THE PROCESS OF EXCITATION 89 



of the combined employment of the most varied kinds of physical 

 as well as chemical indicators. If we use the most varied types of 

 living substance of widely differing properties, showing us the 

 greatest variety of vital manifestations, we may hope by the use 

 of comparative physiological methods, even though with diffi- 

 culty, to separate more and more the essential details of the gen- 

 eral processes of excitation. At present we are still at the very 

 beginning of this task and vast fields of unexplored regions are 

 yet before us. But it is the unknown which has a particular fasci- 

 nation, especially if we succeed from time to time in making new 

 advances. 



If we suppose a living system in a state of metabolism of rest 

 influenced by an instantaneously excitating stimulus, the entire 

 course of excitation extends from the first alteration produced 

 by the stimulation until the complete restitution of the metabolic 

 equilibrium, and we will, therefore, differentiate individually the 

 successive stages of this whole process. 



The very beginning of the chain of alterations produced by the 

 excitating stimulus cannot be studied by any indicator. The 

 changes must first reach a certain dimension by conduction from 

 the point of stimulation before they influence even the most deli- 

 cate indicators. The application of the stimulus is, therefore, 

 followed at first by a measurable "latent period," in which the 

 living substance remains apparently at rest. This latent period 

 has been particularly studied in muscle. After its discovery by 

 Helmholtz 1 it was made the object of innumerable investigations 

 and met with an interest which can only be explained by the exact- 

 ness of the methods employed. Among others Tigerstedf 2 has 

 made the most thorough study of the influence of various factors 

 on the duration of the latent period. These experiments have 

 established the fact that the duration of the latent period varies 

 according to the intensity of the stimulus, temperature, loading 



1 Helmholts: "Messungen iiber den zeitlichen Verlauf der Zuckungen animalischer 

 Muskeln und die Fortpflanzungsgeschwindigkeit der Reizung in den Nerven." Archiv 

 fur Physiologic Jahrgang 1850. 



2 Robert Tigerstedt: "Untersuclmngen uber die Latenzdauer der Muskelzuckung 

 in ihrer Abhangigkeit von verschiedenen Variablen." Arch. f. Physiologic Jahrgang 

 1885 Suppl. 



