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One of us (Irving S. Cutter) has been interested for many years in the production of blood extravasations such as petechiae by trauma. We are all aware that the amount of force or injury necessary to produce such hemorrhagic extravasations varies greatly in different individuals. Investigation of such a problem naturally led us to a study of the blood vessels of the skin and specifically of the capillaries. Using a Muller Weiss skin microscope (Zeiss) we first familiarized ourselves with the technic of capillary observations. The end of the nail bed is by far the most satisfactory location for observation. This, because there is a single row of capillary loops extending towards the finger tip, with no confusing venous network underlying. In this area one is able to follow the arterial and venous sides of the capillary loops for quite a distance and usually to see the blood flow. It was our intention to produce ruptures of the capillary walls by means of measured negative pressure—thus measuring the capillary fragility. Method of Experimentation. The fourth finger of the left hand was used in all cases with the patient sitting and the hand on a level with the heart. The nail bed was lightly cleansed with a 3 to 5% salicylic acid solution, and the finger then inserted into a glass finger holder which is attached to the negative pressure apparatus. The column of mercury is then slowly raised. The finger is removed every few millimeters of mercury to look for signs of capillary rupture.
Cutter et al. (Sat,) studied this question.