Bone marrow; the spinal cord.
[Greek muelos, marrow.]
Terms that refer to bone marrow include myeloma, a malignant tumour of it; myelofibrosis, the replacement of bone marrow by fibrous tissue in some diseases; myelocyte, a cell of the bone marrow that develops into a granulocyte, a type of white blood cell; and myelin, a whitish insulating sheath around many nerve fibres, so named because it resembles marrow.
Myelo- came to refer to the spinal cord because of the concentration of grey matter and nerve fibres associated with it. Examples include myelitis, inflammation of the spinal cord; myelopathy (Greek patheia, suffering), a disease of it; and myelography, x-ray examination of the spinal canal inside the cord by injecting a radio-opaque substance into it.
The adjective myeloid can refer either to bone marrow or to the spinal cord.