AIC (Akaike's information criterion) Criterion, introduced by *Akaike in 1969, for choosing between competing statistical *models. For *categorical data this amounts to choosing the model that minimizes G2 — 2v, where G2 is the *likelihood-ratio goodness-of-fit statistic and v is’
cm.ciiicii.iiiy icimwdi process
the number of 'degrees of freedom associated with the model. An alternative, that usually results in the selection of a simpler model, is the Bayesian information criterion (BIC) for which the quantity minimized is G2 - v In n, where In is the 'natural logarithm and n is the 'sample size. The latter criterion is also called the Schwarz criterion. A third alternative is the Hannan-Quinn criterion for which the quantity to be minimized is G2 - 2v ln(In n). See also MALLOWS CP; STEPWISE PROCEDURES.
Tuesday, August 10, 2010
What is Akaike's information criterion
AIC (Akaike's information criterion) Criterion, introduced by *Akaike in 1969, for choosing between competing statistical *models. For *categorical data this amounts to choosing the model that minimizes G2 — 2v, where G2 is the *likelihood-ratio goodness-of-fit statistic and v is’
cm.ciiicii.iiiy icimwdi process
the number of 'degrees of freedom associated with the model. An alternative, that usually results in the selection of a simpler model, is the Bayesian information criterion (BIC) for which the quantity minimized is G2 - v In n, where In is the 'natural logarithm and n is the 'sample size. The latter criterion is also called the Schwarz criterion. A third alternative is the Hannan-Quinn criterion for which the quantity to be minimized is G2 - 2v ln(In n). See also MALLOWS CP; STEPWISE PROCEDURES.
cm.ciiicii.iiiy icimwdi process
the number of 'degrees of freedom associated with the model. An alternative, that usually results in the selection of a simpler model, is the Bayesian information criterion (BIC) for which the quantity minimized is G2 - v In n, where In is the 'natural logarithm and n is the 'sample size. The latter criterion is also called the Schwarz criterion. A third alternative is the Hannan-Quinn criterion for which the quantity to be minimized is G2 - 2v ln(In n). See also MALLOWS CP; STEPWISE PROCEDURES.
What is Agglomerative clustering
Agglomerative clustering methods. Examples of the distance definitions used in clustering.
What is age-specific
age-specific rate When a rate, such as a *birth rate, incidence rate, or "mortality rate is calculated for individuals of a specified age (or age range) then the rate is described as being age-specific.
What is admissibility
admissibility A term (introduced by *Wald in 1939) used in 'statistical inference in several contexts. A procedure is admissible if there is no alternative procedure that performs at least as well under all circumstances and performs better under some circumstances.
What is additive model
additive model A *model in which the combined effect of the explanatory variables (and their interaction) is equal to the sum of their separate effects.
What is addition law for probabilities
addition law for probabilities (total probability law) Law stating that if two events (see SAMPLE SPACE) A and B are "mutually exclusive then
For example, the *probability that, when a normal six-sided die is rolled, it shows a multiple of 3 is -=s- + - = P(shows 3} + P(shows 6). 3 6 6
The generalization to n mutually exclusive events is the law of total probability: P(Ai U A2 U - U An) = P(A,) + V{A2) + - + V{An). See also UNION.
For example, the *probability that, when a normal six-sided die is rolled, it shows a multiple of 3 is -=s- + - = P(shows 3} + P(shows 6). 3 6 6
The generalization to n mutually exclusive events is the law of total probability: P(Ai U A2 U - U An) = P(A,) + V{A2) + - + V{An). See also UNION.
What is adaptive sampling
adaptive sampling A method of sequential sampling in which the later sampling procedure is affected by the earlier results. Examples include the comparison of rival treatments in *clinical trials and the use of *kriging in the exploration for oil.
What is actuarial statistics
actuarial statistics The branch of Statistics concerned with insurance and loss, including reinsurance, *ruin theory, and *run-off triangles.
What is acquiescence bias
acquiescence bias The tendency of an interviewee to agree with the questioner. For example, if the question is 'Did you vote in the last election?' then the number replying 'Yes' is likely to provide an overestimate of the proportion who voted. On the other hand, if the question had been 'Did you abstain in the last election?', then this would also be likely to provide an overestimate.
