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Medical Dictionary


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leeching (lech′ing)
The former practice of applying leeches to the body to draw blood for therapeutic purposes.

Leede
Carl S., U.S. physician, *1882. See Rumpel-L. sign, Rumpel-L. test, L.-Rumpel phenomenon.

LEEP
Abbreviation for loop electrocautery excision procedure; loop electrosurgical excision procedure.

Leeuwenhoek
Anton van, Dutch microscopist, 1632–1723. See L. canals, under canal.

Lefèvre
Paul, 20th century French dermatologist. See Papillon-L. syndrome.

Le Fort
Léon C., French surgeon and gynecologist, 1829–1893. See L. I fracture, L. II fracture, L. III fracture, L. sound, L. amputation.

left-footed
SYN: sinistropedal.

left-handed
Denoting the habitual or more skillful use of the left hand for writing and for most manual operations. SYN: sinistromanual.

left-sidedness
The normal left-sided location of certain unpaired organs, such as the spleen and most of the stomach. bilateral left-sidedness a syndrome in which normally unpaired organs develop more symmetrically in mirror image; two spleens, one on each side, are usually present, and cardiovascular anomalies are common. SYN: polysplenia syndrome.

leg
1. [TA] Anatomically, the segment of the lower limb between the knee and the ankle; commonly used to mean the entire inferior limb. 2. A structure resembling a l.. SYN: crus (1) [TA] . l. of antihelix SYN: crura of antihelix, under crus. bow-l. See genu varum. elephant l. SYN: elephantiasis. restless legs SYN: restless legs syndrome. rider's l. a strain of the adductor muscles of the thigh. tennis l. a rupture of the gastrocnemius muscle at the musculotendinous junction, resulting from forcible contractions of the calf muscles; commonly seen in tennis players.

Legal
Emmo, German physician, 1859–1922. See L. test.

Legendre
Gaston J., French physician, *1887. See L. sign.

Legg
Arthur T., U.S. surgeon, 1874–1939. See L.-Calvé-Perthes disease.

-legia
Reading, as distinguished from the G. derivatives, -lexis and -lexy, which signify speech. [L. lego, to read]

Legionella (le-ju-nel′la)
A genus of aerobic, motile, nonacid-fast, nonencapsulated, Gram-negative bacilli (family Legionellaceae) that have a nonfermentative metabolism and require l-cysteine HCl and iron salts for growth; they are water-dwelling, airborne-spread, and pathogenic for humans. Over 40 species have been identified; the type species is L. pneumophila. L. bozemanii a bacterial species that causes human pneumonia. L. dumoffii a bacterial species implicated in pneumonia. L. feeleii a bacterial species implicated in pneumonia. L. gormanii a bacterial species implicated in pneumonia. L. longbeachae a bacterial species implicated in pneumonia. L. micdadei a bacterial species that may be acid-fast, and that causes Pittsburgh pneumonia, a variant of Legionnaires disease. Accounts for approximately 60% of L. pneumonias other than those caused by L. pneumophila. SYN: Pittsburgh pneumonia agent. L. pneumophila a bacterial species that is the primary etiologic agent of Legionnaires disease; believed to grow in plumbing systems or in standing water in ventilation systems. The type species of the genus L.. L. wadsworthii a bacterial species implicated in pneumonia.

legionellosis (le-ju-nel-o′sis)
SYN: Legionnaires disease.

legumin (le-goo′min, leg′oo-min)
SYN: avenin.

leguminivorous (le-goo-mi-niv′o-rus)
Feeding on beans, peas, and other legumes.

Lehmann
J.O. Orla, Swedish physician, *1927. See Börjeson-Forssman-L. syndrome.

Leigh
Denis, British psychiatrist, *1915. See L. disease.

