THE NERVE CELL 



181 



158). Another common type of chromatin distribution is to be seen 

 in the ganglion cells from the stellate ganglion of the squid and the 

 electric lobes of the tor- 

 pedo's brain where some, 

 comparatively few, of the 

 chromatin masses are 

 found to be very much 

 larger than the others and 

 to form centers around 

 which these smaller, dust- 

 like particles have gath- 

 ered in clouds that thin 

 out toward the edge (see 

 Figs. 156 and 157). In 



the last example, the tor- FIG. 158. Nerve cells from a sympathetic ganglion (cer- 

 pedo's electric Cell Still vical) of the muskrat, Fiber, tr., trophospongia, nutri- 

 ' . ent channels mostly filled with lymph, x 1000. 



another development is to 



be seen in the collection of a number of these larger particles of chro- 

 matin into a dense mass of irregularly round outline called the chro- 

 matin knot. In this case the chromatin knot is always placed near the 

 periphery of the nucleus and at the opposite side from the nucleolus 



(Fig- 157)- 



These various forms of the chromatin particles and the various man- 

 ners in which they are arranged have not as yet given us any generaliza- 

 tions that enable the nerve tissue to be better understood. They show 

 some changes during the operation of the cell and in certain diseases. 



The nucleolus appears in the nerve cell much as it does in most of the 

 other cells that have this organ well developed. It is nearly always large 

 and sometimes very large (see Figs. 160 and 161). It is usually an 

 exact sphere as far as the eye can determine, and only in the larger forms 

 is any irregularity to be seen. Like other nucleoli, it sometimes contains 

 one or more "vacuoles," or spaces containing non-staining substances. 

 Figure 161 shows one that has a very large vacuole containing a material 

 which shows the differentiation of a chromatic network. Non-staining 

 nucleoli are shown in the sympathetic ganglion cells of the muskrat 

 (see Fig. 158). 



The whole nucleus is, in some instances, found at the periphery of 

 the cell (see Fig. 164) and, while it is usually single, it may be double 

 in some cells or even triple. This is the rule in the cells of the sym- 

 pathetic ganglia of the rodents (Fig. 158), and a frequent exception in 

 the electric nerve cells of the torpedo and the dorsal cells of the winter 

 flounder's spinal cord (see Fig. 160). 



The cytoplasm of a neuron and its processes is of particular interest 



