physical stimulus and the act of sensation lay a multitude of physiological transformations which could not be discovered by introspection.
At the time when we began this work, most writers assumed that each specific quality of sensation arose from stimulation of one particular group of end-organs. The impressions so produced were supposed to be transmitted unchanged to the appropriate cortical centres, where they evoked some single aspect of sensation. Special receptive organs were postulated for tactile, painful and thermal stimuli. With the discovery of the heat-and cold-spots, and with von Frey's further development of the doctrine of punctate sensibility to include touch and pain, a sensory mechanism seemed to have been found capable of satisfying the required conditions.
But our discovery of the functions of deep sensibility at once destroyed this conception of rigid parallelism between peripheral end-organs and receptive centres. For we found that many sensations, usually attributed to "light touch," arose from stimulation of subcutaneous tissues, when the skin was entirely insensitive. Pain also could be evoked in the absence of all cutaneous sensibility.
Obviously both sensations of "touch" and of "pain" could be caused by the excitation of at least two peripheral mechanisms apiece. We were not, therefore, surprised to find that in the skin itself the sensory apparatus for heat is also double; the "heat-spots" respond, it is true, in a specific manner, but they account for one aspect of thermal sensibility only. An appreciation of minor differences in warmth and the power of adaptation to surrounding temperatures are functions of a higher afferent mechanism.
It has long been known that the cold-spots in the skin react to certain degrees of heat; 45° C. applied strictly to one of these spots produces a definite sensation of cold. But if the same stimulus is applied over a wider area, so as to include other end-organs of a different specific reaction, the sensation is one of heat; impulses evoked by exciting the cold-spots are inhibited in the presence of those due to coincident stimulation of the receptive mechanism for heat.
Evidently the afferent impressions produced by the action on the body of some physical force, such as heat, are not only multiform, but may be incompatible with one another. Before they can underlie a single specific aspect of sensation, they must undergo integration within the central nervous system.
Human sense organs have been developed out of the lowliest materials; their functions do not correspond exactly to any of the final categories of sensation, which are the result of innumerable physiological transformations. These changes we have attempted to follow from the periphery to the highest receptive centres. They are of entrancing interest, because they reveal the method by which the sensory functions of man have been evolved from the primitive neural activities of his humbler ancestors.
We believe that "Sensation" was originally a vague undifferentiated state,