What is acceptance sampling
acceptance sampling A method of 'quality control. A random *sample is taken from a "batch of output and the decision to accept or reject the batch is based on either the number of defectives in the sample (inspection by attributes) or on some summary *statistic such as the "sample mean (inspection by variables).In the case of inspection by attributes, the "probability of accepting a batch is a function of the proportion, p, of defectives in the batch. Any acceptance sampling scheme that does not sample 100% of a batch will lead to the occasional rejection of batches with very low proportions of defectives (the producer's risk), and to the occasional acceptance of batches with very high proportions of defectives (the consumer's risk).As p increases, so the probability that a batch will be rejected increases. The lot tolerance percent defective (LTPD) is the value of p (expressed as a percentage) that the sampling scheme would expect to reject on a given proportion (usually 90%) of occasions.The maximum proportion of defectives that is regarded as desirable by the consumer is called the acceptable quality level (AQL). The graph relating the probability of acceptance to p is called the operating characteristic curve (OC-curve). The average quality level of the items in the batches released after inspection is die average outgoing quality (AOQ). The AOQ is usually calculated under the assumption that defective items found during testing will be replaced before the batch is released. The AOQ has minima at p = 0 and p — 1 and its maximum is termed the average outgoing quality limit (AOQL).Suppose that the proportion of defectives remains constant from batch to batch. Eventually a batch will be rejected. The average run length (ARL) is the average number of batches inspected up to and including the
one that is rejected. See QUALITY CONTROL.
one that is rejected. See QUALITY CONTROL.
What is Acceptance-rejection algorithm
Acceptance-rejection algorithm. Uniform random numbers are generated in the intervals (a, b) and (0, M). If the point generated lies between the graph of \[x) and the x-axis, then the value of X is accepted.
What is acceptance-rejection algorithm
acceptance-rejection algorithm A method for generating values of a "continuous random variable for use in a "simulation. Suppose that
the random variable X, which takes values in the interval (a, b), has "probability density function f. Denote the maximum value of f(x) by M. Let u and v be two random numbers *uniformly distributed in the interval (0, l). Write r = a + (b - a)u and s = Mv, so that r and s are uniformly distributed on (a, b) and (0, M), respectively. Calculate f(r). If f(r) > s then r is accepted as a value of X. Otherwise, it is rejected and a new pair of values is taken for u and v.
the random variable X, which takes values in the interval (a, b), has "probability density function f. Denote the maximum value of f(x) by M. Let u and v be two random numbers *uniformly distributed in the interval (0, l). Write r = a + (b - a)u and s = Mv, so that r and s are uniformly distributed on (a, b) and (0, M), respectively. Calculate f(r). If f(r) > s then r is accepted as a value of X. Otherwise, it is rejected and a new pair of values is taken for u and v.
What is acceptance region
acceptance region The set of values of the "statistic, in a "hypothesis test, which lead to acceptance of the "null hypothesis.
What is acceptable risk
acceptable risk In the context of a medical treatment this describes a situation in which the expected benefits outweigh the potential hazards of the treatment.
What is absolute value
absolute value (modulus) The value of a number disregarding its sign. Denoted by a pair of')' signs: thus the modulus of-2.5 is |-2.5 | = 2.5.
What is absolute error
absolute error The "absolute value of the difference between (commonly) an Observation and the value predicted by, or estimated from, some "model.
What is absolute difference
absolute difference The "absolute value of the difference between two numbers. See also MEAN DEVIATION.
Who is Abbe, Ernst Carl
Abbe, Ernst Carl (1840-1905; b. Eisenach, Germany; d. Jena, Germany) German mathematician and physicist. His father was a book printer and factory worker and his childhood was one of privation. Abbe studied at U Jena and U Gottingen, receiving his PhD in 1861. In 1863 he was appointed to a lectureship at Jena on the basis of a dissertation that, in effect, derived the *chi-squared distribution. Following an approach from Carl Zeiss, most of his subsequent work was concerned with optics and astronomy. Alunar crater is named after him, also a minor planet, and several schools in Germany.