Leiner
Karl, Austrian pediatrician, 1871–1930. See L. disease.

leio-
Smooth. [G. leios]

leiomyofibroma (li-o-mi′o-fi-bro′ma)
SYN: fibroleiomyoma.

leiomyoma (li′o-mi-o′ma)
A benign neoplasm derived from smooth (nonstriated) muscle. [leio- + G. mys, muscle, + -oma, tumor] l. cutis cutaneous eruption of multiple small painful nodules composed of smooth muscle fibers; derived from arrector muscles of hair. Solitary, nonpainful l. cutis may arise from cutaneous blood vessels and genital skin. SYN: dermatomyoma. parasitic l. a uterine l. which has become detached from the uterus and adherent to another peritoneal surface from which it derives a blood supply. vascular l. a markedly vascular l., apparently arising from the smooth muscle of blood vessels. SYN: angioleiomyoma, angiomyofibroma, angiomyoma.

leiomyomatosis (li′o-mi′o-ma-to′sis)
The state of having multiple leiomyomas throughout the body. l. peritonealis disseminata a benign condition characterized by multiple small nodules on abdominal and pelvic peritoneum grossly mimicking disseminated ovarian cancer but with histologic characteristics of benign myoma; often associated with recent pregnancy.

leiomyomectomy (li′o-mi-o-mek′to-me)
Surgical resection of a leiomyoma, usually of the uterus.

leiomyosarcoma (li′o-mi′o-sar-ko′ma)
A malignant neoplasm derived from smooth (nonstriated) muscle. [leio- + myosarcoma]

leiotrichous (li-ot′ri-kus)
Having straight hair. [leio- + G. thrix, hair]

leipo-
See lipo-.

Leipzig yellow [C.I. 77600]
SYN: chrome yellow.

Leishman
Sir William B., Scottish surgeon, 1865–1926. See Leishmania, L. chrome cells, under cell, L. stain, L.-Donovan body.

