FATIGUE. 3 1 3 



from the decomposition of the cytoplasm during activity. 

 The signs of fatigue can thus be induced in a dog at 

 rest by injecting into his vessels the blood of a dog 

 that has been wearied (Mosso). The substances thus 

 passed into the blood are not only deterrent to the 

 activity of the cells which produce them, but to the 

 other cells with which they are brought in contact by 

 the circulation. Thus local exercise produces general 

 weariness. Moreover, these substances are probably 

 different, and have a different physiological value 

 according to the tissues in which they originate. The 

 fatigue from nervous activity differs from that due to mus- 

 cular exertion or over-eating. When some error in the 

 elimination or chemical destruction of these substances 

 occurs, there may develop a genuine auto-intoxica- 

 tion the effects of which can apparently be wide- 

 reaching. 1 



Healthy weariness is followed by sleep a condition 

 hard to define. Unconsciousness may be produced by 

 a blow on the head, by drugs, extreme cold, hypnotic 

 suggestion, compression of the carotid arteries, and so 

 on : from these conditions normal sleep is distinguished 

 by the fact that it is refreshing ; and so far as it departs 

 from this character, it fails of its purpose. The animals 

 in which the habit of rhythmic sleep is best developed 

 are the birds and higher mammals, and among these it 

 is especially those in which the encephalon is most 

 elaborated that there is a sharp contrast between 

 activity and repose. In these forms the central cells 

 are proportionately well developed, and thus there is 

 some reason to associate this condition with variations 

 in the functions of this group. At the same time, the 

 higher animals have it in common with the lower, that 



1 Cowles, Neurasthenia and its Mental Symptoms, Shattuck 

 Lecture, Boston, 1891. 



