148 BIOLOGICAL ASPECTS OF HUMAN PROBLEMS 



ture, and if this rise of temperature is sufficiently 

 great, the patient passes into that state of excite- 

 ment which we call delirium, in which there is a 

 marked disorder of personality. There is a remark- 

 able example of the effect on consciousness of poisons 

 locally produced in the blood vessels supplying the 

 cerebral cortex. In the disease known as the African 

 sleeping sickness large numbers of negroes are 

 infected with a peculiar parasite which finds its way 

 in large numbers into the vessels of the cortex and 

 excites there a slow inflammation. The subjects of 

 this disease fall into a deadly sleep from which they 

 can be partially aroused only with difficulty for a 

 few minutes at a time. There has been discovered 

 a preparation of arsenic which destroys these para- 

 sites and rescues the infected subjects from the 

 otherwise fatal sleep. 



Many examples might be cited of poisonous sub- 

 stances which diffuse from the blood into the brain, 

 and give rise to loss of consciousness by passing into 

 solution in certain constituents of the nervous 

 tissues. Alcohol, ether, chloroform, and a host of 

 drugs belong in the class of diffusible anaesthetics 

 and narcotics, which in their action are largely 

 controlled by simple physical laws. A simple 

 but telling experiment can be made to show 

 the readiness with which some of these substances 

 diffuse into the nervous system and out again. If 

 a lively tadpole be placed in a one per cent solution 

 of ordinary ethyl alcohol, he will gradually become 



