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Medical Dictionary


latah (lah′tah)
One of the pathologic startle syndromes. A culture-bound disorder characterized by an exaggerated physical response to being startled or to unexpected suggestion, the subjects involuntarily uttering cries or executing movements in response to command or in imitation of what they hear or see in others. SEE ALSO: jumping disease. [Malay, ticklish]

André, French anatomist, 1877–1947. See L. nerve, L. vein.

latebra (lat′e-bra)
A flask-shaped region in large-yolked eggs extending from the animal pole to a dilated terminal portion near the center of the yolk; it contains the main bulk of the white yolk. [L. hiding place]

latency (la′ten-se)
1. The state of being latent. 2. In conditioning, or other behavioral experiments, the period of apparent inactivity between the time the stimulus is presented and the moment a response occurs. 3. In psychoanalysis, the period of time from approximately age five to puberty.

latent (la′tent)
Not manifest, dormant, but potentially discernible. [L. lateo, pres. p. latens (-ent-), to lie hidden]

laterad (lat′er-ad)
Toward the side. [L. latus, side, + ad, to]

lateral (lat′er-al) [TA]
1. On the side. SYN: lateralis [TA] . 2. Farther from the median or midsagittal plane. SYN: lateralis [TA] . 3. In dentistry, a position either right or left of the midsagittal plane. 4. A radiographic projection made with the film in the sagittal plane; especially, the second view of a chest series. SYN: lateralis [TA] . [L. lateralis, l., fr. latus, side]

lateralis (lat-er-a′lis) [TA]
SYN: lateral (1) , lateral (2) , lateral. [L.]

laterality (lat-er-al′i-te)
Referring to a side of the body or of a structure; specifically, the dominance of one side of the brain or the body. crossed l. right dominance of some members, e.g., arm or leg, and left dominance of other members.

lateralization (lat′er-al-i-za′shun)
The process whereby certain embryological asymmetries of structure (such as the right-side location of the liver and the structure of the great vessels) and function (handedness) are ordained phylogenetically, coded genetically, and realized ontogenetically.

lateriflexion, lateriflection (lat-er-i-flek′shun)
SYN: lateroflexion.

Lateral, to one side. [L. lateralis, lateral, fr. latus, side]

lateroabdominal (lat′er-o-ab-dom′i-nal)
Relating to the sides of the abdomen, to the loins or flanks.

laterodeviation (lat′er-o-de-ve-a′shun)
A bending or a displacement to one side. [latero- + L. devio, to turn aside, fr. via, a way]

lateroduction (lat′er-o-duk′shun)
A drawing to one side; denoting turning of the eyeball away from the midline. SYN: exduction. [latero- + L. duco, pp. ductus, to lead]

lateroflexion, lateroflection (lat′er-o-flek′shun)
A bending or curvature to one side. SYN: lateriflexion, lateriflection. [latero- + L. flecto, pp. flexus, to bend]

lateroposition (lat′er-o-po-zish′un)
A shift to one side.

lateropulsion (lat′er-o-pul′shun)
An involuntary sidewise movement occurring in certain nervous affections. [latero- + L. pello, pp. pulsus, to push, drive]

laterotorsion (lat′er-o-tor′shun)
A twisting to one side; denoting rotation of the eyeball around its anteroposterior axis, so that the top part of the cornea turns away from the sagittal plane. [latero- + L. torsio, a twisting]

laterotrusion (lat′er-o-troo′zhun)
The outward thrust given by the muscles of mastication to the rotating mandibular condyle during movement of the mandible. [latero- + L. trudo, pp. trusus, to thrust]

lateroversion (lat′er-o-ver′shun)
Version to one side or the other, denoting especially a malposition of the uterus. [latero- + L. verto, pp. versus, to turn]

latex (la′teks)
1. An emulsion or suspension produced by some seed plants; it contains suspended microscopic globules of natural rubber. 2. Similar synthetic materials such as polystyrene, polyvinyl chloride, etc. [L. liquid]

lathe (ladh)
A motor-driven machine with a rotating shaft that can be fitted with various types of cutting instruments, grinding stones and polishing wheels; used in finishing and polishing dental appliances.

lathyrism (lath′i-rizm)
A disease occurring in Ethiopia, Algeria, and India, characterized by various nervous manifestations, tremors, spastic paraplegia, and paresthesias; prevalent in districts where vetches, khasari (Lathyrus sativus), and allied species form the main food. Experimentally, a form of bone disease induced in laboratory animals by feeding L. sativus peas, or a principle derived from them, especially β-aminoproprionitrile. SYN: lupinosis. [L. lathyrus, vetch]

lathyrogen (lath′i-ro-jen)
An agent or drug, occurring naturally or used experimentally, that induces lathyrism.

