ch. vii.] SOURCES OF TERRESTRIAL ENERGY. 415 



tional excitement so alters the composition of the blood 

 that infants have been poisoned by milk secreted by their 

 frightened or angry mothers. And lastly may be cited the 

 beautiful experiments of Prof. Lombard, in which the heat 

 evolved by the cerebrum during the act of thinking was not 

 only detected but measured, and found to vary according to 

 the amount of mental activity going on. 



These, though the most conspicuous, are but a few among 

 the facts which force upon the physiologist the conclusion 

 that there is no such thing as a change in consciousness 

 which has not for its correlative a chemical change in 

 nervous tissue. Hence we may the better understand the 

 significance of familiar facts which point to a quantitative 

 correlation between certain states of consciousness and the 

 outward phenomena which give rise to them. A bright 

 light, as measured by the photometer, produces a more 

 vivid state of consciousness than a dim light. Substances 

 which the thermometer declares to be hot are, under normal 

 circumstances, mentally recognized as being hot. The con- 

 sciousness of a sound varies in vividness with the violence 

 of the concussions to which the sound is due. And bodies 

 which are heavy in the balance excite in us correlative 

 sensations of strain when we attempt to move them. Con- 

 versely the molar motions by which our states of feeling 

 are revealed externally, have an energy proportional to the 

 intensity of the feeling ; witness the undulations indicative 

 of pain, which, beginning with a slight twitching of the 

 facial muscles, may end in spasmodic convulsions of the 

 whole body. And of like import is the fact that gentle 

 emotions, like slight electric and narcotic stimuli, agreeably 

 quicken the heart's contractions ; while violent emotions, 

 suddenly awakened, may stop its beating as effectually as a 

 stroke of lightning or a dose of concentrated prussic acid. 



The bearings of such facts as these upon our theories of 

 mental phenomena will be duly considered in future chapters. 



