THEORY OF NEURON ACTION 407 



when touched with a needle point demonstrates that 

 protoplasm is capable of responding to stimulation. Since 

 the result is the same no matter where the protoplasm is 

 touched, it follows that it is also capable of conducting 

 impulses thus set up. In other words, responses to certain 

 forms of stimulation and conduction of impulses is a proD- 

 erty of protoplasm. When we examine the cjiioplasm of 

 a nerve cell we find it to be made up of two kinds of proto- 

 plasm. One kind resembles closely that of the amoeba. 

 The other kind seems to be in the form of threadlike fibrils. 

 The axon protoplasm is almost purely of the fibrillar kind. 

 This fibrillar protoplasm is found to be able to transmit 

 impulses much more rapidly than ordinary protoplasm. 

 In other words, it is protoplasm with a highly developed 

 power of conduction. The discovery of this structural 

 peculiarity of nerve cells enables us to think of them as 

 cells whose protoplasm has been specially developed for 

 conducting impulses. 



Keeping in mind this fact, and also the structure rela- 

 tions in a neuron, the formulation of the theory of neuron 

 action may be expressed in the following laws : 



First law. An impulse may be started wherever there 

 is protoplasm, but externally stimulated impulses have 

 their origin usually in the terminal brushes of the axones 

 or in the dendrites, while the internally stimulated im- 

 pulses arise in the body of the nerve cells. In general, 

 then, the nerve fiber or axon is not a source of impulses, 

 though in cases of inflammation of the sheaths it may' 

 become so. 



Second laiv. An impulse started in any part of the 

 protoplasm of a neuron is conducted by the fibrillar pro- 

 toplasm to every other part of the neuron protoplasm. 



