Deep part of an organ. Fatty, whitish or yellowish substance found inside some animal bones. Inner part of the roots and stems of flowering plants, consisting mainly of parenchymal tissue and surrounded by bundles of woody and sieve vessels. Main substance of a non-material thing. The spinal cord is the portion of the central nervous system housed in the spinal canal. It extends from the foramen magnum to the level where the first lumbar vertebra articulates with the second. It has gray matter in the center and white matter on the periphery.

In the gray matter they are distinguished. The anterior and posterior horns and the intermediate gray matter, traversed by the ependymal duct. The white matter is divided into three spinal cords: anterior, lateral and posterior, the cords in turn are formed by bundles, some sensitive and others motor. Sensory impulses reach the spinal cord through the posterior roots, and motor impulses leave the spinal cord through the anterior roots. The integrative activity of the spinal cord corresponds to the spinal reflexes. There is also the bone marrow, which is found inside the bones and is of two types: the one that is filled in the diaphyseal canal, fatty and yellowish in color (fatty or yellow bone marrow) and the one that fills the spaces of the spongy bones (the short bones and the epiphysis of the long ones), which is red and is responsible for manufacturing the myeloid series of blood cells (red blood cells and leukocytes). The renal medulla is the internal part of the kidney that is pale in color and can be distinguished in a cut from the external part or cortex that has a red-brownish color.

It is made up of chronic-looking units called medullary pyramids, their number in the human kidney oscillating between 12 and 18. It contains loops of Heeln, collecting tubules, straight vessels and abundant intestinal tissue, it receives 100% of the blood flow and its main function is concentration and dilution of urine. Last but not least, there is the adrenal medulla, which is found inside the adrenal gland, it is surrounded by the adrenal cortex and formed by cells derived from sympathoblasts, which secrete adrenaline and noradrenaline, hormones that have sympathomimetric action (similar to the sympatho ).