What is Aalen model
Aalen model A "linear regression model describing the manner in which the "expected value of a response variable, Z(t), depends on explanatory variables Xi(t),..., Xk{t) with time-dependent coefficients
E[Z(f)] = β0(0 +J81(0X,(0 4- - + /3*(0X*M.
E[Z(f)] = β0(0 +J81(0X,(0 4- - + /3*(0X*M.
What is Aalen model
Aalen model A "linear regression model describing the manner in which the "expected value of a response variable, Z(t), depends on explanatory variables Xi(t),..., Xk{t) with time-dependent coefficients
E[Z(f)] = β0(0 +J81(0X,(0 4- - + /3*(0X*M.
E[Z(f)] = β0(0 +J81(0X,(0 4- - + /3*(0X*M.
absolute zero
absolute zero Zeroofthermody-namic temperature (0 kelvin) and the lowest temperature theoretically attainable. It is the temperature at which the kinetic energy of atoms and molecules is minimal. It is equivalent to -273.15°C or -459.67°F. See also ZERO-POINT ENERGY.
What is absolute configuration
absolute configuration A way of denoting the absolute structure of an optical isomer (see OPTICAL ACTIVITY). Two conventions are in use: The D-L convention relates the structure of the molecule to some reference mol¬ecule. In the case of sugars and simi¬lar compounds, the dextrorotatory form of glyceraldehyde (HOCH2CH(OH)CHO), 2,3-dihydroxy-propanal) was used. The rule is as follows. Write the structure of this molecule down with the asymmetric carbon in the centre, the -CHO group at the top, the -OH on the right, the -CH2OH at the bottom, and the -H on the left. Now imagine that the central carbon atom is at the cen¬tre of a tetrahedron with the four groups at the coiners and that the -H and -OH come out of the paper and the -CHO and -CH2OH groups go into the paper. The resulting three-dimensional structure was taken to be that of d-glyceraldehyde and called D-glyceraldehyde. Any com¬pound that contains an asymmetric carbon atom having this configura¬tion belongs to the D-series. One hav¬ing the opposite configuration belongs to the L-series. It is important to note that the prefixes D- and L- do not stand for dextrorotatory and laevorotatory (i.e. they are not the same as d- and I-). In fact the arbitrary configuration assigned to D-glycer¬aldehyde is now known to be the cor¬rect one for the dextrorotatory form, although this was not known at the time. However, all D-compounds are not dextrorotatory. For instance, the acid obtained by oxidizing the -CHO group of glyceraldehyde is glyceric acid (1,2-dihydroxypropanoic acid). By convention, this belongs to the D-series, but it is in fact laevorotatory; i.e. its name can be written as D-glyceric acid or 1-glyceric acid. To avoid confusion it is better to use + (for dextrorotatory) and - (for laevo¬rotatory), as in D-(+)-glyceraldehyde and D-(-}-glyceric acid.The D-L convention can also be used with alpha amino acids (com¬pounds with the -NH2 group on the same carbon as the -COOH group). In this case the molecule is imagined as.being viewed along the H-C bond be¬tween the hydrogen and the asym¬metric carbon atom. If the clockwise order of the other three groups is -COOH, -R, -NH2l the amino acid be¬longs to the D-series; otherwise it be¬longs to the L-series. This is known as the CORN rule.The R-S convention is a convention based on priority of groups attached to the chiral carbon atom. The order of priority is I, Br, Cl, SO3H, OCOCH3, OCH3, OH, NO2, NH2, COOCH3, CONH2, COCH3, CHO, CH2OH, C6H5, C2H5, CH3, H, with hydrogen lowest. The molecule is viewed with the group of lowest priority behind the chiral atom. If the clockwise arrange-ment of the other three groups is in descending priority, the compound belongs to the R-series; if the de¬scending order is anticlockwise it is in the s-series. D-(+)-glyceraldehyde is R-(+)-glyceraldehyde. See illustration.
absolute 1
. Not dependent on or relative to anything else, e.g. ab-solute zero. 2. Denoting a temperature measured on an absolute scale, a scale of temperature based on absolute zero. The usual absolute scale now is that of fhermodynamic '■temperature; its unit, the kelvin, was formerly called the degree absolute (°A) and is the same size as the degree Celsius. In British engineering practice an absolute scale with Fahren-
heit-size degrees has been used: this is the Rankine scale.
heit-size degrees has been used: this is the Rankine scale.
