THE SOMATIC, VISCERAL AND LABYRINTHINE SENSES
The modal senses of touch, pressure, warmth, cold, pain and tickle have bipolar nerve cells as receptors. The cell bodies of these receptors lie either in the spinal ganglia, close to the spinal cord, or in certain ganglia of the head, adjacent to the brain stem; and the dendrites of the receptors terminate in the skin, the subcutaneous tissues, and the mucous membrane of the mouth, the upper part of the gullet, and the anus.
Each of these senses apparently functions for sense data of one quality only, just as does the auditory sense.
The physiological separation of the tactual, rhigotic, thalpotic and algetic senses is sharply shown by certain cases of disease or injury of the spinal cord or brain stem. In some such cases, dermal areas are found which are totally insensitive to pain, although sensitive in the other three modes. In other cases, thalpotic sensitivity alone is lost. In other cases rhigotic sensitivity alone may be lost. In various cases, dermal areas are found in which two forms of sensitivity are lost, and the other two retained.
The adequate stimulus for tickle is very light contact with the skin (or mucous membrane) or with hairs, and is intensified by movement of the contact over the surface. The term tickle is here applied to sense data of a distinct quality, and does not refer to the profound organic processes and experiences which are set up by stimulation of the subcutaneous tissues of the trunk, as by pressure in the region of the ribs or abdomen.
The reaction set up by the tickle-stimulus may be violent as compared with the stimulus intensity, and usually provokes the movement of rubbing the part of the skin tickled. It has been