1 88 HISTOLOGY 



4 



fact that the efferent and afferent fibers both arose from a single process 

 of the cell in such a way that the impulse paths did not lie in the main 

 cell body at all, but passed directly from the dendrite to the neurite. This 

 process, bearing the two fibers, was cut off and the cell body removed 

 from the tissue without disturbing the connection between the fibers or 

 disturbing their relations to the other tissues in any way. Under these cir- 

 cumstances the nerve continued to carry the impulse as before, and kept 

 up its usual function until its death in the otherwise uninjured tissues. 

 Its death was undoubtedly due to the cutting off of its nutrition and other 

 trophic benefits formerly derived from the cell body with its nucleus. 



This latter fact has also been observed in the pathological and ex- 

 perimental cutting off of masses of fibers from their nerve cells. In these 

 cases the fibers die and new processes grow out from the same cells to take 

 their places. 



The nerve fiber is a structure that is only called into existence when 

 either the perceptory or the motor surface of the cell is situated at a dis- 

 tance from the cell body. Consequently its length is variable and in 

 some cases is reduced to the length of an ordinary cell. 



The neurite, as has been previously said, leaves the cell body from an 

 implantation cone, which is the intermediate portion between the cell body 

 and the fiber. This structure may vary from the short conical form seen 

 in most nerve cells to the more extensive kinds that appear in some of the 

 invertebrate nerve cells, as, for instance, the lobster, where the implanta- 

 tion cone is narrowed to the diameter and continues as an extension of the 

 fiber itself into the cell, forming a long curved path that sometimes 

 encircles the nucleus before its substance merges with that of the cyto- 

 plasm and its fibrils are no longer to be distinguished among the neuro- 

 chondria (see Fig. 165). Also see other figures under nerve cells. 



There are interesting accessory tissues found in connection with the 

 nerve fibers and used to provide them with coverings for their protection, 

 and with support and union in their common pathways, the nerve tracts. 

 The latter of these are the neuroglia cells, and will be treated of in the 

 next part but one. The former are the true connective-tissue cells that 

 form the sheaths covering most nerve fibers (Fig. 166). 



These sheath cells have been thought to be of ectodermal origin and 

 to have migrated, along with the nerve process, to the positions in which 

 they are found. It has been proved, however, that they are true connec- 

 tive-tissue cells from the locality through which the nerve fiber has passed 

 in its development. They may be compared closely with the connective- 

 tissue cells that surround some of the nerve-cell bodies in the ganglia. 

 A nerve fiber is not always provided with this covering. Some few kinds 

 have none whatever other than the unspecialized connective or other 

 tissue through which they pass. The simplest form in which a definite 



