Blood Circulation in the Heart
Diagram showing the circulation of blood in the heart. Let a represent the right side of the heart, cthe left side, b the lungs, and d the general system of the body. The arrows point in the direction in which the blood flows. In all the shaded part the blood is dark (oxygen-poor), and in the part that is not shaded it is red (oxygen-rich).
Hooker, Worthington First Book in Physiology For the Use of Schools and Families (New York: Sheldon and Company, 1867) 45