Mechanism of Visceral Pain EXPERIMENTAL morphologists will be interested in the two Lettsomian Lectures on the mechanism of visceral pain delivered by Henry Cohen of Liverpool before the Medical Society of London (see The Lancet, 764; June 10, 1944). In the first lecture, Prof. Cohen surveyed and criticized the clinical and experimental data available at present; in the second he discussed the hypotheses which have been formulated to explain why viscera which are normally insensitive to painful stimuli will often reveal themselves by pain in states of disease. None of these hypotheses satisfies Prof. Cohen. It seems necessary, he believes, to postulate a constant stream of subthreshold impulses from the end organs of all pain fibres to the central nervous system, which do not enter central subconsciousness unless central inhibition is lowered. Even an increase in these may not pass the threshold unless it is reinforced by impulses from somatic structures within the same somatic innervation, and vice versa. The metamere, being the phylogenetic unit, renders it unlikely that impulses from any structure within it will reach closely related areas in the sensorium. When any pain impulses, or the sum of them all, rises above the threshold, pain is localized in the segment. If the somatic component of these impulses is cut off, a much stronger stimulus is required in the viscus if pain is to be experienced, and it may then be felt, not only in the segment, but also in the anæsthetized part.