Leishmania (lesh-man′e-a)
A genus of digenetic, asexual, protozoan flagellates (family Trypanosomatidae) that occur as amastigotes in the macrophages of vertebrate hosts, and as promastigotes in invertebrate hosts and in cultures. Species are largely indistinguishable morphologically, but may be separated by clinical manifestations, geographic distribution and epidemiology, developmental patterns of promastigotes in their sandfly hosts, virulence testing of clones in vivo, the effect of test sera on growth in culture, cross-immunity tests, and serotyping with promastigote excreted factors; strains also can be distinguished by various biochemical analyses. Such procedures have identified all of the recognized groups and confirmed the separation of New World leishmaniasis agents into two species complexes, L. mexicana and L. braziliensis. [W. B. Leishman] L. aethiopica an African species of L. responsible for human cutaneous leishmaniasis in Ethiopia, with a reservoir of human infection in the rock hyraxes, Procavia capensis and Heterohyrax brucei, and in Kenya, with reservoirs in the tree hyrax, Dendrohyrax arboreus, and the giant rat, Cricetomys gambianus; vectors are the sandflies Phlebotomus longipes and P. pedifer. It causes a cutaneous leishmaniasis of three types: classical oriental sore, mucocutaneous leishmaniasis, and diffuse cutaneous leishmaniasis; ulceration is late or absent and healing takes one to three years. L. braziliensis a species that is the causal agent of mucocutaneous leishmaniasis, endemic in southern Mexico and Central and South America, and transmitted by various species of Lutzomyia (New World sandflies); forest rodents and other neotropical arboreal animals serve as reservoir hosts. L. braziliensis is currently divided into three clinically, epidemiologically, and biochemically distinct strains or subspecies: L. b. braziliensis, L. b. guyanensis, and L. b. panamensis. L. braziliensis braziliensis the type subspecies of L. braziliensis and the agent of mucocutaneous leishmaniasis. A natural reservoir of infection remains unknown, but the proven vector in Brazil is Lutzomyia (Psychodopygus) wellcomei; other sandflies may also transmit the infection. L. braziliensis guyanensis a subspecies within the L. braziliensis complex from Brazil and Guyana, and the cause of the cutaneous leishmaniasis condition locally known as “pian bois”; the reservoir host in Brazil is the sloth Choloepus hoffmani and the vector is the sandfly Lutzomyia umbratilis. L. braziliensis panamensis a subspecies of L. braziliensis found in Panama, Colombia, and neighboring regions; it causes ulcerating lesions of cutaneous leishmaniasis which do not heal spontaneously and often involve nearby lymphatic tissues, but nasopharyngeal involvement is rare. The sloth Choloepus hoffmani is the reservoir in Panama and Costa Rica; the sandfly Lutzomyia trapidoi has been proven to be a vector. L. donovani a species that is the causal agent of visceral leishmaniasis in Mediterranean and adjacent countries, the south central section of the former USSR, eastern India, northern China, Kenya, Ethiopia, and the Sudan; also found in Brazil, Argentina, Colombia, and Venezuela; in the Old World, it is transmitted by various species of Phlebotomus; New World vectors are species of Lutzomyia; dogs and other carnivores are known as reservoir hosts in some areas. The intracellular amastigote form multiplies in macrophages and produces a reticuloendothelial hyperplasia grossly affecting the spleen and liver, with other lymphoid tissues being involved as well, resulting in severe hepatosplenomegaly, which usually is fatal if untreated. L. donovani archibaldi L. donovani donovani. L. donovani chagasi a subspecies of L. found in South America, chiefly in Brazil, producing visceral leishmaniasis; infections have been found in domestic dogs and in foxes, though the primary reservoir host is unclear. The vector remains undiscovered, and the taxonomic status of this subspecies is uncertain. L. donovani donovani the type subspecies and agent of visceral leishmaniasis in Asia, Africa, and the Indian subcontinent; a