CHAPTER II. 



THE FUNCTIONS OF THE BRAIN. 



If I begin cliopping the foot of a tree, its branches are 

 unmoved bj my act, and its leaves murmur as peacefully as 

 ever in the wind. If, on the contrary, I do violence to the 

 foot of a fellow-man, the rest of his body instantly responds 

 to the aggression by movements of alarm or defence. The 

 reason of this difference is that the man has a nervous system 

 whilst the tree has none ; and the function of the nervous 

 system is to bring each part into harmonious co-oj)eration 

 with every other. The afferent nerves, when excited by 

 some physical irritant, be this as gross in its mode of oper- 

 ation as a chopping axe or as subtle as the waves of light, 

 conveys the excitement to the nervous centres. The com- 

 motion set up in the centres does not stop there, but dis- 

 charges itself, if at all strong, through the efferent nerves 

 into muscles and glands, exciting movements of the limbs 

 and viscera, or acts of secretion, which vary with the animal, 

 and with the irritant applied. These acts of response have 

 usually the common character of being of service. They 

 ward off the noxious stimulus and support the beneficial 

 one ; whilst if, in itself indifferent, the stimulus be a sign of 

 some distant circumstance of practical importance, the 

 animal's acts are addressed to this circumstance so as to 

 avoid its perils or secure its benefits, as the case may be. 

 To take a common example, if I hear the conductor calling 

 * ±Al aboard ! ' as I enter the depot, my heart first stops, 

 then palpitates, and my legs respond to the air-waves 

 falling on my tympanum by quickening their movements. 

 If I stumble as I run, the sensation of falling provokes a 

 movement of the hands towards the direction of the fall, 

 the effect of which is to shield the body from too sudden a 

 shock. If a cinder enter my eye, its lids close forcibly 

 and a copious flow of tears tends to wash it out. 



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