2 PHYSIOLOGY CHAP. 



fact intimately associated with special states of consciousness. 

 But very few of the impressions that reach us from the outer 

 world, or from our own body, enter completely into consciousness, 

 because the attention can only be focussed upon a small part of 

 the impressions received. 



Sensations are distinguished, according to the most funda- 

 mental difference in the psycho - physical phenomena which 

 constitute them, as internal and external. Internal sensations 

 tell us of the changes within our body and psychical personality ; 

 external sensations bring us news of the outer world, or the 

 changes occurring therein. 



Internal sensations are always vague, indefinite, and often 

 indefinable in character, even when we are fully aware of them ; 

 often, however, they operate unconsciously and modify our 

 mentality, without being distinctly perceived. Such are the 

 sensations of pain, hunger, thirst, nausea, fatigue, sexual desire, 

 etc. The name coenaestliesia (from KOII/OS, common, ouo-0/p-is, 

 sensation) is often applied to the collective internal sensations 

 aroused in the centres by the excitations that reach them from 

 the viscera, muscles, and surface of the skin. 



External sensations are more exact and definite in character, 

 hence they are also known as "specific sensations." They are 

 frequently converted into perceptions, which are objectified 

 sensations, i.e. sensations referred to an external cause by a 

 psychical act which includes a judgment, even if an unconscious 

 one. Consequently external sensations are the substrate of all 

 our higher mental functions and all our knowledge. 



The physiological neural processes that accompany the sensory 

 phenomena arising out of the activity of the senses are, for the 

 most part, not available to external objective observation ; so 

 that in dealing with the senses the physiologist is compelled to 

 borrow freely from the psychological terminology derived from 

 subjective observation, and the missing physiological analysis of 

 phenomena is to some extent replaced by introspective analysis. 

 The justification of this method depends on the validity of the 

 law of psycho-physical parallelism, which assumes not only that 

 functional relations exist between somatic and psychical processes 

 which is indisputable but also that for each state of con- 

 sciousness and each psychical change there is a corresponding 

 state and change in the concomitant neural process which is 

 not and in the present state of our knowledge cannot be 

 demonstrated. The law of psycho-physical parallelism is thus 

 no axiom, as many have supposed, but is merely an empirical 

 and provisory hypothesis, which enables the physiologist in 

 dealing with the highest functions of the nervous system to 

 remain on the positive ground of controllable laws and phenomena, 

 instead of straying into the region of metaphysics and speculating 



