1. A procedure is devised whereby cutaneous blood (so called capillary blood) from a finger-tip can be obtained for gas analyses without coming in contact with the air.
2. Determination was made of the oxygen content of the arterial, cutaneous, and venous blood respectively from a normal resting individual, the arterial and cutaneous blood showing the same oxygen content (97.5 and 96.6 per cent of the total oxygen-combining power of the blood). Venous blood drawn simultaneously was 75 per cent saturated.
3. Using the fact that there is a maximum value for the oxygen-combining power of the blood, we have shown, without doing arterial puncture, that under different conditions (normal and pathological individuals, resting and after exercise) the cutaneous blood and the arterial blood are almost identical as far as the oxygen content is concerned.
4. We think that we are justified in extending the identity, found between the oxygen content of the arterial and cutaneous blood, to other substances in the blood, for instance sugar, salt, uric acid, etc., and also to the reaction of the blood.
5. We are unable to say whether this identity between cutaneous and arterial blood is always true; for example, in a patient with increased venous pressure.
6. In all the experiments we have discarded the first drop of blood, which in some instances was darker than the rest, and always used at least 2 cc. of cutaneous blood. Whether the same results would be obtained with a very small amount of blood, for instance 0.2 to 0.4 cc., we do not know.
7. The experiments show that unless the perfusion of the skin has been extremely great during the experiment, samples of blood obtained from an incision in the skin (of the finger) cannot represent the true capillary blood. The neutral expression cutaneous blood seems therefore for the present preferable to the term capillary blood for samples of blood obtained by cutaneous incisions.