A combined finger-plethysmograph and gradient calorimeter has been devised, especially to study cold-induced vasodilation in the finger. By using an electrical heater and a flow-heater it was shown that the thermal lag depends on thermal capacities, conductivities and temperature distribution, so that heat loss and flow are correlated only in the steady state. The actual drop of blood temperature from inflow to outflow was calculated. Results were: a) skin temperature was a better estimate of outflow than bath temperature, b) with general vasodilation, the inflow temperature was close to deep body temperature, but c) with vasoconstriction, digital arterial temperature might be as low as 14°C, and the calculated minimum blood flow (Greenfield) 70% too low. Plethysmography was possible during the hunting reaction in spite of a 30% decrease in venous distensibility. When the subject was cold, blood flow and heat loss, both in constriction and dilation phases, were reduced two or three times, probably due to the increased viscosity of blood. Submitted on August 3, 1959
