MARROW (מֵחַ, H4671, חֵ֫לֶב֒, H2693, μυελός, G3678) is found in the cavities of the bones. It produces blood platelets to aid in blood clotting, red blood cells for carrying oxygen, and white blood cells for combating infection. There are two kinds of bone marrow, red and yellow. Red marrow preponderates in childhood and represents a more active phase of blood cell formation. Yellow marrow, characterized by more fat tissue, is increased in the healthy adult (Ps 63:5). The marrow in adults reverts to red marrow following serious blood loss or body stress. The marrow cavity of long bones ends at the joints so that it is completely surrounded by bone cortex. This clear demarcation is referred to in Hebrews 4:12.