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I shall then adopt this as the criterion by which to circumscribe the subject - matter of this work so far as action enters into it . Many nervous performances will therefore be unmentioned , as being purely physiological . Nor will the anatomy of the nervous system and organs of sense be described anew . The reader will find in H . N . Martin's ' Human Body , ' in G . T . Ladd's ' Physiological Psychology , ' and in all the other standard Anatomies and Physiologies , a mass of information which we must regard as preliminary and take for granted in the present work . [ 3 ] Of the functions of the cerebral hemispheres , however , since they directly subserve consciousness , it will be well to give some little account . [ 1 ] Cf . Geo . T . Ladd : Elements of Physiological Psychology ( 1887 ) , pt . iii , chap . iii , § § 9 , 12 . [ 2 ] Zur Analysis der Wirklichkeit , p . 489 . [ 3 ] Nothing is easier than to familiarize one's self with the mammalian brain . Get a sheep's head , a small saw , chisel , scalpel and forceps ( all three can best be had from a surgical - instrument maker ) , and unravel its parts either by the aid of a human dissecting book , such as Holden's ' Manual of Anatomy , ' or by the specific directions ad hoc given in such books as Foster and Langley's ' Practical Physiology ' ( Macmillan ) or Morrell's ' Comparative Anatomy and Dissection of Mammalia ' ( Longmans ) . [ Pg 12 ] CHAPTER II . THE FUNCTIONS OF THE BRAIN . If I begin chopping the foot of a tree , its branches are unmoved by 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 - operation with every other . The afferent nerves , when excited by some physical irritant , be this as gross in its mode of operation as a chopping axe or as subtle as the waves of light , conveys the excitement to the nervous centres . The commotion set up in the centres does not stop there , but discharges 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 ' All 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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