Latin square
A statistical design for experiments that removes from experimental error the variation from two sources that may be identified with the rows and columns of a square. The allocation of experimental treatments is such that each treatment occurs exactly once in each row and column. For example, a design for a 5 × 5 square is as follows:

latitude (la′ti-tood)
The range of light or x-ray exposure acceptable with a given photographic emulsion. See l. film. SYN: digital gray scale, gray scale. [L. latitudo, width, fr. latus, wide]

Latrodectus (lat-ro-dek′tus)
A genus of relatively small spiders, the widow spiders, capable of inflicting highly poisonous, neurotoxic, painful bites; they are responsible, along with Loxosceles species (the brown spiders), for most of the severe reactions from spider envenomation. Medically important species are known from Australia, North and South America, South Africa, and New Zealand. Some venomous species, in addition to L. mactans (the black widow spider), are L. bishopi (the red-legged widow spider), L. euracaviensis, L. geometricus, and L. tredecimguttatus. [L. latro, servant, robber, + G. dektes, a biter] L. mactans the black widow spider, a venomous jet-black spider found in protected dark places; it is especially common in the southern U.S.; the full-grown female (slightly more than 1 cm long) has a brilliant red dumbbell- or hourglass-shaped mark on the ventral aspect of the abdomen, and her bite may be extremely painful, producing a syndrome mimicking an acute abdominal crisis; some deaths, though rare, have been reported, particularly in small children; the male spider lacks the hourglass mark and is not venomous.

Abbreviation for long-acting thyroid stimulator.

lattice (lat′is)
A regular arrangement of units into an array such that a plane passing through two units of a particular type or in a particular interrelationship will pass through an indefinite number of such units; e.g., the atom arrangement in a crystal.

latus, gen. lateris, pl .latera (la′tus, lat′er-is, lat′er-a) [TA]
SYN: flank. [L. side]

Wilhelm, Austrian obstetrician, 1863–1945. See L. cesarean section.

laudable (law′da-bl)
A term from the past used to describe a quality of pus (thick and creamy) that suggested the wound would ultimately heal through granulation process and not be associated with sepsis and death. [L. laudabilis, praiseworthy]

laudanine (law′da-nen)
An isoquinoline alkaloid derived from the mother liquor of morphine; it causes tetanoid convulsions, with action similar to that of strychnine.

laudanosine (law′da-no-sen)
An isoquinoline alkaloid obtained from the mother liquor of morphine; it causes tetanic convulsions.

laudanum (law′da-num)
A tincture containing opium. [G. ledanon, a resinous gum]

Stanislas, French surgeon, 1799–1872. See L. hernia.

Jean B.P.N.R., French surgeon, 1749–1818. See L. ganglion.

Pierre E., French physician, 1856–1914. See L.-Cléret syndrome, L.-Bensaude syndrome.

John Zachariah, British ophthalmologist, 1830–1874. See L.-Moon syndrome.

Johann F., German pharmacologist, 1798–1873. See L. canal.

lauric acid (law′rik)
A fatty acid occurring in spermaceti, in milk, and in laurel, coconut, and palm oils as well as waxes and marine fats. SYN: n-dodecanoic acid.

Charles, English chemist, 1836–1913. See L. violet.

Ernst A., German physician, 1803–1837. See L. canal.

Thomas, German anatomist and surgeon, 1758–1826. See L. ligament.

Lauth violet
SYN: thionine.

Abbreviation for lymphadenopathy-associated virus.

lavage (la-vahzh′)
The washing out of a hollow cavity or organ by copious injections and rejections of fluid. [Fr. from L. lavo, to wash] antral l. irrigation of the maxillary sinus through its natural ostium or through a puncture of the inferior meatus. bronchoalveolar l. (BAL) procedure for analyzing the cellular milieu of the alveoli (including microbiology, types of inflammatory cells) by use of a bronchoscope or other hollow tube through which saline is instilled into distal bronchi and then withdrawn.