few cases occur in the south central section of the former USSR, and in Iran, Iraq, and possibly Yemen; the dog and jackal are animal reservoirs. The form in Africa may be this subspecies, though the name L. donovani archibaldi is also used. L. donovani infantum a strain or subspecies of L. donovani that causes visceral leishmaniasis in young children in Mediterranean countries; the reservoir is the domestic dog. L. furunculosa former name for L. tropica. L. major a species responsible for zoonotic cutaneous leishmaniasis in a large area of the Mediterranean region and Asia Minor. The animal reservoirs are usually ground squirrels, such as Rhombomys opimus in the former USSR and elsewhere in south central Asia, and other rodents in northwest India, the Middle East, and northern Africa; proven sandfly vectors include Phlebotomus papatasi, P. duboscqi, and P. salehi. SYN: L. tropica major. L. mexicana the agent of many forms of cutaneous leishmaniasis, now considered a complex of several subspecies or possibly species, each with distinctive DNA and enzyme characteristics, distribution, and vector-reservoir host association, resulting in distinct manifestations of human leishmaniasis; reservoir hosts are extremely diverse and include a wide array of arboreal rodents as well as marsupials, primates, and small carnivores. Typical disease forms caused by this species are chiclero ulcer and diffuse cutaneous leishmaniasis, in contrast with mucocutaneous leishmaniasis, more characteristic of L. braziliensis infection. SYN: L. tropica mexicana. L. mexicana amazonensis a particularly widespread form of L. mexicana in the Amazon basin (Bolivia, Brazil, Colombia, Ecuador, and southern Venezuela), where it infects a variety of forest rodents, the reservoirs of human infection. The disease is rare in humans, but the single or multiple lesions, when induced, rarely heal spontaneously; the disseminated form is common, but nasopharyngeal involvement does not occur. The vector is the sandfly Lutzomyia flaviscutellata. L. mexicana garnhami a subspecies of L. mexicana, found in western Venezuela, causing single or multiple lesions in humans that heal spontaneously in about six months; the probable sandfly vector is Lutzomyia townsendi. L. mexicana mexicana a species described from Mexico, Guatemala, and Belize; agent of a form of New World cutaneous leishmaniasis called chiclero ulcer, associated with chicle gum and mahogany forest workers. The New World sandfly, Lutzomyia olmeca, is a proven vector of this subspecies. L. mexicana pifanoi a strain of L. mexicana accorded species status by those who consider it responsible for the diffuse or disseminated form of cutaneous leishmaniasis. It is responsible for this condition in Venezuela, where it was described, but it is now recognized that several species and subspecies of L. cause similar disseminated forms of leishmaniasis in widely separated regions (L. mexicana amazonensis, L. aethiopica); absence or suppression of the cell-mediated immune response in the host is also an important factor in induction of diffuse cutaneous leishmaniasis. SYN: L. pifanoi. L. mexicana venezuelensis a recently described subspecies of L. mexicana from Venezuela that causes indolent, nodular, single lesions of cutaneous leishmaniasis to develop, sometimes with curable disseminated cutaneous leishmaniasis; infection has also been found in equines. L. peruviana species of L. found infecting humans in the high Andean valleys of Peru and Bolivia; cause of a distinct form of New World cutaneous leishmaniasis called uta. L. pifanoi SYN: L. mexicana pifanoi. L. tropica species that is the causal agent of anthroponotic cutaneous leishmaniasis; formerly endemic throughout the Mediterranean basin, the Middle East, parts of the southern section of the area formerly known as the USSR and elsewhere in Asia, and also reported from western Africa; it is transmitted by Phlebotomus papatasi, P. sergenti, and related species of sandflies; small rodents such as various ground squirrels serve as reservoir hosts. L. tropica major SYN: L. major. L. tropica mexicana SYN: L. mexicana.

