent of the 

 curve of contraction, represents as it were a reflected picture of 

 the actual excitation processes similar to an object seen in a dis- 

 torting mirror ; the first and the last parts of the process are not 

 even perceptible. Later, when organ physiology was extended 

 into a cell physiology the processes of excitation were studied in 

 numerous simple organisms, such as the plant cell, the rhizopoda, 

 the infusoria, etc. Later, in this way, by the use of compara- 

 tive methods many essential facts were discovered. However, 

 even the single cell, in spite of its minuteness, is, compared with 

 the size of a molecule, a gigantic system, and it would be a grave 

 error if we should consider this system even in its simplest aspect 

 as homogeneous. In order, therefore, to analyze the vital activi- 

 ties in the cell, cell physiology must endeavor to penetrate into 

 molecular conditions. For this purpose the indicators employed 

 must be essentially of a chemical nature, capable of magnifying 

 the processes of molecular dimension to such a degree that we 

 are enabled to base conclusions upon these not otherwise directly 

 perceptible phenomena. To obtain a sufficient magnification we 

 must necessarily place somewhat larger quantities of living sub- 

 stance under observation and apply a stimulus of such frequency 

 or length of duration that the chemical alterations as a result of 

 excitation are so increased as to be plainly perceptible with the 

 aid of our chemical indicators. Unfortunately, we do not pos- 

 sess specific chemical indicators for every individual molecular 

 constituent process of the cell and so cannot dispose with the help 

 of indicators of the combined happenings in a greater quantity of 

 living substance. It remains for us to obtain data concerning the 

 cycle of excitation processes in the living substances by the aid 