Michail D., Russian histologist, 1846–1902. See L. nucleoid.

Laverania (lav-er-a′ne-a)
Old generic name for malaria-causing and other hematozoan protozoa. L. falciparum is a distinctive generic name for Plasmodium falciparum, and is preferred by some who believe that crescentic gametocytes should be the basis for classifying the causal agent of falciparum malaria in a separate genus. See Plasmodium, Haemoproteus. [C. Laveran, Fr. protozoologist and Nobel laureate, 1845–1922]

laveur (la-vur′)
An instrument for irrigation or lavage. [Fr.]

law (law)
1. A principle or rule. 2. A statement of fact detailing a sequence or relation of phenomena that is invariable under given conditions. SEE ALSO: principle, rule, theorem. [A.S. lagu] Alexander l. states that a jerky nystagmus becomes worse when gazing in the direction of the fast component. all or none l. SYN: Bowditch l.. Ångström l. a substance absorbs light of the same wavelength as it emits when luminous. Arndt l. obsolete l. stating that weak stimuli excite physiologic activity, moderately strong ones favor it, strong ones retard it, and very strong ones arrest it. Arrhenius l. SYN: Arrhenius doctrine. laws of association principles formulated by Aristotle to account for the functional relationships between ideas; the l. of contiguity (association) proved most useful to experimental psychologists, culminating in modern studies of respondent conditioning. l. of average localization visceral pain is most accurately localized in the least mobile viscera and least accurately in the most mobile. Avogadro l. equal volumes of gases contain equal numbers of molecules, the conditions of pressure and temperature being the same. SYN: Ampère postulate, Avogadro hypothesis, Avogadro postulate. Baer l. the general organ characteristics found in all members of a group appear earlier in embryogenesis than the special organ characteristics that distinguish specific members of the group; this l. is the predecessor of the recapitulation theory. Baruch l. the effect of any hydriatric procedure is in direct proportion to the difference between the temperature of the water and that of the skin; when the temperature of the water is above or below that of the skin the effect is stimulating; when the two temperatures are the same the effect is sedative. Beer l. the intensity of a color or of a light ray is inversely proportional to the depth of liquid through which it is transmitted; it is concluded that the absorption is dependent upon the number of molecules in the path of the ray. Cf.:Beer-Lambert l.. Beer-Lambert l. the absorbance of light is directly proportional to the thickness of the media through which the light is being transmitted multiplied by the concentration of absorbing chromophore; i.e., A = εbc where A is the absorbance, ε is the molar extinction coefficient, b is the thickness of the solution, and c is the concentration. Behring l. parenteral administration of serum from an immunized person provides a relative, passive immunity to that disease ( i.e., prevents it, or favorably modifies its course) in a previously susceptible person. Bell l. the ventral spinal roots are motor, the dorsal are sensory. SYN: Bell-Magendie l., Magendie l.. Bell-Magendie l. SYN: Bell l.. Bernoulli l. when friction is negligible, the velocity of flow of a gas or fluid through a tube is inversely related to its pressure against the side of the tube; i.e., velocity is greatest and pressure lowest at a point of constriction. SYN: Bernoulli principle, Bernoulli theorem. Berthollet l. salts in solution will always react with each other so as to form a less soluble salt, if possible. biogenetic l., l. of biogenesis SYN: recapitulation theory. Blagden l. the depression of the freezing point of dilute solutions is proportional to the amount of the dissolved substance. Bowditch l. consistently total response to any effective stimulus. SYN: all or none l.. Boyle l. at constant temperature, the volume of a given quantity of gas varies inversely with its absolute pressure. SYN: Mariotte l.. Broadbent l. lesions of the upper segment of the motor tract cause less marked paralysis of muscles that habitually produce bilateral movements than of those that commonly act independently of the opposite side. Bunsen-Roscoe l. in two photochemical reactions, e.g., the darkening of a photographic plate or film, if the products of the intensity of illumination and the time of exposure are equal, the quantities of chemical material undergoing change will be equal; the retina for short periods of exposure obeys this l.. SYN: reciprocity l., Roscoe-Bunsen l.. Charles l. all gases expand equally on heating, namely, 1273.16 of their 0°C volume for every additional degree Celsius. SYN: Gay-Lussac l.. l. of constant numbers in ovulation the number of ova discharged at each ovulation is nearly constant for any given species. l. of contiguity when two ideas or psychologically