leishmania, pl .leishmaniae (lesh-man′e-a)
A member of the genus L..

leishmaniae
Plural of leishmania.

leishmaniasis (lesh′ma-ni′a-sis)
Infection with a species of Leishmania resulting in a clinically ill-defined group of diseases traditionally divided into four major types: 1) visceral l. (kala azar); 2) Old World cutaneous l.; 3) New World cutaneous l.; 4) mucocutaneous l.. Each is clinically and geographically distinct and each has in recent years been subdivided further into clinical and epidemiological categories. Transmission is by various sandfly species of the genus Phlebotomus or Lutzomyia. See tropical diseases, under disease. SYN: leishmaniosis. acute cutaneous l. SYN: zoonotic cutaneous l.. American l., l. americana SYN: mucocutaneous l.. anergic l. SYN: diffuse cutaneous l.. anthroponotic cutaneous l. a form of Old World cutaneous l., usually with a prolonged incubation period and confined to urban areas. SYN: chronic cutaneous l., dry cutaneous l., urban cutaneous l.. canine l. a mild infection of dogs, usually confined to the muzzle or ears, produced by human disease-causing species of Leishmania; dogs therefore are important reservoirs of human infection, such as with visceral l. in the Mediterranean region. chronic cutaneous l. SYN: anthroponotic cutaneous l.. cutaneous l. infection with promastigotes (leptomonads) of Leishmania tropica and of L. major inoculated into the skin by the bite of an infected sandfly, Phlebotomus (commonly P. papatasi); it is endemic in parts of Asia Minor, northern Africa, and India, and is known by innumerable names, including tropical sores, tropical ulcers, and other indications of locality ( e.g., Aleppo, Baghdad, Delhi, or Jericho boil; Aden ulcer; Biskra button); the ulcer begins as a papule that enlarges to a nodule and then breaks down into an ulcer. Leishmanial cells are seen within histiocytes in hematoxylin and eosin–stained tissue sections. Two distinctive clinical and epidemiological diseases are recognized: the more common and widespread zoonotic rural disease with a moist acute form, caused by L. major, with reservoir rodent hosts, and an urban, anthroponotic, dry, chronic form of l. caused by L. tropica, without a reservoir host, and now largely controlled. See zoonotic cutaneous l., anthroponotic cutaneous l.. SYN: Old World l.. diffuse l. SYN: diffuse cutaneous l.. diffuse cutaneous l. l. caused by several New and Old World species and strains of Leishmania (L. mexicana amazonensis, L. m. pifanoi, possibly L. m. garnhami and L. m. venezuelensis; in Ethiopia, L. aethiopica, and unidentified leishmanial agents in Namibia and Tanzania). The condition is associated with a suppressed cell-mediated immune response, so that the nonulcerating, nonnecrotizing cutaneous lesions can spread widely over the body; great numbers of parasite-filled macrophages are found in the dermal lesions. Healing does not appear to occur unless an acquired cellular hypersensitivity can develop. SYN: anergic l., diffuse l., disseminated cutaneous l., l. tegumentaria diffusa, pseudolepromatous l.. disseminated cutaneous l. SYN: diffuse cutaneous l.. dry cutaneous l. SYN: anthroponotic cutaneous l.. infantile l. visceral l. in infants, from Leishmania donovani infantum. lupoid l. SYN: l. recidivans. mucocutaneous l. a grave disease caused by Leishmania braziliensis braziliensis, endemic in southern Mexico and Central and South America, except for the equatorial region of Chile; the organism does not invade the viscera, and the disease is limited to the skin and mucous membranes, the lesions resembling the sores of cutaneous l. caused by L. mexicana or L. tropica; the chancrous sores heal after a time, but some months or years later, fungating and eroding forms of ulceration may appear on the tongue and buccal or nasal mucosa; many variants of the disease exist, marked by differences in distribution, vector, epidemiology, and pathology, which suggest that it may in fact be caused by a number of closely related etiologic agents. SEE ALSO: espundia. SYN: American l., l. americana, bubas, nasopharyngeal l., New World l.. nasopharyngeal l. SYN: mucocutaneous l.. New World l. SYN: mucocutaneous l.. Old World l. SYN: cutaneous l.. pseudolepromatous l. SYN: diffuse cutaneous l.. l. recidivans a partially healing leishmanial lesion caused by Leishmania tropica and characterized by an extreme form of cellular immune response, intense granuloma production, fibrinoid necrosis without caseation, and frequent development of satellite lesions that continue the production of granulomatous tissue without healing, sometimes over a period of many years; organisms are difficult to demonstrate but can be cultured. SYN: lupoid l.. rural cutaneous l. SYN: zoonotic cutaneous l.. l. tegumentaria diffusa SYN: diffuse cutaneous l.. urban cutaneous l. SYN: anthroponotic cutaneous l.. visceral l. 1. a chronic disease, occurring in India, China, Pakistan, the Mediterranean littoral, the Middle East, South and Central America, Asia, and Africa caused by Leishmania donovani and transmitted by the bite of an appropriate species of sandfly of the genus Phlebotomus or Lutzomyia; the organisms grow and multiply in macrophages, eventually causing them to burst and liberate amastigote parasites which then invade other macrophages; proliferation of macrophages in the bone marrow causes crowding out of erythroid and myeloid elements, resulting in leukopenia, and anemia, splenomegaly, and hepatomegaly which are characteristic, along with enlargement of lymph nodes; fever, fatigue, malaise, and secondary infections also occur; different strains of L. donovani occur; L. infantum in Eurasia, L. chagasi in Latin America. 2. visceral l. caused by Leishmania tropica, cultured from bone marrow aspirates of some military patients following the Gulf War. SYN: Assam fever, black sickness, Burdwan fever, cachectic fever, Dumdum fever, kala azar, tropical splenomegaly. wet cutaneous l. SYN: zoonotic cutaneous l.. zoonotic cutaneous l. a form of cutaneous l. characterized by rural distribution of human cases near infected rodents, particularly communal ground squirrels; characterized by acute rapidly developing dermal lesions that become severely inflamed, with moist necrotizing sores or ulcers that heal in 2–8-months after a 2–4-month incubation period; among nonimmune immigrants, multiple lesions may develop, which heal more slowly and leave disabling or disfiguring scars. A strong delayed hypersensitivity and involvement of immune complexes play a role in necrosis, which is part of the healing process and of the strong specific immunity that follows. SYN: acute cutaneous l., rural cutaneous l., wet cutaneous l..

leishmaniosis (lesh′man-e-o′sis)
SYN: leishmaniasis.

leishmanoid (lesh′ma-noyd)
Resembling leishmaniasis. dermal l. SYN: post-kala azar dermal l.. post-kala azar dermal l. a chronic, progressive, granulomatous, nonulcerating hypopigmented nodular cutaneous eruption that may appear 6 months to 5 years after spontaneous or drug cure of visceral leishmaniasis (kala azar); this condition was first described in India and is most characteristic of kala azar in that country. SYN: dermal l..

