Human physiology has given much of its attention to those systems which control in a multicellular organism the essential internal conditions: to respiration as it provides optimal concentrations of carbon dioxide, hydrogen ions and oxygen, to circulation as it maintains adequate blood flowrates and pressures, and to production and loss of energy as they are balanced through a regulatory mechanism for the maintenance of optimal body temperature.For the purpose of analysis, three main components may be distinguished with any regulatory system in physiology:1. Specific sensory-receptor organs register the physical or chemical quantity that is to be regulated. They produce nerve impulses commensurate with the magnitude of this stimulus.2. One or more effector organs act in response to the stimulus. This results in a return of the physical or chemical quantity registered toward the optimal level whereby the stimulus is reduced or abolished at the site of registration and elsewhere.3. A coordinating center in the central nervous system receives the afferent nerve impulses. It produces efferent impulses which initiate or maintain the regulatory action of the effector organs.A physiological control mechanism cannot be considered clarified until its effector organs, center of coordination, and receptor sensory structures have been identified, and until the quantitative relations between causes and effects, that is, between physical or chemical stimuli and physiological responses, have been demonstrated. In this paper an attempt is described to clarify experimentally one of these mechanisms: the so-called "physical heat regulation"* of man. We shall also discuss the registration of temperature and the sense of temperature, -functions no less important than the other senses to survival and performance at various intensities of exertion, on earth with its many climates. Such an attempt begins at the present state of knowledge and requires a brief history of the discoveries made in this field.For the regulation of temperature in experimental animals and man physiologists have detected, at very early times in some instances and more recently in others, the following possible components suited for the sensory, central, or effector assign-

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