| periaqueductal gray | Central gray matter surrounding the cerebral aqueduct in the mesencephalon. Physiologically it is probably involved in rage reactions, the lordosis reflex, feeding responses, bladder tonus, and pain. (12 Dec 1998) |
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| Gray | <radiobiology, unit> The new international system unit (SI unit) of absorbed dose of radiation (Gy), 1 Gy = 1 J kg-1 = 100 rad. (16 Dec 1997) |
| gray cataract | A cataract of gray colour, usually seen in senile, mature, or cortical cataract. (05 Mar 2000) |
| gray collie syndrome | An autosomal recessive, inherited immunodeficiency of gray collie dogs characterised by overwhelming recurrent bacterial infections, bleeding, and coat colour dilution. Synonym: gray collie syndrome. (05 Mar 2000) |
| gray columns | The three somewhat ridge-shaped masses of gray matter (anterior, posterior, and lateral columns) that extend longitudinally through the centre of each lateral half of the spinal cord; in transverse sections these columns appear as gray horns and are therefore commonly called ventral or anterior, dorsal or posterior, and lateral horn, respectively. Synonym: columnae griseae. (05 Mar 2000) |
| gray degeneration | Degeneration of the white substance of the spinal cord, the fibres of which lose their myelin sheaths and become darker in colour. (05 Mar 2000) |
| gray fibres | A fibre having no myelin covering (CNS); a naked axon; in the PNS represented by all axons lying in troughs in a single Schwann cell (Schwann cell unit); a slow conducting fibre. Synonym: gray fibres, nonmedullated fibres, Remak's fibres. (05 Mar 2000) |
| gray hepatisation | The second stage of hepatisation in pneumonia, when the exudate is beginning to degenerate prior to breaking down; the colour is a yellowish gray or mottled. (05 Mar 2000) |
| gray induration | A condition occurring in lungs during and after pneumonic processes in which there is failure of resolution; there is a conspicuous increase in fibrous connective tissue in the walls of the alveoli, and also within the alveoli (e.g., fibrous organization of exudate); in contrast to brown induration, there is usually not a prominent degree of pigmentation, unless chronic passive congestion is also present. (05 Mar 2000) |
| gray infiltration | A term sometimes used for the relatively rapidly formed, semisolid, gray or gray-white exudate (chiefly necrotic cells and remnants of tissue, and macrophages) resulting from unusually acute, overwhelming, diffuse tuberculous infection in the lung. Synonym: gelatinous infiltration. (05 Mar 2000) |
| gray layer of superior colliculus | Term applied to any one of the three major layers of gray matter of the superior colliculus that alternate with layers composed chiefly of nerve fibres: 1) the superficial gray layer of superior colliculus, above the largely white layer of the incoming fibres of the optic tract (optic layer); 2) the middle gray layer of superior colliculus, placed between the optic layer and a more deeply located layer of fibres, the stratum lemnisci; 3) the deep gray layer of superior colliculus, between the stratum lemnisci and the central gray substance surrounding the cerebral aqueduct, and containing the large nerve cells from which most of the colliculus descending connections (tectobulbar, tectopontine, and tectospinal tract) originate. Synonym: stratum griseum colliculi superioris, stratum cinereum colliculi superioris. (05 Mar 2000) |
| gray level histogram | <microscopy> In digital image processing, a histogram that depicts the number of pixels at each gray value. The histogram can be used to measure the areas that have given ranges of gray values or to adjust image contrast by histogram stretching or equalisation. See: Image histogram. (05 Aug 1998) |
| gray matter | The cortex of the brain which contains nerve cell bodies. The gray matter is in contrast to the white matter, the part of the brain that contains myelinated nerve fibres. The gray matter is so named because it in fact appears gray. The white matter is white because that is the colour of myelin, the insulation covering the nerve fibres. In the mysterious affair at styles (1920), agatha christie first quoted the fictional belgian detective hercule poirot in regard to his gray matter: 'this affair must be unravelled from within.' he tapped his forehead. 'these little grey cells. It is up to them as you say over here.' (12 Dec 1998) |
| gray rami communicantes | Short nerves arising from the lateral aspect of the sympathetic trunk conducting nonmyelinated postsynaptic sympathetic nerve fibres from the sympathetic trunk to the initial portions of all 31 pairs of ventral primary rami of spinal nerves for distribution by all parts (including the dorsal primary ramus) of the spinal nerve. The gray rami are the parietal branches of the sympathetic trunks since all postsynaptic fibres to be distributed to the body wall (including limbs) must pass through them. Synonym: communicating branches of sympathetic trunk, communicating rami of sympathetic trunk. (05 Mar 2000) |
| gray scale | <microscopy> The various shades of gray or luminance values in a video picture. As industrial test standards, gray wedges are used with discrete steps incrementing in brightness by factors of ~ 7. 3 (05 Aug 1998) |