338 MAN --AN ADAPTIVE MECHANISM 



These are illustrations of the principle which we 

 have sought to make clear in the foregoing chapters 

 - the fact that the nervous system acts as a whole ; 

 that it can respond to but one stimulus at a time ; and 

 that when the nervous system is preempted by one 

 stimulus to the point of exhaustion, it can respond to 

 no other until an approximately normal condition of 

 the kinetic system has been regained through rest. 

 The lack of the power to laugh or weep, and the 

 absence of pain among the exhausted, the senile and 

 those weakened by disease demonstrate the fact that 

 the kinetic system is exhausted ; that it has been 

 integrated for response to a stimulus stronger than 

 that to a motor activity for which laughter or weep- 

 ing is a relieving substitute ; and that consequently 

 the energy required for laughter or weeping is no 

 more forthcoming than it is forthcoming for any other 

 form of motor activity under the same conditions. 



A striking contrast to the absence of laughter or 

 weeping when the brain thresholds have been raised by 

 extreme exhaustion is found in the hypersusceptibility 

 to both laughter and weeping, shown in cases of 

 Graves' disease and in neurasthenia, both of which 

 are inevitably marked by weak inhibition and a low 

 threshold to all stimuli. 



In Graves' disease the motor mechanism is in an 

 exalted state of activity. These patients, therefore, 

 as would be expected, exhibit an extreme susceptibility 

 to laughter and weeping because of the fact that the 

 motor mechanism is constantly integrated by the most 

 trivial stimuli. In Graves' disease the flood gates 

 of tears are open. The susceptibility to pain is also 



