A reasoned explanation of known facts or phenomena that serves as a basis of investigation by which to seek the truth. SEE ALSO: hypothesis, postulate. [G. theoria, a beholding, speculation, t., fr. theoros, a beholder]
- adsorption t. of narcosis that a drug becomes concentrated at the surface of the cell as a result of adsorption, and thus alters permeability and metabolism.
- aerodynamic t. generally accepted t. that the vibration of the vocal folds in phonation is produced by the flow of exhaled air past lightly approximated vocal folds; opposed to the now untenable concept that vocal fold motion in phonation results from contraction of the intrinsic muscles of the larynx at the frequency of the vocal fold vibration.
- Altmann t. a t. that protoplasm consists of granular particles (called bioblasts) that are clustered and enclosed in indifferent matter.
- Arrhenius-Madsen t. that the reaction of an antigen with its antibody is a reversible reaction, the equilibrium being determined according to the law of mass action by the concentrations of the reacting substances.
- atomic t. that chemical compounds are formed by the union of atoms in certain definite proportions; in its modern form, first advanced in 1803 by John Dalton.
- Baeyer t. that carbon bonds are set at fixed angles (109° 28′) and that those carbon rings are most stable that least distort those angles; for this reason, planar rings composed of 5 or 6 carbon atoms ( e.g., cyclopentane, benzene) are more common than rings containing less than 5 or more than 6 carbon atoms.
- balance t. in social psychology, a t. that assumes that steady and unsteady states can be specified for cognitive units ( e.g., an individual and his or her attitudes or acts) and that such units tend to seek steady states (balance); e.g., balance exists when both parts of a unit are evaluated the same, but disequilibrium arises when both parts are not evaluated the same, which causes either cognitive reevaluation of the parts or their segregation. SEE ALSO: cognitive dissonance t., consistency principle.
- beta-oxidation-condensation t. that the two carbon fragments split from the fatty acid molecule by beta-oxidation are converted to acetic acid and then condensed to acetoacetic acid.
- Bohr t. that spectrum lines are produced 1) by the quantized emission of radiant energy when electrons drop from an orbit of a higher to one of a lower energy level, or 2) by absorption of radiation when an electron rises from a lower to a higher energy level.
- Brøonsted t. that an acid is a substance, charged or uncharged, liberating hydrogen ions in solution, and that a base is a substance that removes them from solution ( e.g., NH4+, CH3COOH, and HSO4 are acids; NH3, CH3COO, and SO4 are bases); useful in the concept of weak electrolytes and buffers. Cf.:Brøonsted acid, Brøonsted base.
- Burn and Rand t. that stimulation of sympathetic fibers results first in the production of acetylcholine in the postganglionic nerve endings, which then release norepinephrine to act on the active site of the effector cell.
- Cannon t. SYN: emergency t..
- Cannon-Bard t. the view that the feeling aspect of emotion and the pattern of emotional behavior are controlled by the hypothalamus.
- catastrophe t. a branch of mathematics dealing with large changes in the total system that may result from a small change in a critical variable in the system; an example is the change in the physical properties of H2O as the temperature reaches 0 or 100°C; many applications of catastrophe t. occur in clinical medicine and in epidemiology.
- cellular immune t. a concept, put forth by Elie Metchnikoff, that cells, not antibodies, were responsible for the immune response of an organism.
- chaos t. a branch of mathematics dealing with events and processes that cannot be predicted precisely on the basis of conventional mathematical theories or laws; some biological processes, e.g., spread of malignant disease, appear to conform to chaos t., at least sometimes.
- chemiosmotic t. a hypothesis proposing that cellular energy requiring processes such as ATP synthesis and ion pumping may be driven by a pH and membrane potential gradient; proposed by Peter Mitchell in 1961.
- cloacal t. the belief sometimes held by neurotic adults or children that a child is born, as a stool is passed, from a common opening.
- clonal deletion t. the elimination of certain T cell populations in the thymus that have receptors for self-antigens (forbidden clones). See immunologic tolerance.
