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A MANUAL OF PHYSIOLOGY 



constant changes in fatigue are a diminution in size and irregularity 

 in outline of the nucleus (which also stains more deeply than before), 

 a shrinking of the protoplasm, a diminution in the amount of the 

 stainable substance of Nissl, with the breaking up of some of the Nissl 

 bodies (p. 662) and the diffusion of their stainable material through the 

 cell. The shrinking of the protoplasm is preceded by an increase 

 in its volume, and this is the sole change in moderate activity. It is 

 only when activity begins to pass into fatigue that other alterations 

 occur. Histological alterations may also be caused in sympathetic 

 ganglion cells by prolonged artificial stimulation of the nerves con- 

 nected with the ganglia. 

 i Experiments on fatigue 



changes in the cells of 

 the spinal ganglia after 

 electrical excitation of the 

 posterior root-fibres are 

 less decisive, some ob- 

 servers having obtained 

 positive, others negative 

 results (p. 711). 



Theories of the Causa- 

 tion of Sleep. (i) Some 

 have suggested that sleep 

 is induced by the using 



U P of substances neces - 



sarv for the functional 

 activity of the neurons, 

 e.g., the stored-up or in- 

 tramolecular oxygen, or 

 by the action of the waste 

 products of the tissues, 

 and especially lactic acid, 

 when they accumulate be- 

 yond a certain amount in 

 the blood, or in the ner- 

 vous elements themselves. 

 (2) Others have looked 

 for an explanation to vas- 

 cular changes in the brain, 

 but so far are the possible 

 causes of such changes 

 from being understood, that it is even yet a question whether in sleep 

 the brain is congested or anaemic. Certain writers have settled this 

 question by the summary statement that when the brain rests the 

 quantity of blood in it must be supposed to be diminished, as in 

 other resting organs. But this is a fallacious argument. For when 

 the whole body rests, as it does in sleep, it has as much blood in it 

 as when it works ; in sleep, therefore, if some resting organs have 

 less blood than in waking life, other resting organs must have more ; 

 and it is the province of experiment to decide which are congested 



FIG. 272. EFFECT OF FATIGUE ON NERVE- 

 CELLS (BARKER, AFTER MANN). 



Two motor cells from lumbar cord of dog fixed in 

 sublimate and stained with toluidin blue ; a, from 

 rested dog: i, pale nucleus, 2, dark Nissl spindles; 



3, bundles of nerve fibrils. , from the fatigued dog : 



4, dark shrivelled nucleus ; 5, pale spindles. 



