344 MAN --AN ADAPTIVE MECHANISM 



have been evolved to react to acidity in opposite ways ; 

 namely, the histologic changes in the several parts of 

 the brain, produced by the intravenous injection of 

 hydrochloric acid, acid sodium phosphate, etc., and 

 by such acid-producing work as muscular exertion, 

 emotion, physical injury, etc., are uniformly unequal. 

 These changes are striking in the cerebral cortex and 

 slight in the medulla. 



It is probable that the remarkable phenomenon of 

 anesthesia the coexistence of unconsciousness and 

 life is made possible by this antithetic relation be- 

 tween the cortex and the medulla. Within a few 

 seconds after the beginning of nitrous oxid anesthesia 

 the acidity of the blood is increased. This rapid 

 acidulation is synchronous with almost instantaneous 

 unconsciousness and increased respiration. If the 

 amount of oxygen in the inhaled mixture be increased, 

 a decrease in acidity occurs, together with lighter 

 anesthesia and a decreased respiratory rate. 



If our premises are sound, we are justified in con- 

 cluding that the state of anesthesia is in part, at least, 

 the result of an induced acidity of the blood. If the 

 reduction of alkalinity be slight, then the anesthesia is 

 light, and the force of the nerve impulses is lessened, 

 but the patient is still conscious of them. As the alka- 

 linity of the blood continues to decrease, associative 

 memory is lost, and the patient is said to be uncon- 

 scious ; the centers governing the voluntary muscles are 

 not wholly inhibited, however, since cutting the skin 

 causes movements. If the alkalinity be further de- 

 creased, there is loss of muscular tone and even the 

 strong contact ceptor stimuli of a surgical operation do 