What is ab-initio calculation
ab-initio calculation A method of calculating atomic and molecular structure directly from the first principles of quantum mechanics, without using quantities derived from experiment (such as ionization energies found by spectroscopy) as parameters. Ab-initio calculations require a large amount of numerical computation; the amount of computing time required increases rapidly as the size of the atom or molecule increases. The development of computing power has enabled the properties of both small and large molecules to be calculated accurately, so that this form of calculation can now replace semi-empirical calculations. Ab-initio calculations can, for example, be used to determine the bond lengths and bond angles of molecules by calculating the total energy of the molecule for a variety of molecular geometries and finding which conformation has the lowest energy.
Saturday, August 7, 2010
What is acetazolamide
acetazolamide n. a "carbonic anhydrase inhibitor used mainly in the treatment of glaucoma to reduce the pressure inside the eyeball and also as a preventative for epilep¬tic seizures and altitude sickness. It is ad¬ministered by mouth or injection; side-effects include drowsiness and numb¬ness and tingling of the hands and feet. Trade name: Diamox.
What is acetabulum
acetabulum (cotyloid cavity) n. either of the two deep sockets, one on each side of the "hip bone, into which the head of the thigh bone (femur) fits at the *hip joint.
What is acervulus cerebri
acervulus cerebri a collection of granules of calcium-containing material that is sometimes found within the *pineal gland as its calcification proceeds (normally after the 17th year): 'brain sand'.
What is acentric
acentric n. (in genetics) a chromosome or fragment of a chromosome that has no *centromere. Since acentrics cannot attach to the *spindle they are usually lost during cell division. They are often found in cells damaged by radiation, —acentric adj.
What is ACE inhibitor
ACE inhibitor angiotensin-converting en¬zyme inhibitor: any one of a group of drugs used in the treatment of raised blood pres¬sure and heart failure. ACE inhibitors act by interfering with the action of the enzyme that converts the inactive *angiotensin I to the powerful artery constrictor angiotensin II. The absence of this substance allows ar¬teries to widen and the blood pressure to drop. ACE inhibitors are administered by mouth; they include *captopril, *enalapril, perindopril (Coversyl), and ramipril (Tritace). Possible side-effects include weakness, dizzi¬ness, loss of appetite, and skin rashes.
What is acebutolol
acebutolol n. a "beta blocker drug used to treat high blood pressure, angina pec-
toris, and irregular heart rhythms. It is ad¬ministered by mouth. Possible side-effects include breathing difficulty, especially in asthmatics, and cold hands and feet. Trade name: Sectral.
toris, and irregular heart rhythms. It is ad¬ministered by mouth. Possible side-effects include breathing difficulty, especially in asthmatics, and cold hands and feet. Trade name: Sectral.
What is accommodative insufficiency
accommodative insufficiency a weak¬ness of the * accommodation reflex, some¬times a result of injury, disease, or the effect of medication.
What is accommodation reflex
accommodation reflex (convergence
reflex) the constriction of the pupils and inward turning of the eyes that occurs when an individual focuses on a near ob¬ject.
reflex) the constriction of the pupils and inward turning of the eyes that occurs when an individual focuses on a near ob¬ject.
