MENTAL DEVELOPMENT 295 



" Body and Mind." — Although the brain is the organ which 

 stores the recollection of past experiences and the bonds that 

 unite them, thereby enabling the individual to adapt himself 

 to environment, yet strictly speaking the mind is directly de- 

 pendent upon the vital activities and harmonious interactions 

 of all the organs and tissues of the body ; for of what use 

 would the brain be without the peripheral sense organs and 

 the nerves which connect them with the spinal cord and 

 brain ? These are the avenues of intelligence, as was clearly 

 recognised by Aristotle in his famous dictum : " Nihil in 

 intellectu quod non fuerit prius in sensu." But another funda- 

 mental function of the brain besides perception of the external 

 world and its surroundings is the consciousness of the in- 

 dividual's own personality, his appetites and desires, which 

 are due in great part to the organic sensibility of the nerves 

 of the body and internal organs, which without cessation are 

 continually carrying messages to the brain, making us aware 

 of our existence and our needs. The quality of the blood and 

 the presence in it of subtle bio-chemical substances produced by 

 secreting glands and the viscera have a profound influence 

 upon states of consciousness and mental activity. It is the 

 consciousness of feelings connected with the preservation of 

 the individual and the preservation of the species which con- 

 stitutes the fundamental biological source of all vital activity, 

 and is thus poetically expressed by Schiller in the following 

 lines : 



Durch Hunger und durch Liebe, 

 Erhalt sich die Weltgetriebe. 



The mental states concerned with the consciousness of appetites 

 and desires and the control of the instincts and habits asso- 

 ciated with their gratification, the avoidance of pain and the 

 obtaining of pleasure essential for the preservation of the life 

 of the individual and reproduction are the mainspring of 

 human activities, passions, and emotions. 



Plan of a Simple Nervous System. — Let us now consider for 

 a few moments the general plan of a nervous system. 



The nervous system of all animals with a nervous system is 

 constructed on the same plan. As we rise in the zoological 

 scale it consists of more and more complex systems and groups 

 of neurones. A neurone is a nervous unit which consists of 

 a nerve-cell with branching processes ; one process becomes 



