THE CENTRAL NERVOUS SYSTEM 



organs periods of action alternate at more or less frequent intervals 

 with periods of relative repose. In all the higher animals the central 

 nervous system enters once at least in the twenty-four hours into 

 the condition of rest which we call sleep. What the cause of this 

 regular periodicity is we do not know. It is accompanied by 

 changes in the microscopical appearance of the nerve-cells. Thus, 

 Hodge found differences between the cells of certain portions of the 

 cerebral cortex in birds, and of certain ganglia in the honey-bee after 



a long day of work and after 

 a night's rest. Mann, Lugaro, 

 and other observers, found 

 similar differences in the cells 

 of the cerebral cortex and the 

 anterior horn, and Dolley in 

 the Purkinje's cells of the 

 cerebellum in dogs fatigued by 

 muscular exercise as compared 

 with rested dogs (Fig. 382). 



According to Dolley, there is, 

 as a result of continued activity, 

 at first a steady increase of the 

 basic chromatic material. This 

 increase affects first the extra- 

 nuclear chromatin, the Nissl sub- 

 stance, which, according to the 

 most modern view, is really 

 nuclear substance distributed 

 through the cytoplasm, and func- 

 tions as such (Goldschmidt) . The 

 size and number of the granules 

 are increased, and some of the 

 chromatic material is diffused 

 Fig . 382. Effect of Fatigue on Nerve-Cells throughout the cytoplasm, as in- 

 (Barker, after Mann). Two motor cells dicated by diffuse staining. Then 

 from lumbar cord of dog fixed in sublimate the intranuclear chromatin also 

 and stained with toluidin blue, a, from undergoes an increase, and the 

 rested dog; i, pale nucleus; 2, dark Nissl size of the cell is increased too. 

 spindles ; 3, bundles of nerve fibrils. In moderate activity the change 

 6, from the fatigued dog 54, dark shrivelled goes no farther. At this stage 

 nucleus; 5, pale spindles. t h e cell is hyperchromatic i.e., 



as compared with a normal 



resting cell it contains an excess of chromatin. The production of 

 chromatin having reached the maximum of which trie nucleus is 

 capable, and functional activity, which entails the using up of the 

 extranuclear chromatin, still continuing, the total chromatin content 

 begins to diminish, first in the nucleus, through the passage of 

 its chromatin into the cytoplasm to recruit the Nissl substance, then 

 in the cytoplasm as well. Accompanying the disappearance of the 

 chromatic material there is diminution in the size of both cell and 

 nucleus, but especially of the nucleus, so that the normal proportion 

 between volume of cell and volume of nucleus (nucleus-plasma relation 

 of Hertwig) is disturbed in favour of the cytoplasm. Both cell and 