perceived events have once occurred in close association they are likely to so occur again, the subsequent occurrence of one tending to elicit the other; this l. figures prominently in modern theories of conditioning and learning. l. of contrary innervation SYN: Meltzer l.. Coppet l. solutions having the same freezing point have equal concentrations of dissolved substances. Courvoisier l. painless enlargement of the gallbladder with jaundice is likely to result from carcinoma of the head of the pancreas and not from a stone in the common duct, because in the latter the gallbladder is usually scarred from infection and does not distend. SYN: Courvoisier sign. Dale-Feldberg l. an identical chemical transmitter is liberated at all the functional terminals of a single neuron. Dalton l. each gas in a mixture of gases exerts a pressure proportionate to the percentage of the gas and independent of the presence of the other gases present. SYN: l. of partial pressures. Dalton-Henry l. in dissolving a mixture of gases, a liquid will absorb as much of each gas in the mixture as if that were the only gas dissolved. l. of definite proportions the relative weights of the several elements forming a chemical compound are invariable. SYN: Proust l.. l. of denervation when a structure is denervated, its irritability to certain chemical agents is increased; e.g., the greater sensitivity of the pupil to acetylcholine after section and degeneration of the third nerve, and of the nictitating membrane to adrenaline after excision of the superior cervical ganglion. Descartes l. SYN: l. of refraction. Donders l. the rotation of the eyeball is determined by the distance of the object from the median plane and the line of the horizon. Draper l. a chemical change is produced in a photochemical substance only by those light rays that are absorbed by that substance. Du Bois-Reymond l. SYN: l. of excitation. Dulong-Petit l. the specific heats of many solid elements are inversely proportional to their atomic weights. Einthoven l. in the electrocardiogram the potential of any wave or complex in lead II is equal to the sum of its potentials in leads I and III. SYN: Einthoven equation. Elliott l. adrenaline acts upon those structures innervated by sympathetic nerve fibers. l. of excitation a motor nerve responds, not to the absolute value, but to the alteration of value from moment to moment, of the electric current; i.e., rate of change of intensity of the current is a factor in determining its effectiveness. SYN: Du Bois-Reymond l.. Faraday laws 1. the amount of an electrolyte decomposed by an electric current is proportional to the amount of the current; 2. when the same current is passed through several electrolytes, the amounts of the different substances decomposed are proportional to their chemical equivalents. Farr laws a set of mathematical formulae, axioms, and laws first enunciated in the annual reports submitted by William Farr to the Registrar General of England and Wales from 1839 to 1883. The laws deal with the relationship of incidence to prevalence, the natural history of epidemics, and mathematical features of common types of epidemic. [Derived from the writings of William Farr, British medical statistician] Fechner-Weber l. SYN: Weber-Fechner l.. Ferry-Porter l. the critical fusion is directly proportional to the logarithm of the light intensity. Fick laws of diffusion 1. the direction of movement of solutes by diffusion is always from a higher to a lower concentration and the diffusive flux JA of solute A across a plane at x is proportional to the concentration gradient of A at x; i.e., JA = −D(CA/x); 2. the increase of concentration of solute A with time, CA/t, is directly proportional to the change in the concentration gradient, i.e., CA/t = D(fl2/x2). Flatau l. a l. concerning the excentric position of the long spinal tracts; the greater the distance the nerve fibers run lengthwise in the cord, the more they tend to be situated toward its periphery. Galton l. in a population mating at random, the progeny of a parent with an extreme value for a measurable phenotype will tend on average to have values nearer the population mean than in the extreme parent. SEE ALSO: l. of regression to mean. SYN: l. of regression to mean. Gay-Lussac l. SYN: Charles l.. Godélier l. tuberculosis of the peritoneum is always associated with tuberculosis of the pleura on one or both sides. Gompertz l. the proportional relationship of mortality to age; after age 35–40, the increase in mortality with age tends to be logarithmic. Graham l. the relative rapidity of diffusion of two gases varies inversely as the square root of their densities, i.e., their molecular weights. Grasset l. SYN: Landouzy-Grasset l.. l. of gravitation SYN: Newton l.. Guldberg-Waage l. SYN: l. of mass action. Haeckel l. SYN: recapitulation theory. Halsted l. transplanted tissue will grow only if there is a lack of that tissue in the