NERVE ELEMENTS AND THEIR FUNCTIONS. 89 



of fibrillae in the protoplasm of nerve cells has been known for 

 nearly fifty years, it is only in the last few years that they have 

 been extensively studied and described. They have been found 

 in many classes of animals and in many kinds of cells in the 

 nervous system of invertebrates and vertebrates, so that their 

 existence as structures characteristic of nervous tissue is quite 

 certain. Further, it has been clearly shown that when two nerve 

 cells are connected by strands of protoplasm the neurofibrillae 

 may extend from one cell into the other. Neurofibrillae have been 

 described as running through two, three or more cells without 

 interruption. With regard to the origin, structural character and 

 function of these fibrillae, further study is needed. It is said that 

 they are formed outside of nerve cells and grow into them, that 

 they may extend beyond the limits of the protoplasm of nerve cells 

 as in nerve-muscle endings, and that they are the medium of 

 conduction of nerve impulses, while the nerve cells perform only 

 the incidental function of nutrition. Indeed the neurofibrillae 

 are regarded by some as quite new structures in the nervous 

 system in addition to nerve cells; structures which are more 

 intimately and essentially concerned in specific nervous functions 

 than are the nerve cells themselves. 



Certainly in the present state of knowledge this view is extreme. 

 It seems more reasonable to regard the neurofibrillae as a dense 

 portion of the colloid substances in the cytoplasm of nerve cells, 

 whose definite form and arrangement are conditioned upon the 

 intimate structure of the protoplasm as a whole. Thus if the 

 protoplasm has the structure of a foam, then the colloid substances 

 forming the walls of the vesicles and filling the solid angles, in 

 elongated strands such as dendrites and neurites would certainly 

 appear as threads. In more rounded cell-bodies the colloid sub- 

 stance would be more irregularly arranged and there would be 

 the appearance of branching and crossing of threads, as is actually 

 the case with the neurofibrillae. When the dendrites of two cells 

 fuse together the neurofibrillae would of course continue from 

 one cell into the other. In the most minute end branches of 

 neurites the plasm may be so slight in amount that in prepa- 

 rations in which the colloid substance alone is sharply stained, 



