Determination of spleen indices. The ventral surface of the mouse was divided into four equal quadrants progressing from the left rib cage to the right rib cage. The terminus of each quadrant was assigned a splenic index of 1, 2 (median line of the mouse), 3, and 4 (right rib cage) (see solid lines in figure). Representations of spleens with splenic indices of 1 (black), 2 (dark gray), 3 (light gray), and 4 (white) bordered by black are illustrated. The costal margin is drawn to improve clarity.