Full context:   These terms only partly correspond to modern medical terminology, in which there is no distinction between black and yellow bile, and phlegm has a very different meaning. It was believed that the humors were the basic substances from which all liquids in the body were made. Robin Fåhræus (1921), a Swedish physician who devised the erythrocyte sedimentation rate, suggested that the four humors were based upon the observation of blood clotting in a transparent container. When blood is drawn in a glass container and left undisturbed for about an hour, four different layers can be seen: a dark clot forms at the bottom (the "black bile"); above the clot is a layer of red blood cells (the "blood"); above this is a whitish layer of white blood cells (the "phlegm"); the top layer is clear yellow serum (the "yellow bile").