host. Hardy-Weinberg l. if mating occurs at random with respect to any one autosomal locus in a population in which the gene frequencies are equal in the two sexes, and the factors tending to change gene frequencies (mutation, differential selection, migration) are either absent or negligible, then in one generation the probabilities of all possible genotypes will on average equal the same proportions as if the genes were assembled at random. The l. does not apply to two or more loci jointly, nor to X-linked traits where the initial gene frequencies differ in the two sexes. l. of the heart the energy liberated by the heart when it contracts is a function of the length of its muscle fibers at the end of diastole. SYN: Starling l.. Heidenhain l. glandular secretion is always accompanied by an alteration in the structure of the gland. Hellin l. twins occur once in 89 births, triplets once in 892, and quadruplets once in 893. If the frequency of twins in a population is p, the frequency of triplets is p2, and the frequency of quadruplets is p3. Henry l. at equilibrium, at a given temperature, the amount of gas dissolved in a given volume of liquid is directly proportional to the partial pressure of that gas in the gas phase (this is only true for gases that do not react chemically with the solvent). Herring l. states that paired agonist muscles from each eye operating in the same field of gaze receive equal innervation while paired antagonist muscles receive equal inhibition. Hess l. the amount of heat generated by a reaction is the same whether the reaction takes place in one step or several steps; i.e., ΔH values (and thus ΔG values) are additive. Hilton l. the nerve supplying a joint supplies also the muscles which move the joint and the skin covering the articular insertion of those muscles. Hooke l. the stress applied to stretch or compress a body is proportional to the strain, or change in length thus produced, so long as the limit of elasticity of the body is not exceeded. l. of independent assortment genes that are not alleles assort independently when the gametes are formed; traits at linked loci are an exception. SYN: Mendel second l.. l. of intestine SYN: myenteric reflex. inverse square l. as applied to point sources, the intensity of radiation diminishes in proportion to the square of the distance from the source. isodynamic l. for energy purposes, the different foodstuffs may replace one another in accordance with their caloric values when burned in a calorimeter. Jackson l. loss of mental functions due to disease retraces in reverse order its evolutionary development. Koch l. SYN: Koch postulates, under postulate. Lambert l. 1. each layer of equal thickness absorbs an equal fraction of the light that traverses it. Cf.:Beer-Lambert l.. 2. the illumination of a surface on which the light falls normally from a point source is inversely proportional to the square of the distance from the source. Landouzy-Grasset l. in lesions of one hemisphere, the patient's head is turned to the side of the affected muscles if there is spasticity and to that of the cerebral lesion if there is paralysis. SYN: Grasset l.. Lapicque l. the chronaxie is inversely proportional to the diameter of an axon. Laplace l. the equilibrium relationship between transmural pressure difference (ΔP), wall tension (T), and radius of curvature (R) in a concave surface; for a sphere: ΔP = 2T/R; for a cylinder: ΔP = T/R. Le Chatelier l. if external factors such as temperature and pressure disturb a system in equilibrium, adjustment occurs in such a way that the effect of the disturbing factors is reduced to a minimum. SYN: Le Chatelier principle. Listing l. when the eye leaves one object and fixes upon another, it revolves about an axis perpendicular to a plane cutting both the former and the present lines of vision. Louis l. tuberculosis in any organ is associated with tuberculosis in the lung. Magendie l. SYN: Bell l.. Marey l. the pulse rate varies inversely with the blood pressure; i.e., the pulse is slow when the pressure is high; an expression of baroreceptor reflex influences on heart rate. Marfan l. the healing of localized tuberculosis protects against subsequent development of pulmonary tuberculosis. Mariotte l. SYN: Boyle l.. mass l. SYN: l. of mass action. l. of mass action the rate of a chemical reaction is proportional to the concentrations of the reacting substances; when the forward reaction rate equals the reverse reaction rate ( i.e., at equilibrium) then, at constant temperature, the product of the concentrations of all the products divided by the product of the concentrations of all the reactants is itself a constant (Keq). SYN: Guldberg-Waage l., mass l.. Meltzer l. all living functions are continually controlled by two opposite forces: augmentation or action on the one hand, and inhibition on the other. SYN: l. of contrary innervation. Mendeléeff l. the properties of elements