COMMON SENSATIONS, 567 



\ibly different pressures have the ratio 1 : 3, with the mus- 

 -cular sense differences of ^ can be perceived. 



Common Sgnsations. Under this name are included the 

 sensations which weocThot mentally attribute to the prop- 

 erties of external objects, but to conditions of our own 

 Bodies; of them we may here consider min, hunger, and 



Pain arises when powerful mechanical or thermal stimuli 

 acton the skin, and when a sensory nerve-trunk (except the 

 optic, auditory, or olfactory) is directly excited. Most 

 commonly we derive the feeling through cutaneous or sub- 

 cutaneous nerves, and hence it has been supposed that pain 

 is not a sensation of distinct modality due to the excitation 

 of special nerve-fibres, but is dependent on excessive excite- 

 ment of ordinary tactile fibres; and pressure or temperature 

 sensations do undoubtedly gradate into painful as the 

 stimuli increase. If this be so, pain is a sort of incoordi- 

 nate or "convulsive" sensation. We shall see in Chapter 

 XXXV. that, when a sensory nerve is normally excited in 

 a decapitated animal, regular purpose-like reflex movements 

 result: but if the stimulus be very powerful, or a nerve- 

 trunk be. directly excited, then inco-ordinate convulsions 

 occur r the afferent impulses radiate farther in the centre 

 and produce a new and useless result. We may suppose 

 something similar to occur in the cutaneous nerves of an 

 animal still possessing sensory brain-centres, if the stimuli 

 acting on the skin are such as to excite the end organs very 

 powerfully, or the sensory fibres directly without the inter- 

 mediation of end organs: that a new sensation should be 

 thus aroused, different from tactile though gradually shad- 

 ing off into them, is a phenomenon comparable with the 

 production of new color sensations by combinations of the 

 fundamental ones. In such case, too, we could understand 

 the difference of kinds of "pain" in a more general sense 

 of the word; muscular cramp, dazzling, and disagreeably 

 shrill or inharmoniously combined tones, might all be 

 looked upon as inco-ordinate sensations, each with a charac- 

 acter of its own determined by the central apparatus ex- 

 cited. 