What is accommodation
accommodation n. adjustment of the shape of the lens to change the focus of the eye. When the ciliary muscle (see CILIARY BODY) is relaxed, suspensory ligaments at¬tached to the ciliary body and holding the lens in position are stretched, which causes the lens to be flattened. The eye is then able to focus on distant objects. To focus the eye on near objects the ciliary muscles contract and the tension in the ligaments is thus lowered, allowing the lens to become rounder.
accident
accident n. a traumatic incident involving any part of the body. Accident and emer¬gency (A and E) medicine is a specialized area of patient care.
accessory nerve
accessory nerve (spinal accessory nerve)
the eleventh "cranial nerve (XI), which arises from two roots, cranial and spinal. Fi¬bres from the cranial root travel with the nerve for only a short distance before branching to join the vagus and then form¬ing the recurrent laiyngeal nerve, which supplies the internal laryngeal muscles. Fi¬bres from the spinal root supply the ster-nomastoid and trapezius muscles, in the neck region (front and back).
the eleventh "cranial nerve (XI), which arises from two roots, cranial and spinal. Fi¬bres from the cranial root travel with the nerve for only a short distance before branching to join the vagus and then form¬ing the recurrent laiyngeal nerve, which supplies the internal laryngeal muscles. Fi¬bres from the spinal root supply the ster-nomastoid and trapezius muscles, in the neck region (front and back).
What is accessory muscles
accessory muscles muscles of the shoul¬der girdle and chest wall that (in addition to the intercostal muscles and the diaphragm) are utilized by people with *respiratory dis¬tress to help the flow of air in and out of the lungs. Use of these muscles can be a sign of the degree of difficulty that the patient is in, for example in cases of asthma or *air-way obstruction. .
What is acceptor
acceptor n. (in biochemistry) a substance that helps to bring about oxidation of a re¬duced 'substrate by accepting hydrogen ions.
What is acatalasia
acatalasia n. an inborn lack of the en¬zyme 'catalase, leading to recurrent infec¬tions of the gums (gingivitis) and mouth. It is most common in the Japanese.
What is Acarina
Acarina n. the group of arthropods that in¬cludes the *mites and 'ticks.
Acarus (Tyroglyphus) n. a genus of mites. The flour mite, A. siro (T.farinae), is nonpara-sitic, but its presence in flour can cause a se¬vere allergic dermatitis in flour-mill workers.
Acarus (Tyroglyphus) n. a genus of mites. The flour mite, A. siro (T.farinae), is nonpara-sitic, but its presence in flour can cause a se¬vere allergic dermatitis in flour-mill workers.
What is acariasis
acariasis n. an infestation of mites and ticks and the symptoms, for example al¬lergy and dermatitis, that their presence may provoke.
What is acardia
acardia n. congenital absence of the heart. The condition may occur in conjoined twins; the twin with the heart controls the circulation for both.
What is acapnia
acapnia (hypocapnia) Ft. a condition in which there is an abnormally low concen¬tration of carbon dioxide in the blood. This may be
caused by breathing that is excep¬tionally deep in relation to the physical ac¬tivity of the individual.
caused by breathing that is excep¬tionally deep in relation to the physical ac¬tivity of the individual.
What is acanthosis
acanthosis n. an increase in the number of 'prickle cells in the innermost layer of the epidermis, leading to thickening of the
epidermis. Acanthosis nigricans is associated with "insulin resistance; it is characterized by papillomatous growths, mainly in the armpits, giving the skin a pigmented ap¬pearance and a velvety texture. It may be benign or malignant. Pseudoacanthosis ni¬gricans is more common and is associated with obesity.
epidermis. Acanthosis nigricans is associated with "insulin resistance; it is characterized by papillomatous growths, mainly in the armpits, giving the skin a pigmented ap¬pearance and a velvety texture. It may be benign or malignant. Pseudoacanthosis ni¬gricans is more common and is associated with obesity.
What is acanthion
acanthion n. the rip of the spine formed where projecting processes of the upper jaw bones (maxillae) meet at the front of the face.
What is Acanthamoeba
Acanthamoeba n. a genus of amoebae that are commonly found in soil and conta¬minated water and cause painful corneal in¬fection and ulcers in humans, usually resulting from improper sterilization of contact lenses in contact lens solutions.
