SELF-PRESERVATION AND THE MENTAL LIFE 147 



lively insusceptible, who show what we call mental 

 stability, we find, in general, that they show 

 freedom from nervous and mental diseases, good 

 nutrition, freedom from intoxication, from alcohol 

 and syphilis, and a tendency to longevity. Could 

 nature give us a more impressive hint as to what 

 society should strive to do for the elimination of 

 mental disorder? 



Let us now review the influences that are capable 

 of disturbing the mental functions, especially in the 

 susceptible, but also in those who are relatively 

 insusceptible. It is quite impossible to get a fair 

 conception of the nature of mental and nervous 

 disease, if we fail to scrutinize these influences. 



There are two main paths by which the brain may 

 be approached, and, so far as we are aware, only 

 two. These are the blood vessels with their lym- 

 phatic connections and the nerves of sensation. A 

 liberal supply of blood is necessary to maintain the 

 functions of the brain, and the blood vessels which 

 serve to transmit this blood serve also as the 

 avenues for the approach of injurious substances, 

 or substances capable of modifying profoundly 

 the functions of the brain. A physiological example 

 is the carbon dioxide of the blood, an excess of 

 which, carried to the brain, quickly ushers in loss 

 of consciousness. Then there are the bacterial 

 poisons that give rise to fever. These poisons, on 

 coming in contact with a definite part of the brain, 

 cause nervous changes that result in a rise in tempera- 



