PHYSICAL BASIS OF SENSATION 653 



conveys are varied in their character. There are, for 

 instance, the immediate effects of stimulus, whether positive 

 or negative, and also the persistent after-effects of stimuli 

 previously absorbed. The central apparatus, however, is not 

 acted on by these impulses from any single circuit alone, 

 but from many circuits at the same time, the whole resulting 

 in a vague tremor of generalised consciousness. The 

 individual sensation evoked by any particular stimulus bears 

 to the rippling surface of this consciousness the relation of 

 a larger or smaller wave. 



Thus we see that there are two conditions which will 

 contribute to the intensity of the sensation evoked by an 

 individual stimulus. There will be, first to revert to the 

 simile of the galvanometer the enhancement of the con- 

 ductivity of the particular circuit involved ; and, secondly, 

 the suppression of all interference caused by the semi- 

 conscious activity of other circuits. By the action of the 

 will, producing the condition of attention or expectation, 

 the excitability of the receptive or responsive points, and the 

 conductivity of particular channels, may be exalted, while 

 they may be depressed in others by the reverse process of 

 inhibition. The extent to which it is claimed that this 

 power of inhibition may, with practice, be carried, would 

 appear almost unimaginable. I have myself known of an 

 authenticated instance in which the pulsation of the heart was 

 arrested and renewed at will. In India, indeed, it has been 

 held, from very remote times, that such practices are capable 

 of reduction to a science. It is thus believed to be possible 

 that all nervous impulses due to external causes may one by 

 one be inhibited, until the attention is concentrated on a 

 given point, in complete isolation from any interference 

 whatever by the physical organism. Regarding the physical 

 aspects of these processes of inhibition and concentration, 

 more will be said at the end of this chapter. 



There are again other elements calculated to bring about 

 further variations in the sensitiveness of the instrument 

 which lie more or less beyond the control of the observer. 



