482 PHYSIOLOGY 



unit, by the aggregation of a number of which a conscious state is produced. 

 As we have seen, the primitive function of the whole nervous system is 

 reaction. The neural life of an animal is composed of a series of reactions, 

 some simple, some complex, and becoming ever more complicated as we 

 ascend the animal scale. The first reactions of a baby, for instance, will 

 be those by which it procures nourishment and satisfies a need. The earliest 

 event in its dawning consciousness will be, not a sensation of sweetness or 

 of colour, but that of a thing which can satisfy its needs. It will have had 

 to try many gustatory experiments before, out of the sum of its material 

 experiences, it will be able to choose a number of like factors which can 

 be grouped together as ' sweet.' Judgment of quality of sensation involves 

 a power of abstraction and of classifying similar elements in different neural 

 events or reactions and the referring of these elements to the external world. 

 It is very difficult however to divest ourselves of/ the mental standpoint 

 reached as the result of many years' continual trials, successes and failures, 

 and constant care has to be exercised if we are not to fall into the common 

 conception of the ego, the personality or soul, as a sort of sentient god sitting 

 somewhere in the brain or, as Descartes suggested, in the pineal gland, and 

 receiving by means of one part or other of his servile material brain a blue 

 sensation from the eyes, or an auditory impression, or a tactile impression, 

 and then, if he feels so inclined, pressing the stop in a pyramidal cell to let out 

 a voluntary motor response. An elementary unit in psychical life, as in 

 neural life, must be a complete reaction. It is from the reaction and not 

 from the sensation that a constructive psychology will have to be built up. 

 RELATIONSHIP BETWEEN STIMULUS AND SENSATION. Although 

 the sensation is not a reproduction of the stimulus, it is a symbol of the 

 stimulus, and can be used to inform us of events occurring in the world 

 around. Like stimuli, falling on the same end-organ, always evoke like 

 sensations, other conditions being equal. An orderly sequence of 

 sensations may therefore be interpreted as indicating a corresponding 

 orderly sequence of physical occurrences in the world around us. Since 

 our sensations are merely symbols of the physical conditions which give rise 

 to them, it is important to inquire how far they correspond quantitatively 1o 

 differences in the energy of the afferent stimuli, i.e. how alterations in the 

 strength of stimulus will affect the intensity of the resulting sensation. 

 Whatever form of stimulus be applied and whatever sense organs be affected, 

 a certain minimum intensity of stimulus is necessary for it to be effect i\v, 

 i.e. to produce a minimum sensation. This strength, which varies with 

 (lifTerent sense organs, is spoken of as the ' liminal intensity,' or ' threshold 

 value ' of stimulus or sensation respectively. As the strength of the stimulus 

 is increased above this minimal amount the resulting sensation also increases. 

 The change in intensity of sensation does not however continue indefinit el v. 

 When the stimulus is increased to a certain amount the resultant sensation 

 becomes maximal, and a further increase in the stimulus evokes no further 

 increase in sensation. In fact fatigue of the sense organs or recipient 

 centres of the brain rapidly sets in, so that the sensation diminishes even 



