The core, quintessence, or most essential part of something.

the bundle of nerve fibers and associated tissue enclosed in the spine that connects the body to the brain

tooth pulp

poliomyelitis

myelin sheath (membrane surrounding axon of nerve cell)

(anatomy) medulla oblongata

the brain as an organ or the seat of thought and intellect

myelitis; inflammation of the spinal cord

a cryptocrystalline form of silica, often used as a gemstone

the essential character or core spirit of something, such as an art form or a piece of literature

red bone marrow; hematopoietic marrow

the true essence; the core meaning

penetrating to the marrow; deeply affected or moved to the core; thoroughly ingrained

(mineralogy) chrysoprase

myelodysplastic syndrome (MDS)

(medicine) bone marrow transplant

(anatomy) medullary cavity; marrow cavity of a bone

pith knot (in timber)

the physical substance of the brain or gray matter

myeloperoxidase (also known as MPO)

(idiom) to strike at one's core; to deeply hurt someone

(medicine) multiple myeloma