IRRITABILITY 



Fig. 14. 



Contractions of the musculus extensor digitorum communis longus of the dog, brought about by rhyth- 

 mic stimulation of the nervus peroneus. The muscle is in the condition of tonic excitation which 

 proceeds from the center. The arrows indicate the point where reflex inhibition of the central tonus 

 is produced. The height of the single contraction undergoes no diminution. 



respect to the conduction of excitation in the following points : 

 In regard to the rapidity with which the excitation is conducted, 

 the extent of the area over which it spreads, and the intensity 

 with which it extends. These conditions may be best illustrated 

 by citing two extreme examples. The one is formed by the 

 rhizopods, the other by the nerve fibers. Between these two 

 extremes we have manifold gradations in the conditions of con- 

 ductivity. Not all cell forms are suitable objects for the study of 

 conductivity. There are forms of rhizopods which are as favor- 

 able to investigation as the nerve ; this is due to the fact that, 



