Mice can be vitally stained with many of the phthalein indicators. The staining is diffuse, appearing to interfere not at all with health in the case of the majority of the dyes. The color phenomena show that these retain the character of indicators. A special technique has been evolved for the determination of the hues of the various organs, which are readily modified by extraneous influences. The ability to recognize that the pH has thus been altered is a signal advantage of the indicator method.

Phthaleins of slightly alkaline range or one that trenches slightly on acidity have been employed for the work here reported. Cresol red, phenol red, and brom phenol red have proved especially useful. The observations with the three agree closely in pointing to the existence of notable differences between the reaction of the blood and that within the tissues generally.

The hue of blood plasma from the right heart is such as to suggest that its reaction lies at about pH 7.38 ordinarily, whereas that of the most alkaline of the tissues, judging from its color, the connective tissue, would appear to have a pH of 7.2 or slightly less. The tendons seem to be nearly but not quite so alkaline. The other stained tissues without exception, are of a hue which would indicate that the reaction lies beyond the range of phenol red on the acid side, that is to say is at least as acid as pH 6.6. In a subsequent paper observations which accord with these findings, carried out with indicators of frankly acid range, will be described.

On the exposure of tissues to air, without disturbance of the circulation, some of them become alkaline. In the case of connective tissue, at least, the change is a consequence of the escape of carbon dioxide. The gas passes readily in and out, exerting a practically immediate influence on the color of the tissue bared by eversion of a skin flap; and so much may be absorbed on exposure to pure carbon dioxide, when the surface is large, that a general acidosis results.

The precise interpretation of the color changes in terms of pH waits necessarily upon further work.

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