- clonal selection t. a t. which states that each lymphocyte has membrane-bound immunoglobulin receptors specific for a particular antigen and once the receptor is engaged, proliferation of the cell occurs such that a clone of antibody-producing cells (plasma cell) is produced.
- cognitive dissonance t. a t. of attitude formation and behavior describing a motivational state that exists when an individual's cognitive elements (attitudes, perceived behaviors, etc.) are inconsistent with each other, such as the espousal of the Ten Commandments concurrent with the belief that it is all right to cheat on one's taxes; a test which indicates that persons try to achieve consistency (consonance) and avoid dissonance which, when it arises, may be coped with by changing one's attitudes, rationalizing, selective perception, and other means. SEE ALSO: balance t., consistency principle.
- decay t. a t. of forgetting based on the premise that an engram or memory trace dissipates progressively with time during the interval when it is not activated.
- dipole t. a t. in which the activation current of the heart is conceived as a single net moving dipole, the positive pole leading.
- duplicity t. of vision that the cones of the retina function in bright light and the rods function in dim light.
- Ehrlich t. side-chain t..
- emergency t. a t. of the emotions, advanced by W.B. Cannon, that animal and human organisms respond to emergency situations by increased sympathetic nervous system activity including an increased catecholamine production with associated increases in blood pressure, heart and respiratory rates, and skeletal muscle blood flow. SEE ALSO: relaxation response. SYN: Cannon t..
- enzyme inhibition t. of narcosis that narcotics inhibit respiratory enzymes by suppression of the formation of high-energy phosphate bonds within the cell.
- Flourens t. an older t. that thought is a process depending upon the action of the entire cerebrum.
- Frerichs t. that uremia represents a toxic condition caused by ammonium carbonate, which is formed as the result of the action of a plasma enzyme on the increased amounts of urea.
- Freud t. a comprehensive t. of how personality is formed and develops in normal and emotionally disturbed individuals; e.g., that an attack of conversion hysteria is due to a psychic trauma which was not adequately reacted to at the time it was received, and persists as an affect memory. SEE ALSO: psychoanalysis.
- game t. the branch of mathematical logic concerned with the range of possible reactions to a particular strategy; each reaction can be assigned a probability and each reaction can lead to a counter-reaction by the “adversary” in the game. Used mainly in systems analysis, game t. has some applications in disease surveillance and control; it is one of the underlying theories in clinical decision analysis.
- gastrea t. SYN: Haeckel gastrea t..
- gate-control t. a t. to explain the mechanism of pain; small-fiber afferent stimuli, particularly pain, entering the substantia gelatinosa can be modulated by large-fiber afferent stimuli and descending spinal pathways so that their transmission to ascending spinal pathways is blocked (gated). SYN: gate-control hypothesis.
- germ t. the t., now a doctrine, that infectious diseases are due to the presence and functional activity of microorganisms within the body.
- germ layer t. the concept that young embryos differentiate three primary germ layers (ectoderm, mesoderm, and endoderm), each of which has the potentiality of forming different characteristic structures and organs in the developing body.
- Haeckel gastrea t. that the two-layered gastrula is the ancestral form of all multicellular animals. SYN: gastrea t..
- Helmholtz t. of accommodation that the ciliary muscle relaxes for near vision and allows the anterior aspect of the lens to become more convex.
- Helmholtz t. of color vision SYN: Young-Helmholtz t. of color vision.
- Helmholtz-Gibbs t. Gibbs-Helmholtz equation.
- Helmholtz t. of hearing SYN: resonance t. of hearing.
- Hering t. of color vision that there are three opponent visual processes: blue-yellow, red-green, and white-black.
- hydrate microcrystal t. of anesthesia a t. of narcosis pertaining to nonhydrogen-bonding agents; postulates the interaction of the molecules of the anesthetic drug with water molecules in the brain. SYN: Pauling t..
- implantation t. of the production of endometriosis that, at the time of menstruation, cells of the uterine mucosa pass through the fallopian tubes and escape into the pelvic cavity where they implant themselves on the peritoneum.