What is acalculia
acalculia n. an acquired inability to make simple mathematical calculations. It is a symptom of disease in the 'parietal lobe of the brain. See GERSTMANN"S SYNDROME.
acantha n. 1. a spine projecting from a 'vertebra. 2. the 'backbone.
acantha n. 1. a spine projecting from a 'vertebra. 2. the 'backbone.
absorption
absorption n. (in physiology) the uptake of fluids or other substances by the tissues of the body. Digested food is absorbed into the blood and lymph from the alimentary canal. Most absorption of food occurs in the small intestine - in the jejunum and ileum - although alcohol is readily absorbed from the stomach. The small intestine is lined with minute finger-like processes (see VIILUS), which greatly increase its surface area and therefore the speed at which ab¬sorption can take place. See also ASSIMILA¬TION, DIGESTION.
What is Absidia
Absidia n. a genus of fijngi that some¬times cause disease in humans (see PHY-COMYCOSIS).
What is abscess
abscess n. a localized collection of pus and necrotic tissue anywhere in the body, sur¬rounded and walled ofTby damaged and inflamed tissues. A *boil is an example of an abscess within the skin. The usual cause is local bacterial infection, often by staphylo-cocci, that the body"s defences have failed to overcome. In a cold abscess, due to tuber¬cle bacilli, there is swelling, but little pain or inflammation (as in acute abscesses).
Antibiotics, aided by surgical incision to re¬lease pus where necessary, are the usual forms of treatment.The brain and its meninges have a low re¬sistance to infection and a cerebral abscess is liable to follow any penetration of these by microorganisms. The condition is fatal unless relieved by aspiration or surgical drainage.
Antibiotics, aided by surgical incision to re¬lease pus where necessary, are the usual forms of treatment.The brain and its meninges have a low re¬sistance to infection and a cerebral abscess is liable to follow any penetration of these by microorganisms. The condition is fatal unless relieved by aspiration or surgical drainage.
What is abruptio placentae
abruptio placentae (placental abruption) bleeding from a normally situated placenta causing its complete or partial detachment from the uterine wall after the 24th week of gestation. There may be varying degrees of shock; 'disseminated intravascular coag¬ulation is a further complication. Abruptio placentae is often associated with hyperten¬sion and pre-eclampsia.
What is abreaction
abreaction n. the release of strong emo¬tion commonly associated with a buried memory. While this can happen sponta¬neously, it is usually deliberately produced by a therapist using psychotherapy or hyp¬nosis. The technique is used as a treatment for conversion disorder, anxiety state, and other neurotic conditions, especially when they are thought to be caused by "repres¬sion of memories or emotions.
What is abrasion
abrasion n. 1. a graze: a minor wound in
which the surface of the skin or a mucous membrane is worn away by rubbing or scraping. 2. the wearing of the teeth, particularly at the necks by overvigorous brushing. It is frequently enhanced by *ero-sion. 3. any nibbing or scraping action that produces surface wear.
which the surface of the skin or a mucous membrane is worn away by rubbing or scraping. 2. the wearing of the teeth, particularly at the necks by overvigorous brushing. It is frequently enhanced by *ero-sion. 3. any nibbing or scraping action that produces surface wear.
What is Abrams-Griffiths number
Abrams-Griffiths number (bladder out¬let obstruction index, BOOI) a scale used to estimate the degree of bladder outflow ob¬struction (*BOO) in a patient: >1.5 indicates obstruction.
What is abdrtus
abdrtus n. a fetus, weighing less than 500 g, that is expelled from the uterus either dead or incapable of surviving.
What is abortion
abortion n, 1. (induced abortion, termina¬tion of pregnancy) the removal of an em¬bryo or fetus from the uterus at a stage of pregnancy when it is deemed incapable of independent survival (i.e. at any time be¬tween conception and the 24th week of pregnancy]. One of the most commonly performed gynaecological procedures in the UK, it must be carried out within the terms of the Abortion Act 1967 and the subse¬quent Abortion Regulations 1991. Two doc¬tors must agree that termination of pregnancy is necessaiy for one of the rea¬sons specified in the Regulations, which sometimes allow termination beyond the legal age of viability (24 weeks) in special circumstances (e.g. substantial risk of dis¬ability in the child if born). The procedure must be carried out in an approved hospital or clinic. Drugs used for medically induced abortion include *mifepristone and prostaglandin analogues (e.g. *gemeprost). Surgical methods of termination of preg¬nancy include vacuum aspiration of the products of conception through an intra-uterine cannula, "dilatation and curettage, and uterine evacuation by ovum forceps. Termination carries little risk early in preg¬nancy, but complications (e.g. bleeding, in¬fection) are more likely to occur after the 13th week of pregnancy. 2. (spontaneous abortion) see MISCARRIAGE.
