The one constant factor in the production of cyanosis is diminution of oxygenation. This may be the result of respiratory affections which hinder the access of air or diminish the area of the aerating surface, or it may, on the other hand, be due to circulatory disorders lessening the amount of the blood flowing to the lungs. These latter affections furnish the most conspicuous examples of cyanosis, and in them the symptom is entirely produced by hindrance to the blood flow. Such is in reality the original conception advanced by Morgagni and supported by numerous observers. The other view, apparently suggested by Corvisart, that the condition is due to admixture of arterial and venous blood, is absolutely untenable, as in the vast majority of cases there is no possibility of such intermingling. In general terms the blood in cyanosis may be said to be of high specific gravity—from 1070 to 1080. The amount of hæmoglobin rises considerably—often reaching above 100 per cent. The erythrocytes increase in number so as frequently to exceed 7,500,000 per cubic millimetre, while the leucocytes, although not so commonly altered in number, often reach such a figure as 12,000 per cubic millimetre. The increase of these constituents of the blood was first observed by Toeniessen. Malassez showed that there are more red corpuscles in the blood of the superficial than of the deeper parts of the body. Toeniessen and Penzoldt suggested that this might be due to loss of fluid from the surface, while the interior receives a constant supply of fluid from the alimentary tract.