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the body substance that consists of nerve cells, or neurons, and of auxiliary cells, or neuroglia. Nerve cells are the primary functional units of nerve tissue. In evolution, nerve tissue arose as the neurons combined to form ganglia; it is not found in organisms with a primitive nervous system whose neurons are scattered among the other cells of the body.
The ganglia of invertebrates are usually only externally bathed in hemolymph, and these nutritional conditions account for the structure of invertebrate ganglia. Thus, the cell bodies, or perikarya, of trophic neurons are arranged along the periphery of a ganglion; the ganglionic center is occupied by a neuropil. The most highly developed invertebrate nerve tissue is found in cephalopods, which have a vascular circulatory system that directly supplies the nerve tissue with blood.
Capillary blood circulation in nerve tissue is especially well represented in the vertebrates. The gray matter, composed of neuropils and the cell bodies of neurons, and the white matter, composed mostly of myelinated nerve fibers, constitute the brain and spinal cord in the vertebrates.
REFERENCESZavarzin, A. A. Ocherki po evoliutsionnoi gistologii nervnoi sistemy. Moscow-Leningrad, 1941.
Peters, A., S. Paley, and H. Webster. Ultrastruktura nervnoi sistemy. Moscow, 1972. (Translated from English.)
Ramón y Cajal, S. Histologie du système nerveux de I’homme et des vertébrés, vols. 1–2. Paris, 1909–11.
D. A. SAKHAROV