- incasement t. SYN: preformation t..
- information t. in the behavioral sciences, a system for studying the communication process through the detailed analysis, often mathematical, of all aspects of the process including the encoding, transmission, and decoding of signals; not concerned in any direct sense with the meaning of a message.
- instructive t. a t. that states that an antibody learns or acquires its specificity after contact with a particular antigen.
- kern-plasma relation t. a t. enunciated by Hertwig (1903) that a definite relation as to size normally exists in every cell between the mass of nuclear material and that of the protoplasm. [Ger. kern, kernel, nucleus]
- Knoop t. that the catabolism of fatty acid s occurs in stages in each of which there is a loss of two carbon atoms as a result of oxidation at the β-carbon atom, e.g.,
- Ladd-Franklin t. SYN: molecular dissociation t..
- lamarckian t. that acquired characteristics may be transmitted to the descendants and that experience, and not biology alone, can change and thereby influence genetic transmission.
- learning t. any of several prominent theories designed to explain learning, especially those promulgated by Pavlov, Thorndike, Guthrie, Hull, Kohler, Spence, Miller, Skinner, and their modern followers. SEE ALSO: conditioning.
- libido t. Freud's t. that a person's psychic life results mainly from instinctual or libidinal needs and the attempts to satisfy them.
- Liebig t. that the hydrocarbons that oxidize readily and burn are aliments that produce the greatest quantity of animal heat.
- lipoid t. of narcosis that narcotic efficiency parallels the coefficient of partition between oil and water, and that lipoids in the cell and on the cell membrane absorb the drug because of this affinity. SYN: Meyer-Overton t. of narcosis.
- mass action t. that large areas of brain tissue function as a whole in learned or intelligent action.
- t. of medicine the science, as distinguished from the art, or practice, of medicine.
- membrane expansion t. that adsorption of anesthetics into membranes so alters membrane volume and/or configuration that membrane function is affected in such a way as to produce anesthesia.
- Metchnikoff t. the phagocytic t., that the body is protected against infection by the leukocytes and other cells that engulf and destroy the invading microorganisms.
- Meyer-Overton t. of narcosis SYN: lipoid t. of narcosis.
- miasma t. an explanation of the origin of epidemics, based on the false notion that they were caused by air of bad quality, e.g., emanating from rotting vegetation in marshes or swamps.
- Miller chemicoparasitic t. that dental caries is caused by microorganisms of the mouth fermenting dietary carbohydrates and producing acids that demineralize the teeth.
- mnemic t. SYN: mnemic hypothesis.
- molecular dissociation t. a t., pertaining to color vision, that gray is the earliest of color sensations, from which are derived, by molecular change, two paired substances that, respectively, detect yellow and blue, and that the yellow gives rise to paired substances for detection of red and green. SYN: Ladd-Franklin t..
- myoelastic t. a t. stating that sound of the human voice is produced by vibrations of the vocal cords resulting from folding upward due to air pressure below, and subsequent movement downward due to elastic tension of cords.
- neurochronaxic t. t. stating that variations in the frequency of the human voice are produced by changes in the rate of contractions of the laryngeal muscles; no longer believed to be true.
- Ollier t. a t. of compensatory growth; after resection of the articular extremity of a bone, the articular cartilage of the other bone entering into the structure of the joint takes on an increased growth.
- omega-oxidation t. that the oxidation of fatty acid s commences at the CH3 group, i.e., the terminal or omega-group; beta-oxidation then proceeds at both ends of the fatty acid chain.
- overproduction t. SYN: Weigert law.
- oxygen deprivation t. of narcosis that narcotics inhibit oxidation, which causes the cell to be narcotized.
- Pauling t. SYN: hydrate microcrystal t. of anesthesia.
- permeability t. of narcosis that the permeability of the cell membrane is decreased by narcotic concentrations of aliphatic and other central nervous system depressants.
