3. The Epicritic critical and discriminating system of cutaneous nerves develops later in phylogenetic history than the pro topathic system, if we may judge from its regeneration after experimental section (as the superficial radial nerve in the arm of Henry Head). The epicritic system responds to light touch; to warmth, between and C. and to coolness, between 26° and 29° C. It enables one to locate the exact point stimulated (tactile localization), to distinguish two or more points of simultaneous contact (tactile discrimination), and to recognize size, shape and form (stereognosis). The connection of the roots of this system with the spinal cord is very loosely segmental, its root-fibers spread widely above and below the segment to which the nerve properly belongs. The activity of the epicritic extero-ceptors exercises a regulating and inhibiting influence over the protopathic system; it abolishes vividness, radiation and false reference and thus greatly reduces the intensity of pain without raising its threshold.
4. The intero-ceptors include the afferent nerves supplying the internal organs—the alimentary, respiratory and genitourinary tracts. Near the external orifices the protopathic and epicritic systems are distributed as in the lips, tongue, throat, larynx, upper part of the oesophagus and lower part of the rectum. The intero-ceptors form the afferent side of a great reflex system; few of their impulses pass over the threshold of consciousness. The work of Hertz on "The Sensibility of the Alimentary
Tract" throws much light on this subject. All internal organs are insensitive to tactile stimuli and, excepting the oesophagus and anal canal, they are insensitive to cold and heat stimuli. The stomach and intestines are responsive to stimulation with alcohol, but not to dilute mineral and organic acids. The intero-ceptors are very responsive to tension; under normal conditions, tension is the only stimulus of the intero-ceptors that gives rise to pain.
These four systems of peripheral neurones carry their proper impulses into the cerebrospinal axis in as many combinations; but, when the second order neurones are reached, the impulses are re-grouped and all of the same variety traverse one specific path, in obedience to Sherrington's law of integration. Then, impulses of light and deep touch flow in one path; pain impulses from the protopathic extero-ceptors, the intero-ceptors and deep systems flow through one path; impulses due to coolness (26° to C.) and to cold (anything below 26° C.) traverse one separate path; impulses produced by warmth to 37° C.) and by high heat (above C.) journey in one special path and localizing impulses from the epicritic and deep mechanisms travel the same specific path.