154 BIOLOGICAL ASPECTS OF HUMAN PROBLEMS 



rational basis of deranged nervous activity. We 

 have seen that the cerebral centers may be influ- 

 enced through two pathways, the blood stream and 

 the sensory nervous mechanism, and it has been 

 pointed out that a variety of chemical and mechani- 

 cal agencies are capable of operating as disturbing 

 factors. I have, however, failed to emphasize three 

 considerations which are essential to enable us to 

 think clearly of the phenomena of nervous and 

 mental derangement in their bearing on human 

 longevity and efficiency. These are, first, the multi- 

 plicity of disturbing factors operating by way of the 

 blood stream and the sensory system; second, the 

 nature of the erroneous life habits that set these 

 disturbing factors in action; and, third, the powerful 

 effect of hypnotic suggestion on the functional ac- 

 tivities of the brain. 



There is a tendency to ascribe mental derange- 

 ments (including in this term the slight manifesta- 

 tions which make borderland cases) to single causes, 

 although in reality the factors are commonly mul- 

 tiple. Often it happens that some one agent is 

 conspicuous, and the true determinant of the actual 

 disorder, but careful study will usually show that 

 there are associated auxiliary elements in the chain 

 of causation. Psychical causes, mechanical causes, 

 and toxic causes are often associated in bringing on 

 the derangement, although one or other of these 

 types of noxious influence may have been operative 

 longer than the others. Thus a chronic intestinal 