Leiter
Russell G., U.S. psychologist, *1901. See L. International Performance Scale.

Lejeune
Jerôme J.L.M., French cytogeneticist, 1926–1994. See L. syndrome.

Lembert
Antoine, French surgeon, 1802–1851. See L. suture, Czerny-L. suture.

lemic (le′mik)
Relating to plague or any epidemic disease. [G. loimos, plague]

Leminorella (lem′in-o-rel′a)
A genus in the family Enterobacteriaceae containing two species, L. grimontii and L. richardii, that have been isolated from clinical material, primarily from fecal samples; its clinical importance is unclear at present.

Lemli
Luc, 20th century U.S. pediatrician. See Smith-L.-Opitz syndrome.

lemmoblast (lem′o-blast)
In an embryo, a cell of neural crest origin capable of forming a cell of the neurilemma sheath. [G. lemma, husk, + blastos, germ]

lemmocyte (lem′o-sit)
One of the cells of the neurolemma. [G. lemma, husk, + kytos, cell]

lemniscus, pl .lemnisci (lem-nis′kus, -nis′i) [TA]
A bundle of nerve fibers ascending from sensory relay nuclei to the thalamus. SYN: fillet (1) . [L. from G. lemniskos, ribbon or fillet] acoustic l. SYN: lateral l.. auditory l. SYN: lateral l.. gustatory l. the uncrossed secondary-sensory fiber system ascending from the rhombencephalic gustatory nucleus to the parabrachial nuclei (rostral pontine level) and directly to the thalamic gustatory nucleus (ventral postero-medial nucleus, pars parvicellularis). lateral l. [TA] a bundle of ascending fibers that originate from the cochlear and auditory relay nuclei of the rhombencephalon, enter the trapezoid body, a transverse fiber stratum in which about half their number decussate, and from here turn rostrally along the lateral side of the spinothalamic tract; in the midbrain, it arches dorsally and enters the inferior colliculus in which all of its fibers terminate; the auditory pathway is transsynaptically extended from here by the brachium of the inferior colliculus to the medial geniculate body of the thalamus, from which in turn the auditory radiation leads to the auditory cortex; intercalated in the trapezoid body and along the ascending trajectory of the l. are several cell groups in which part of the fibers synapse. SYN: l. lateralis [TA] , acoustic l., auditory l., auditory tract, lateral fillet. l. lateralis [TA] SYN: lateral l.. medial l. [TA] a band of white fibers originating from the gracile and cuneate nuclei and decussating in the lower medulla; thence it passes upward through the center of the medulla oblongata, close to the median raphe; on entering the pons it spreads out laterally to form a flat band ascending over the dorsal border of the pontine nuclei; in the mesencephalon it passes over the dorsal border of the substantia nigra and is displaced laterally by the red nucleus; passing medial to the medial geniculate body, the bundle enters and terminates in the ventral posterior nucleus of the thalamus. Throughout their course, the fibers retain a somatotopic order such that those originating from the gracile nucleus and representing the lower extremity lie lateral to those originating in the cuneate nucleus and representing the arm. The medial l. conveys somatic-sensory information involved in tactile discrimination (two-point discrimination), position sense, and vibration sense. SYN: l. medialis [TA] , medial fillet, Reil band (2) , Reil ribbon. l. medialis [TA] SYN: medial l.. spinal l. [TA] SYN: spinothalamic tract. l. spinalis [TA] SYN: spinothalamic tract. trigeminal l. [TA] collective term denoting the fibers ascending from the sensory nuclei of the trigeminus; one such fiber system originates from the spinal trigeminal nucleus and from the principal sensory nucleus, decussates, and ascends as the anterior trigeminothalamic tract [TA] (ventral trigeminothalamic tract [TAalt]) in close association with the medial l. with which it enters the ventrobasal complex to terminate in the ventral posteromedial nucleus; a second, uncrossed, fiber group originating from the principal sensory nucleus that follows an ascending course through central parts of the mesencephalic tegmentum as the posterior trigeminothalamic tract [TA] (dorsal trigeminothalamic tract [TAalt]) also to terminate in the ventral posteromedial nucleus. The trigeminal l. conveys tactile, pain, and temperature impulses from the skin of the face, the mucous membranes of the nasal and oral cavities, and the eye, as well as proprioceptive information from the facial and masticatory muscles. SYN: l. trigeminalis [TA] . l. trigeminalis [TA] SYN: trigeminal l..

