CHAPTER III. 



THE PHENOMENON OF CONDUCTION PROPERTIES 

 OF THE NERVE FIBER. 



Conduction. When living matter is excited or stimulated in 

 any way the excitation is not localized to the point acted upon, 

 but is or may be propagated throughout its substance. This prop- 

 erty of conducting a change that has been initiated by a stimulus 

 applied locally is a general property of protoplasm, and is exhib- 

 ited in a striking way by many of the simplest forms of life. A 

 light touch, for instance, applied to a vorticella will cause a retrac- 

 tion of its vibrating cilia and a shortening of its stalk. In the most 

 specialized animals, such as the mammalia, this property of con- 

 duction finds its greatest development in the nervous tissue, and 

 indeed, especially in the axis cylinder processes of the nerve cells, 

 the so-called nerve fibers. But this property is exhibited also to 

 a greater or less extent by other tissues. When a muscular mass 

 is stimulated at one point the excitation set up may be propagated 

 not only through the substance of the cells or fibers directly affected, 

 but from cell to cell for a considerable distance. In the heart 

 tissue especially and in plain muscle it has been shown that a 

 change of this sort may be conducted independently of the phe- 

 nomenon of visible contraction. A stimulus applied to the venous 

 end of a frog's heart, for instance, may, under certain conditions, 

 be conducted through the auricular tissue without causing in it a 

 visible change, and yet arouse a contraction in the ventricular 

 muscle (Engelmann). The change thus conducted may be spoken 

 of as a muscle impulse. Under normal conditions a muscle fiber 

 is stimulated through its motor nerve fiber at some point near the 

 middle of its course, but the stimulus thus applied must be con- 

 ceived as arousing a muscle impulse that travels over the length of 

 the muscle fiber and precedes the change of contraction. Similarly 

 it can be shown that ciliary cells can convey an impulse from cell 

 to cell. A stimulus applied to one point of a field of ciliary epi- 

 thelium may set up a change that is conveyed as a ciliary impulse 

 to distant cells. The universality of this property of conduction 

 in the simpler, less differentiated forms of life, and its presence in 

 some form in many of the tissues of the higher forms would justify 

 the assumption that the underlying change is essentially the same 

 in all cases. But in nerve fibers this property has become special- 



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