The blood viscosity depends on the number of the corpuscular elements pres e n t ?and on the viscosity of the plasma. It is rapidly altered by conditions which concentrate or dilute the fluid of the blood, and is very sensitive to slight variations in the carbon dioxide. Such changes are readily neutralised * in health, but under disease conditions persistent alterations of the blood viscosity frequently occur. It is probable that many of the symptoms found in such cases, may be directly attributable to the altered blood viscosity.

For a proper understanding of the physics of the circulation it is essential to have a knowledge of the part played by blood viscosity}and of the modifications it may undergo in disease.