over, pain cannot be produced by such stimuli as the prick of a pin, sufficient to evoke sensations from protopathic parts on the surface of the body. Internal surfaces cannot respond to artificial stimuli, to which they have never been exposed during the life of the individual or the race.
Even if a stimulus is able to evoke impulses from these sheltered parts of defective sensibility, it does not usually produce a sensation, in consequence of the concurrent activity of the sensory organs of the skin. But a sensation may be produced, whenever these visceral impulses become sufficiently strong to overcome this inhibition, or when the central resistance to their passage is in any way lessened. Once the path has been opened, the resistance to potentially painful impulses is lowered, and a weaker visceral stimulus will evoke a sensation. To this diminished resistance is probably due the production of pain by otherwise inadequate stimuli in cases of long-continued visceral irritation.
Since the internal organs are totally devoid of epicritic sensibility, a sensation produced within the visceral area will tend to show the same peculiarities as one evoked from a part supplied with deep and protopatliic sensibility only. If the stimulus consists of pressure or of the movement of muscles, the patient will recognise to some extent its true locality, in proportion as the part is supplied with end-organs from the deep afferent system. When, however, the stimulus evokes pain the sensation will tend to be referred into remote parts.
Now, just as one part of the affected area on H.'s hand seemed to be linked with some other remote portion, so visceral sensory surfaces seem to be closely associated with somatic segmental areas. When pain is evoked, it is not localised in the organ stimulated, but is referred to some area on the surface of the body.
Thus, the retention, on the primary level, of afferent impulses, which if not inhibited, would lead to incorrect localisation, has a protective object. To the normal organism they would be worse than useless, but in disease they underlie widespread pain and uncontrollable muscular reflexes.
The sensory processes discussed in this chapter take place on the physiological level. Psychological analysis fails entirely to disclose the struggle of sensory impulses revealed by our experiment. Integration occurs as impulses pass from the periphery towards the higher centres; the change is a constant one from a complex to a simpler and more specific grouping. Sensation, the final end of the process, assumes forms simpler than any sensory impulses.
We believe that the essential elements exposed by our analysis owe their origin to the developmental history of the nervous system. They reveal the means by which an imperfect organism has struggled towards improved functions and psychical unity.
