| locum tenens | Synonym: locum tenant. Origin: L. One holding a place (05 Mar 2000) |
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| locus | <genetics> The site in a linkage map or on a chromosome where the gene for a particular trait is located. Any one of the alleles of a gene may be present at this site. (18 Nov 1997) |
| locus ceruleus | A shallow depression, of a blue colour in the fresh brain, lying laterally in the most rostral portion of the rhomboidal fossa near the cerebral aqueduct; it lies near the lateral wall of the fourth ventricle and consists of about 20,000 melanin-pigmented neuronal cell bodies whose norepinephrine-containing axons have a remarkably wide distribution in the cerebellum as well as in the hypothalamus and cerebral cortex. Synonym: substantia ferruginea, locus cinereus, locus ferrugineus. (05 Mar 2000) |
| locus cinereus | A shallow depression, of a blue colour in the fresh brain, lying laterally in the most rostral portion of the rhomboidal fossa near the cerebral aqueduct; it lies near the lateral wall of the fourth ventricle and consists of about 20,000 melanin-pigmented neuronal cell bodies whose norepinephrine-containing axons have a remarkably wide distribution in the cerebellum as well as in the hypothalamus and cerebral cortex. Synonym: substantia ferruginea, locus cinereus, locus ferrugineus. (05 Mar 2000) |
| locus coeruleus | Bluish region in the superior angle of the fourth ventricle floor, corresponding to melanin-like pigmented nerve cells which lie lateral to the ponto-mesencephalic central gray (griseum centrale). It is also known as nucleus pigmentosus pontis, locus cinereus, and locus ferrugineus. (12 Dec 1998) |
| locus control region | A regulatory region first identified in the human beta-globin locus but subsequently found in other loci. The region is believed to regulate transcription by opening and remodeling chromatin structure. It may also have enhancer activity. (12 Dec 1998) |
| locus ferrugineus | A shallow depression, of a blue colour in the fresh brain, lying laterally in the most rostral portion of the rhomboidal fossa near the cerebral aqueduct; it lies near the lateral wall of the fourth ventricle and consists of about 20,000 melanin-pigmented neuronal cell bodies whose norepinephrine-containing axons have a remarkably wide distribution in the cerebellum as well as in the hypothalamus and cerebral cortex. Synonym: substantia ferruginea, locus cinereus, locus ferrugineus. (05 Mar 2000) |
| locus minoris resistentiae | A place of less resistance, in latin. For example, a damaged heart valve may act as a locus minoris resistentiae where bacteria released into the blood stream (bacteraemia) tend to settle. (12 Dec 1998) |
| locus niger | A large cell mass, crescentic on transverse section, extending forward over the dorsal surface of the crus cerebri from the rostral border of the pons into the subthalamic region; it is composed of a dorsal stratum of closely spaced pigmented (i.e., melanin-containing) cells, the pars compacta, and a larger ventral region of widely scattered cells, the pars reticulata; the pars compacta in particular includes numerous cells that project forward to the striatum (caudate nucleus and putamen) and contain dopamine, which acts as the transmitter substance at their synaptic endings; other, apparently non-dopaminergic cells of the substantia nigra project to a rostral part of the ventral nucleus of thalmus, the middle layers of the superior colliculus and to restricted parts of the reticular formation of the midbrain; the nigrostriatal projection is reciprocated by a massive striatonigral fibre system with multiple neurotransmitters, chief among which is gamma-aminobutyric acid (GABA); substantia n. Receives smaller afferent projections from the subthalamic nucleus, the lateral segment of the globus pallidus, the dorsal nucleus of the raphe and the pedunculopontine nucleus of the midbrain. The pars reticulata forms part of the output system for the striate body. The substantia n. Is involved in the metabolic disturbances associated with Parkinson's disease and Huntington's disease. Synonym: locus niger, nucleus niger, Soemmerring's ganglion. (05 Mar 2000) |
| locus of control | A theoretical construct designed to assess a person's perceived control over his/her own behaviour; classified as internal if the person feels in control of events, external if others are perceived to have that control. (05 Mar 2000) |
| locus perforatus anticus | A region at the base of the brain through which numerous small branches of the anterior and middle cerebral arteries (lenticulostriate arteries) enter the depth of the cerebral hemisphere; it is bordered medially by the optic chasm and anterior half of the optic tract, rostrally and laterally by the lateral olfactory stria; its anteromedial part corresponds to the olfactory tubercle. Synonym: substantia perforata anterior, locus perforatus anticus, olfactory area. (05 Mar 2000) |
| locus perforatus posticus | The bottom of the interpeduncular fossa at the base of the midbrain, extending from the anterior border of the pons forward to the mamillary bodies, and containing numerous openings for the passage of perforating branches of the posterior cerebral arteries. Synonym: substantia perforata posterior, locus perforatus posticus, Malacarne's space. (05 Mar 2000) |
| locust | 1. <zoology> Any one of numerous species of long-winged, migratory, orthopterous insects, of the family Acrididae, allied to the grasshoppers; especially, (Edipoda, or Pachytylus, migratoria, and Acridium perigrinum, of Southern Europe, Asia, and Africa. In the United States the related species with similar habits are usually called grasshoppers. See Grasshopper. These insects are at times so numerous in Africa and the south of Asia as to devour every green thing; and when they migrate, they fly in an immense cloud. In the United States the harvest flies are improperly called locusts. See Cicada. Locust beetle, an African bird; the beefeater. 2. [Etymol. Uncertain. <botany> The locust tree. See Locust Tree (definition, note, and phrases). <botany> Locust bean, a commercial name for the sweet pod of the carob tree. Origin: L. Locusta locust, grasshopper. Cf. Lobster. Source: Websters Dictionary (01 Mar 1998) |
| locust gum | <botany> The Carob, a leguminous tree of the Mediterranean region; also, its edible beans or pods, called St. John's bread. The Honey mesquite (Prosopis juliflora), a small tree found from California to Buenos Ayres; also, its sweet, pulpy pods. A valuable gum, resembling gum arabic, is collected from the tree in Texas and Mexico. Origin: Sp. Algarroba, fr. Ar. Al-kharrbah. Cf. Carob. Source: Websters Dictionary (01 Mar 1998) |
| locust tree | <botany> A large North American tree of the genus Robinia (R. Pseudacacia), producing large slender racemes of white, fragrant, papilionaceous flowers, and often cultivated as an ornamental tree. In England it is called acacia. The name is also applied to other trees of different genera, especially to those of the genus Hymenaea, of which H. Courbaril is a lofty, spreading tree of South America; also to the carob tree (Ceratonia siliqua), a tree growing in the Mediterranean region. <botany> Honey locust tree, a small swamp tree (Gleditschia monosperma), of the Southern United States. Origin: Etymol. Uncertain. Source: Websters Dictionary (01 Mar 1998) |