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The various physical factors concerned in transporting fluid through the human capillary wall have received relatively little attention in spite of the value that such information might have in clarifying the complicated mechanism of fluid balance and the even more involved mechanism of edema formation. The present lack of information is to be ascribed, in all probability, to the difficulty of obtaining quantitative data concerning the filtration and absorption of small amounts of tissue fluid. At present the plethysmograph offers the only available means of detecting small changes in the volume of extravascular fluid.The ordinary plethysmograph was found by Krogh, Landis and Turner (1) to be quite unsuitable for measuring the volume of tissue fluid accumulating during short periods of slight or moderate venous congestion. Therefore, a so-called "pressure plethysmograph " was devised to exclude spontaneous variations in arm volume referable to changing vasomotor tone. Pressure was exerted on the surface of the segment of forearm within the plethysmograph in order to collapse the blood vessels before the final volume was measured. Under these conditions the state of contraction or dilatation of the blood vessels did not, within certain limits, interfere with the reasonably accurate measurement of changes in the volume of tissue fluid.It was found in normal subjects that fluid was filtered into the tissues of the forearm when the venous pressure exceeded 15 cm. water. Above an average venous pressure of 17 cm. water the rate of filtration was directly proportional to the increase in venous pressure. WVhen the subject stood motionless the colloid osmotic pressure of the blood rose and the rate of filtration produced by given grades of venous congestion fell. Thus changes in capillary pressure and in the colloid osmotic pressure of the blood influenced the movement of fluid through the capillary walls of normal human subjects in a manner conforming to the Starling hypothesis.If capillary blood pressure and the colloid osmotic pressure of the blood were the sole factors involved in fluid balance it would be difficult to explain how it is possible for the human being to avoid dependent 105

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Several tests have been proposed, and are now in general use, for estimating the grade of organic occlusion existing in the blood vessels of an extremity. The principle underlying these various methods is similar. The surface temperature of the distal portion of the cool, exposed extremity is measured thermo-electrically. Dilatation of the peripheral vessels is then produced and the coincident rise in surface temperature is recorded. The level to which the temperature rises with complete vasodilatation has been determined in individuals with normal peripheral circulation. If the surface temperature fails to rise to this normal level, the arteries supplying the part are regarded as being unable to dilate due to organic changes in their walls.One of the earliest methods of producing vasodilatation in the extremities in man was suggested by Brown (1) and consists in producing fever by the intravenous injection of typhoid vaccine. Morton and Scott (2) induced vasodilatation in the lower limbs by means of spinal anesthesia, and later proposed the use of general anesthesia, ether, nitrous oxide or ethylene (3), for the same purpose. Injection of peripheral nerves with novocaine has also been used to produce local vasodilatation by White (4), Lewis (5), and Scott and Morton (6). Still more recently Lewis and Pickering (7) studied the vasodilatation in the hands produced by warming the body. The subject was seated in a small hot air chamber with the head and hands projecting into a cool room. Heating the air in the chamber to a temperature of 460 C. or more was followed by a rise in the skin temperature of the hands. They suggest that the method may also be used to produce vasodilatation in the feet.The following report describes the effects which warming one or two extremities exerts on the skin temperature of the other extremities, with special emphasis on the vasodilatation in the feet produced by immersing the forearms in warm water. The mechanism by which this vasodilatation is induced has been studied in normal subjects. The simplicity of the method and apparatus, as well as the constancy of the reaction in normal subjects, suggests that the procedure may have a certain value in the 1019

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