lemon (lem′on)
The fruit of Citrus limon (family Rutaceae); a source of citric and ascorbic acid; the freshly expressed juice of the ripe fruit is used as a refrigerant diuretic in fever, in the form of lemonade. SYN: limon. [L. limon]

lemon yellow
SYN: chrome yellow.

Lendrum
A.C., 20th century Scottish pathologist. See L. phloxine-tartrazine stain, Fraser-L. stain for fibrin.

Lenègre
Jean, 20th century French cardiologist. See L. disease, L. syndrome.

length (l)
Linear distance between two points. arch l. the amount of space required for the permanent teeth as measured from the mesial aspect of the first molar on one side to the mesial aspect of the first molar on the opposite side, as measured through the contact points along an imaginary line of the dental arch. available arch l. the amount of space available for the permanent teeth around the dental arch from first permanent molar to first permanent molar. crown-heel l. (CH, CHL) l. of an outstretched embryo or fetus from skull vertex to heel. See Streeter developmental horizon(s). crown-rump l. (CR, CRL) a measurement from the skull vertex to the midpoint between the apices of the buttocks of an embryo or fetus, that permits approximation of embryonic or fetal age. greatest l. measurement from the cranial to caudal end of the embryo prior to folding. required arch l. the sum of the mesiodistal widths of the permanent teeth from first permanent molar to first permanent molar. resting l. the l. at rest from which a muscle develops maximum isometric tension. spinal l. (SL) a measurement from the distal surface of the embryo where the plane passes through the developing eye (this is the cranial limit of the spinal cord) down to the rump.

Lenhossék
Michael (Mihály) von, Hungarian anatomist, 1863–1937. See L. processes, under process.

lenitive (len′i-tiv)
1. Soothing; relieving discomfort or pain. 2. Rarely used term for a demulcent. [L. lenio, pp. lenitus, to soften, fr. lenis, mild]

Lennert
Karl, *1921. See L. lymphoma, L. classification.

Lennox
William G., U.S. neurologist, 1884–1960. See L. syndrome, L.-Gastaut syndrome.

Lenoir
Camille A.H., French anatomist, *1867. See L. facet.

