Means have been described for the study of pressure conditions in normal and pathological skin of living human beings and mice. The true pressure in normal skin cannot be measured directly by any of the means hitherto described, because there is insufficient free fluid to make manometric determinations. However, for practical purposes, the intracutaneous pressure has been approximately estimated by introducing into skin exceedingly small amounts of a relatively unabsorbable fluid, a mixture of Locke's solution and a vital dye, and then finding the least pressure required to overcome the resistance of the skin to the passage of this fluid through it at the lowest rate measurable with accuracy by the apparatus at hand. In the present paper measurements of this pressure have been termed the interstitial resistance.

In normal skin the interstitial pressure, as estimated by measurements of the interstitial resistance, is low, slightly less, on the average, than 1.7 cm. of water in the skin of the mouse, and less than 3.1 cm. of water in human skin. It remains unchanged in states of active hyperemia.

In edematous skin the interstitial pressure can be directly measured by determination of the edema fluid pressure. It has been compared with determinations of the interstitial resistance and found to be only 0.5 cm. of water lower in both the mouse and man.

Under the conditions of our experiments, in skin rendered slowly edematous by the introduction of irritant chemicals or their topical application, little rise in pressure took place. On the other hand, in rapidly forming edema of the skin the edema fluid pressure and the intradermal interstitial resistance rose and became great enough to hinder materially the further escape of fluid from the blood vessels. The edema fluid pressure rose in proportion to the rapidity with which the edema formed. When a rapidly formed edema subsided, the edema fluid pressure and interstitial resistance fell, but if inflammation and induration followed later, the interstitial resistance became high again. As these conditions subsided the interstitial resistance fell, at times to normal levels, even in the presence of edema.

In mouse skin injured by squeezing according to a standard procedure, with result in pronounced edema, the intradermal interstitial resistance rose within a few hours to levels of 10 to 15 cm. of water. In those instances in which the injury progressed to induration, the interstitial resistance rose to such high levels that it seemed impossible that fluid could continue to escape from the capillaries. Such a state of affairs may be of great importance in determining whether necrosis follows trauma.

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