THE GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM 103 



prolonged muscular exertion produces toxins (carbon dioxid, lactic acid, and 

 others) which are dissolved in the blood and exert a profound depressing 

 influence upon all of the tissues of the body. If the blood of a fatigued 

 animal be injected into or transfused with a perfectly fresh animal of the 

 same species, the latter immediately manifests all the signs of fatigue. 



It is often taught that a change of work is physiologically equivalent to 

 complete rest. It is true that, so long as one is well within the limits of 

 extreme fatigue, a change of work will prolong efficiency far beyond that 

 which would be possible in continuous activity of a single nervous or mus- 

 cular mechanism. Nevertheless experiment shows that mental efficiency 

 is greatly impaired in extreme muscular fatigue, and, conversely, muscular 

 power is greatly weakened after long sustained mental work. Glandular 

 secretions are also apparently often reduced in extreme fatigue, thus, for 

 instance, reducing the efficiency of the digestive organs. These effects are 

 doubtless clue to the accumulation of toxic products in the blood, producing 

 a true "fatigue of depression" throughout the entire body. 



It has been suggested that the local feelings of muscular fatigue are due 

 to excitations of the organs of the muscular sense in the muscle spindles (p. 

 87) ; but the evidence for this does not seem very convincing. 



The experiments of Dolley suggest to him, further, that the more highly 

 differentiated nerve-centers are more susceptible to the structural altera- 

 tions of fatigue than are those of the lower reflex systems. It is a well- 

 known fact that sustained mental work produces the subjective evidences 

 of fatigue more promptly than does muscular work, and that during severe 

 mental training one is more apt to go "stale" than during physical training. 

 This principle has been widely recognized in the provision of short work- 

 ing hours and frequent holidays for pupils and teachers in our schools; it 

 should be still further extended, especially in commercial and professional 

 life. Its neglect is in large measure responsible for the prevalence of neu- 

 rasthenia and other forms of nervous breakdown. 



The early fatigue of the higher voluntary centers is particularly evident 

 in young children, where continuous sustained attention is impossible except 

 for very short periods. By training, these periods can be greatly length- 

 ened, the nervous mechanism involved here probably being the acquisi- 

 tion of a wider range of associations related with the subject which occupies 

 the focus of attention, so that individual neurons or systems of neurons 

 which participate in the functional complex may be temporarily rested while 

 other related systems are brought into maximum activity, without thereby 

 interrupting the continuous progress of the train of thought. 



The neurological basis of sleep is at present wholly unknown, 

 though the physiological phenomena seem to be in many respects 

 analogous with those of fatigue. Of the various theories which 

 have been suggested, the two which have excited greatest interest 

 are: (1) the belief that some soluble toxin is produced during 

 waking hours which induces sleep by a process similar to that of 

 the "fatigue of depression," and (2) the doctrine of the retraction 

 of the neuron, which teaches that during sleep (and according 

 to some authors in less measure during fatigue also) the dendrites 

 of the neurons retract toward their cell bodies and away from 