are periodical functions of their atomic weights; i.e., if the elements are arranged in the order of their atomic weights, every element in the series will be related in respect to its properties to the eighth in order before or after it. SYN: periodic l.. Mendel first l. SYN: l. of segregation. Mendel second l. SYN: l. of independent assortment. l. of the minimum growth and development of plants and animals are determined by the availability of that essential nutrient which is present in the smallest amount. Müller l. each type of sensory nerve ending, however stimulated (electrically, mechanically, etc.), gives rise to its own specific sensation; moreover, each type of sensation depends not upon any special character of the different nerves but upon the part of the brain in which their fibers terminate. SYN: l. of specific nerve energies. l. of multiple proportions SYN: l. of reciprocal proportions. Nasse l. an early statement of the pattern of X-linked recessive inheritance: hemophilia affects only boys but is transmitted through mothers and sisters. Neumann l. in compounds of analogous chemical constitution, the molecular heat, or the product of the specific heat and the atomic weight, is always the same. Newton l. the attractive force between any two bodies is proportional to the product of their masses, and inversely proportional to the square of the distance between their centers. SYN: l. of gravitation. Nysten l. rigor mortis affects first the muscles of the head and spreads toward the feet. Ochoa l. the content of the X-chromosome tends to be phylogenetically conserved. Ohm l. in an electric current passing through a wire, the intensity of the current (I) in amperes equals the electromotive force (E) in volts divided by the resistance (R) in ohms: I = E/R. l. of partial pressures SYN: Dalton l.. Pascal l. fluids at rest transmit pressure equally in every direction. periodic l. SYN: Mendeléeff l.. Pflüger l. SYN: l. of polar excitation. Plateau-Talbot l. when successive light stimuli follow each other sufficiently rapidly to become fused, their apparent brightness is diminished. Poiseuille l. in laminar flow, the volume of a homogeneous fluid passing per unit time through a capillary tube is directly proportional to the pressure difference between its ends and to the fourth power of its internal radius, and inversely proportional to its length and to the viscosity of the fluid. l. of polar excitation a given segment of a nerve is irritated by the development of catelectrotonus and the disappearance of anelectrotonus, but the reverse does not hold; i.e., excitation occurs at the cathode when the circuit is closed and at the anode when it is opened. SYN: Pflüger l.. l. of priority use of the earliest published name (senior synonym) of two or more names of an organism as the correct name. Profeta l. the subject of congenital syphilis is immune to the acquired disease. Proust l. SYN: l. of definite proportions. Raoult l. the vapor pressure of a solution of a nonvolatile nonelectrolyte is that of the pure solvent multiplied by the mole-fraction of the solvent in the solution. l. of recapitulation SYN: recapitulation theory. l. of reciprocal proportions the relative weights in which two substances form a chemical union singly with a third are the same as, or simple multiples of, those in which they unite with each other; a corollary of the l. of definite proportions. SYN: l. of multiple proportions. reciprocity l. SYN: Bunsen-Roscoe l.. l. of referred pain pain arises only from irritation of nerves which are sensitive to those stimuli that produce pain when applied to the surface of the body. l. of refraction for two given media, the sine of the angle of incidence bears a constant relation to the sine of the angle of refraction. SYN: Descartes l., Snell l.. l. of regression to mean SYN: Galton l.. Ribot l. of memory in progressive dementias, remote memories tend to be preserved whereas recent memories are lost. Ricco l. for small images, light intensity × area = constant for the threshold. Roscoe-Bunsen l. SYN: Bunsen-Roscoe l.. Rosenbach l. 1. in affections of the nerve trunks or nerve centers, paralysis of the flexor muscles appears later than that of the extensors; 2. in cases of abnormal stimulation of organs with rhythmical functional periodicity, there is often a grouping of the individual acts with corresponding lengthening of the pauses, in such a way that the proportion of total rest and activity remains nearly the same. Rubner laws of growth 1. the l. of constant energy consumption: the rapidity of growth is proportional to the intensity of the metabolic processes; 2. the l. of the constant growth quotient: in most young mammals, 24% of the entire food energy, or calories, is used for growth; in humans, only 5% is thus used. Schütz l. SYN: Schütz rule. second l. of thermodynamics the entropy of the universe moves toward a maximum; similarly, the entropy