| 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) |
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| 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) |
| gray-scale ultrasonography | The display of the ultrasound echo amplitude or signal intensity as different shades of gray, improving image quality compared to the obsolete black and white presentation. (05 Mar 2000) |
| gray substance | 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 syndrome | <syndrome> Gray baby syndrome, gray appearance of an infant at birth and during the neonatal period which can be caused by transplacental toxic effects of the drug chloramphenicol taken by the mother during late pregnancy; the syndrome may be fatal. (05 Mar 2000) |
| gray tuber | Layer of gray matter in the hypothalamus that also forms part of the floor of the third ventricle. (12 Dec 1998) |
| gray tubercle | Greater tubercle of humerus, the larger of the two tubercles next to the head of the humerus; it gives attachment to the supraspinatus, infraspinatus, and teres minor muscles. Synonym: tuberculum majus humeri, greater tuberosity of humerus, tuberculum cinereum. (05 Mar 2000) |
| gray wedge | <microscopy> An elongated rectangular pattern whose brightness changes from black through shades of gray to white along its length. In calibration wedges, the brightness may vary linearly or logarithmically in discrete steps. (05 Aug 1998) |
| gray wing | A prominence in the floor of the inferior fovea of the fourth ventricle that overlies the dorsal motor nucleus of the vagus. Synonym: trigonum nervi vagi, ala cinerea, ashen wing, gray wing, trigone of vagus nerve, va'gi eminentia. (05 Mar 2000) |
| middle gray layer of superior colliculus | See: gray layer of superior colliculus. Molecular layer, term applied to any layer of brain tissue that contains few nerve-cell bodies and is composed largely of terminal arborizations of dendrites and axons; notable examples are the superficial layer (first layer) of the cerebral cortex and the molecular layer of cerebellum. Synonym: plexiform layer, stratum moleculare. (05 Mar 2000) |
| pontine gray matter | The massive gray matter filling the basilar pons. The nuclei are of fairly homogeneous architecture and project to the cortex of the contralateral cerebellar hemisphere by way of the middle cerebellar peduncle. Their main afferents come from the entire extent of the cerebral neocortex by way of the longitudinal pontine bundles (corticopontine fibres); thus, the pontine nuclei form a major way-station in the impulse conduction from the cerebral cortex of one hemisphere to the posterior lobe of the opposite cerebellum. Synonym: nuclei pontis, pontine gray matter. (05 Mar 2000) |
| heterotopic gray matter | <radiology> Presence of cortical neurons in an abnormal location secondary to arrest of migrating neuroblasts from ventricular walls to brain surface between 7-24 weeks of gestational age, frequency: 3% of healthy population associated with: agenesis of corpus callosum, aqueductal stenosis, microcephaly, schisencephaly, encephalocele X-ray: single/multiple bilateral subependymal nodules along lateral ventricles Differential diagnosis: subependymal spread of neoplasm, subependymal haemorrhage, vascular malformation, tuberous sclerosis, intraventricular meningioma, neurofibromatosis (12 Dec 1998) |