What is abortifacient
abortifacient n. a drug that induces abor¬tion or miscarriage. See MIFEPRISTONE,
PROSTAGLANDIN.
PROSTAGLANDIN.
What is ablation
ablation ri. the removal or destruction of tissue, a pait of the body, or an abnormal growth. For example, ablation therapy is an
alternative to surgery for the treatment of breast cancer; it allows accurate targeting of tumours guided by MR scanning. See also
ENDOMETRIAL ABLATION, RADIOFREQJJENCY ABLATION.
alternative to surgery for the treatment of breast cancer; it allows accurate targeting of tumours guided by MR scanning. See also
ENDOMETRIAL ABLATION, RADIOFREQJJENCY ABLATION.
What is abiotrophy
abiotrophy n. degeneration or loss of function without apparent cause; for exam¬ple, retinal abiotrophy is progressive degen¬eration of the retina leading to impaired vision, occurring in genetic disorders such as *retinitis pigmentosa.
What is aberration
aberration n. (in optics) a defect in the image formed by a lens. In chromatic aber¬ration the image formed by a lens has coloured fringes as a result of the different extent to which light of different colours is refracted by glass. It is corrected by using an •achromatic lens. In spherical aberration, the image is blurred because rays from the object come to a focus in slightly different positions as a result of the curvature of the lens: the rays passing more peripherally through the lens are bent more than those passing through centrally. This occurs even with monochromatic light.
What is aberrant adj
aberrant adj. abnormal: usually applied to a blood vessel or nerve that does not follow its normal course.
What is abductor
abductor n. any muscle that moves one part of the body away from another or from the midline of the body.
What is abduct vb
abduct vb. to move a limb or any other part away from the midline of the body. —abduction n.
What is abdominal thrusts
abdominal thrusts (Heimlich manoeuvre) a manoeuvre for the treatment of choking in which the patient is held firmly around the midriff just under the ribcage. The hands of the rescuer are held as a fist and short sharp thrusts into the patient's upper abdomen are made in order to dislodge the obstructing article from the airway.
abducens nerve the sixth *cranial nerve
(VI), which supplies the lateral rectus mus¬cle of each eyeball, responsible for turning the eye outwards.
abducens nerve the sixth *cranial nerve
(VI), which supplies the lateral rectus mus¬cle of each eyeball, responsible for turning the eye outwards.
What is abdomin
abdomin- (abdomino-) combining form denoting the abdomen. Examples: abdomi-nalgia (pain in the abdomen); abdomtno-thoraric (relating to the abdomen and thorax).
What is abdomen
abdomen n. the part of the body cavity below the chest {see THORAX), from which it is separated by the 'diaphragm. The ab¬domen contains the organs of digestion -stomach, liver, intestines, etc. - and excre¬tion - kidneys, bladder, etc.; in women it also contains the ovaries and uterus. The re¬gions of the abdomen arc shown in the il¬lustration: terms in brackets are those used in anatomy textbooks (rather than in day-to¬day medical practice) to designate these re¬gions, —abdominal adj.
Monday, August 2, 2010
What is savannah
savannah An extensive tropical vegeta¬tion dominated by grasses with varying admixtures of tall bushes and/or trees in open formation. Savannah occurs in di¬verse tropical environments, although most experience a dry season. Much savan¬nah is no doubt *climatic climax, although extensive tracts are anthro¬pogenic *fire climaxes and others are *edaphically controlled; it is generally difficult to distinguish one type from the other.
What is savannah woodland
savannah woodland A savannah in which trees and 'shrubs form a generally light canopy. The trees and bushes are generally 'deciduous, yet 'evergreens are usually also well represented. Some tall trees occur, but most are stunted and gnarled. They frequently have thick, corky, fire-resistant bark.
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