of any isolated microcosm ( e.g., a chemical reaction) proceeds spontaneously only in that direction that yields an increase in entropy, entropy being maximal at equilibrium. To quote G.N. Lewis, “Every process that occurs spontaneously is capable of doing work; to reverse any such process requires the expenditure of work from the outside.” l. of segregation factors that affect development retain their individuality from generation to generation, do not become contaminated when mixed in a hybrid, and become sorted out from one another when the next generation of gametes is formed. SYN: Mendel first l.. Sherrington l. every dorsal spinal nerve root supplies a particular area of the skin, the dermatome (3) , which is, however, invaded above and below by fibers from the adjacent spinal segments. l. of similars similia similibus curantur. Snell l. SYN: l. of refraction. Spallanzani l. the younger the individual the greater is the regenerative power of its cells. l. of specific nerve energies SYN: Müller l.. Starling l. SYN: l. of the heart. Stokes l. 1. a muscle lying above an inflamed mucous or serous membrane is frequently the seat of paralysis; 2. a relationship of the rate of fall of a small sphere in a viscous fluid; applicable to centrifugation of macromolecules; 3. the wavelength of light emitted by a fluorescent material is longer than that of the radiation used to excite the fluorescence. Tait l. an obsolete dictum that an exploratory laparotomy should be performed in every case of obscure pelvic or abdominal disease that threatens health or life. Thoma laws the development of blood vessels is governed by dynamic forces acting on their walls as follows: an increase in velocity of blood flow causes dilation of the lumen; an increase in lateral pressure on the vessel wall causes it to thicken; an increase in end-pressure causes the formation of new capillaries. van't Hoff l. 1. in stereochemistry, all optically active substances have one or more multivalent atoms united to four different atoms or radicals so as to form in space an unsymmetrical arrangement; 2. the osmotic pressure exerted by any substance in very dilute solution is the same that it would exert if present as gas in the same volume as that of the solution; or, at constant temperature, the osmotic pressure of dilute solutions is proportional to the concentration (number of molecules) of the dissolved substance; i.e., the osmotic pressure, Π, in dilute solutions is Π = RTΣci, where R is the universal gas constant, T is the absolute temperature, and ci is the molar concentration of solute i; 3. the rate of chemical reactions increases between two- and three-fold for each 10°C rise in temperature. Vogel l. when a phenotype may be transmitted by various modes of mendelian inheritance, the dominant will have the least deleterious phenotype, the recessive the most, and the X-linked intermediate between the two. wallerian l. after section of the posterior root of a spinal nerve between the root ganglion and the spinal cord, the central portion degenerates; after division of the anterior root, the peripheral portion degenerates; the trophic center of the posterior root is therefore the ganglion, that of the anterior root the spinal cord. Weber l. SYN: Weber-Fechner l.. Weber-Fechner l. the intensity of a sensation varies by a series of equal increments (arithmetically) as the strength of the stimulus is increased geometrically; if a series of stimuli is applied and so adjusted in strength that each stimulus causes a just perceptible change in intensity of the sensation, then the strength of each stimulus differs from the preceding one by a constant fraction; thus, if a just perceptible change in a visual sensation is produced by the addition of 1 candle to an original illumination of 100 candles, 10 candles will be required to produce any change in sensation when the original illumination was one of 1000 candles. SYN: Fechner-Weber l., Weber l.. Weigert l. the loss or destruction of a part or element in the organic world is likely to result in compensatory replacement and overproduction of tissue during the process of regeneration or repair (or both), as in the formation of callus when a fractured bone heals. SYN: overproduction theory. Williston l. as the vertebrate scale is ascended, the number of bones in the skull is reduced. Wolff l. every change in the form and the function of a bone, or in its function alone, is followed by certain definite changes in its internal architecture and secondary alterations in its external conformation; these changes usually represent responses to alterations in weight-bearing stresses.

Robert D., English physician, 1892–1968. See L.-Seip syndrome.

lawrencium (Lr, Lw) (law-ren′se-um)
An artificial transplutonium element; atomic no. 103; atomic wt. 262.11. [E.O. Lawrence, U.S. physicist and Nobel laureate, 1901–1958]


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