lens (lenz) [TA]
1. A transparent material with one or both surfaces having a concave or convex curve; acts upon electromagnetic energy to cause convergence or divergence of light rays. 2. [TA] The transparent biconvex cellular refractive structure lying between the iris and the vitreous humor, consisting of a soft outer part (cortex) with a denser part (nucleus), and surrounded by a basement membrane (capsule); the anterior surface has a cuboidal epithelium, and at the equator the cells elongate to become l. fibers. SYN: crystalline l.. [L. a lentil] achromatic l. a compound l. made of two or more lenses having different indices of refraction, so correlated as to minimize chromatic aberration. acoustic l. in ultrasonography, a l. used to focus or diverge a sound beam; may be simulated by electronic manipulation of signals. aplanatic l. a l. designed to correct spherical aberration and coma (q.v.). SYN: periscopic meniscus. apochromatic l. a compound l. designed to correct both spherical and chromatic aberrations. aspheric l. a l. with a paraboloidal surface that eliminates spherical aberration. astigmatic l. SYN: cylindrical l.. bandage contact l. a contact l. placed on the cornea to cover a defect. biconcave l. a l. that is concave on two opposing surfaces. SYN: concavoconcave l., double concave l.. biconvex l. a l. with both surfaces convex. SYN: convexoconvex l., double convex l.. bifocal l. a l. used in cases of presbyopia, in which one portion is suited for distant vision, the other for reading and close work in general; the reading addition may be cemented to the l., fused to the front surface, or ground in one-piece form; other bifocal lenses are the flat-top Franklin type, or blended invisible. cataract l. any l. prescribed for aphakia. l. clock SYN: Geneva l. measure. compound l. an optical system of two or more lenses. concave l. a diverging minus power l.. SYN: minus l.. concavoconcave l. SYN: biconcave l.. concavoconvex l. a converging meniscus l. that is concave on one surface and convex on the opposite surface. contact l. a l. that fits over the cornea and sclera or cornea only; used to correct refractive errors. convex l. a converging l.. SYN: plus l.. convexoconcave l. a minus power l. having one surface convex and the opposite surface concave, with the latter having the greater curvature. convexoconvex l. SYN: biconvex l.. corneal l. contact l. of plastic without scleral portions. crystalline l. SYN: l. (2) . cylindrical l. (cyl., C) a l. in which one of the surfaces is curved in one meridian and less curved in the opposite meridian; e.g., a teaspoon or a football. SYN: astigmatic l.. decentered l. a l. so mounted that the visual axis does not pass through the axis of the l.. dislocation of l. SYN: ectopia lentis. double concave l. SYN: biconcave l.. double convex l. SYN: biconvex l.. eye l. the upper of the two planoconvex lenses of Huygens ocular. SYN: ocular l.. field l. the lower of the two planoconvex lenses of Huygens ocular. foldable intraocular l. a l. often made of silicone or an acrylic polymer that may be doubled over for implantation into the eye following cataract removal. Fresnel l. a l. with a surface consisting of a concentric series of zones that duplicate the power of a l. or prism but with less thickness. SYN: lighthouse l.. Hruby l. a non–contact l. mounted on a slitlamp used for evaluating the retina. immersion l. an objective (for a microscope) constructed in such a manner that the lower l. may be moved downward into direct contact with a fluid which is placed on the object being examined; by using a fluid with a refractive index closely similar to that of glass, the loss of light is minimized. lighthouse l. SYN: Fresnel l.. meniscus l. a l. having a spherical concave curve on one side and a spherical convex curve on the other. SYN: articular crescent, articular meniscus, intraarticular cartilage (2) , meniscus articularis, meniscus (1) . minus l. SYN: concave l.. multifocal l. a l. with segments providing two or more powers; commonly, a trifocal l.. ocular l. SYN: eye l.. omnifocal l. a l. for near and distant vision in which the reading portion is a continuously variable curve. orthoscopic l. a spectacle l. corrected for distortion and curvature of the periphery. periscopic l. a l. with 1.25 D base curve. photochromic l. a light-sensitive spectacle l. that reduces light transmission in sunlight and increases transmission in reduced light. planoconcave l. a l. that is flat on one side and concave on the other. planoconvex l. a l. that is flat on one side and convex on the other. plus l. SYN: convex l.. safety l. a l. that meets government specifications of impact resistance; the increased impact resistance required for safety lenses is obtained by tempering, by an ion-exchange process, or by using laminated or plastic lenses. slab-off l. a spectacle l. with a base-up prism below; used in unequal myopia to equalize image displacement when reading. spherical l. (S, sph.) a l. in which all refracting surfaces are spherical. spherocylindrical l. a combined spherical and cylindrical l., one surface being spherical, the other cylindrical. SYN: spherocylinder. toric l. a l. in which both meridians are curved but not to the same degree. trial lenses a series of cylindrical and spherical lenses used in testing vision. trifocal l. a l. with segments of three focal powers: distant, intermediate, and near.

lens clock
See under lens.

lensectomy (len-sek′to-me)
Removal of the lens of the eye by an infusion-aspiration cutter; often done by puncture incision through the pars plana in the course of vitrectomy. [lens + G. ektome, excision]




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