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HISTOLOGY 



plasm of the nerve cell. The cells that produce these fibrils are some of 

 the connective-tissue elements that have moved into the nerve tissue and, 

 in connection with the neuroglia cells, are used to give it support and 

 tensile strength. In some of the lower animals they form a thick cover- 



FlG. 164. Dorsal nerve cells in cord of a flounder, Achirus. Shows cytoplasmic connection 

 between cells and an eccentrically placed nucleus. X 800. 



ing for the cell, and their fibrils are so woven into the outer texture of the 

 cell that they are with difficulty distinguished from the neuro-fibrils 

 except by special staining methods. The cell bodies of these connective- 

 tissue cells may lie entirely within the nerve cell. The mollusk, 

 Helix (Fig. 162), shows such conditions, as well as the incidental entrance 

 of connective tissue noted above in the pediculate fishes (see Fig. 161) 

 and in the stellate ganglion cell of Loligo (see Fig. 156). 



Pigment normally appears in a number of nerve cells as a collection 

 of small brown granules occupying some particular portion of the cyto- 

 plasm (Fig. 163). This is of unknown use to the cell. In disease as well 

 as in old age the amount of this pigment is largely increased. 



A centrosphere, sometimes containing 

 a centrosome, has been described in nerve 

 cells by McClure, Lewis, and others. It is 

 possible to look upon these structures 

 either as vestigial organs left from the 

 last division or as centers of some present 

 kinetic operations connected with the activ- 

 ities of the cell. The former view seems 

 not to be in accord with what we know 

 of the persistence of the centrosome after 

 cell division. And yet we shall consider 

 it to be the best view because this struc- 

 ture is only occasionally found, even in the 

 same kind of cell. Many nerve cells show 

 protoplasmic processes which appear to 

 unite with similar processes from other cells. This is well illustrated 

 by the "dorsal nerve cells" from the cord of Achirus lineatus (Fig. 



FIG. 165. Nerve cell from the brain 

 of a lobster, Homarus, showing 

 an implantation cone (imp.c.) that 

 reaches far into the cell. 



