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 

 apphed 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, 

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

 so-called nerve fibers. But the 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 and in plain muscle it has been shown that a change of 

 this sort may be conducted independently of the phenomenon 

 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). Similarly, it can be shown that ciliary 

 cells can convey a stimulus from cell to cell. A stimulus applied 

 to one point of a field of ciliary epithelium 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 degree in 

 many of the tissues of the higher forms would justify the as- 

 sumption that the underlying change is essentially the same in 

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

 ized to the highest degree, and in this tissue it may be studied, 

 therefore, with the greatest success and profit. 



Structure of the Nerve Fiber.— The peripheral nerve fiber, 

 as we find it in^the nerve trunks and nerve plexuses of the body, 

 may be either meduUated or non-meduUated. All the nerve fibers 

 that arise histologically from the nerve-cells of the central nervous 



77 



