A soft and fatty vascular tissue that fills the interior cavities of most bones. There are two kinds of marrow, yellow and red. In adults, the long, rounder bones are filled with yellow, or inactive, marrow composed mainly of fat, and the flat bones of the skull, the ribs, the  sternum, and the pelvis contain red, or active, marrow. Red marrow plays an important role in the formation of blood. It yields the oxygen-carrying red blood corpuscles, the important clotting agents called platelets, and a large percentage of white corpuscles, which primarily serve as fighters of infection. As a blood-forming organ, the marrow has a direct effect upon an individual’s health and vigor. Hence, Job (21:24) appropriately alludes to a well-nourished and healthy person under the figure of one whose bone marrow “is being kept moist.”

Animal bone marrow was apparently used for food by the Israelites. (Compare Mic 3:2, 3.) It has a very high nutritional value, being rich in protein, fats, and iron. Jehovah’s banquet for all the peoples, therefore, fittingly includes symbolic “well-oiled dishes filled with marrow.”​—Isa 25:6.

In Hebrews 4:12 “the word of God” is compared to a weapon that is sharper than any two-edged sword and can penetrate the thoughts and motives of an individual, piercing, as it were, clear to the marrow, the innermost part of the bones.