- place t. a t. of pitch perception that states that the region of the basilar membrane of the cochlea that is set into vibration depends on the frequency of the sound. SEE ALSO: resonance t. of hearing.
- Planck t. SYN: quantum t..
- quantum t. that energy can be emitted, transmitted, and absorbed only in discrete quantities (quanta), so that atoms and subatomic particles can exist only in certain energy states. SYN: Planck t..
- recapitulation t. the t. formulated by E.H. Haeckel that individuals in their embryonic development pass through stages similar in general structural plan to the stages their species passed through in its evolution; more technically phrased, the t. that ontogeny is an abbreviated recapitulation of phylogeny. SYN: biogenetic law, law of biogenesis, Haeckel law, law of recapitulation.
- Reed-Frost t. of epidemics a mathematical t. to explain how epidemics originate and continue.
- reed instrument t. a no longer tenable t. stating that in human voice production the larynx functions in a manner similar to a reed musical instrument.
- reentry t. that extrasystoles are due to reentry of an impulse initiated by the sinus or AV junctional impulse, to which the extrasystole is coupled, into the ectopic focus.
- resonance t. of hearing that the basilar membrane of the cochlea acts as a resonating structure, with low frequency tones activating it in the apical turn and high frequency tones activating it in the basal turn. No longer considered correct; superseded by von Bekesy traveling wave t.. SYN: Helmholtz t. of hearing.
- scientific t. a t. that can be tested and potentially disproved; failure to disprove or refute it increases confidence in it, but it cannot be considered as proven.
- Semon-Hering t. SYN: mnemic hypothesis.
- sensorimotor t. in the developmental t. of Piaget, the postulation that during the first 18 months of life there occurs a transformation of action into thought; at first there is a gradual shift from inborn to acquired behavior, then from body-centered to object-centered activity, ultimately permitting intentional behavior and inventive thinking.
- side-chain t. Ehrlich postulated that cells contained surface extensions or side chains (haptophores) that bind to the antigenic determinants of a toxin (toxophores); after a cell is stimulated, the haptophores are released into the circulation and become the antibodies. SEE ALSO: receptor. SYN: Ehrlich postulate.
- somatic mutation t. of cancer that cancer is caused by a mutation or mutations in the body cells (as opposed to germ cells), especially nonlethal mutations associated with increased proliferation of the mutant cells.
- Spitzer t. an interpretation of the partitioning of the heart of mammalian embryos primarily on the basis of recapitulations of the adult structural pattern of lower forms; most frequently cited in relation to the partitioning of the truncus arteriosus to form ascending aorta and pulmonary trunk, which is achieved by the phylogenetic development of the lungs.
- stringed instrument t. a no longer tenable t. stating that in human voice production the vocal cords function in a manner similar to the strings in a stringed musical instrument.
- surface tension t. of narcosis that substances which lower the surface tension of water pass more readily into the cell and cause narcosis by decreasing metabolism.
- telephone t. a t. of pitch perception that states that the cochlea possesses no faculty of sound analysis, but that the frequency of the impulses transmitted over the auditory nerve fibers corresponds to the frequency of the sound vibrations, and is the sole basis for pitch discrimination; a t. no longer tenable. SEE ALSO: traveling wave t..
- thermodynamic t. of narcosis that the interposition of narcotic molecules in nonaqueous cellular phase causes changes that interfere with facilitation of ionic exchange.
- traveling wave t. generally held t. that a wave travels from the base to the apex of the basilar membrane of the cochlea in response to acoustic stimulation, and that the site of maximal displacement of the basilar membrane depends on the frequency of the stimulating tone with higher frequencies causing maximal displacement near the base and lower frequencies causing maximal displacement near the apex.
- van't Hoff t. that substances in dilute solution obey the gas laws. Cf.:van't Hoff law.
- Warburg t. that the development of cancer is due to irreversible damage to the respiratory mechanism of cells, leading to the selective multiplication of cells with increased glycolytic metabolism, both aerobic and anaerobic.
- Wollaston t. a t. that the semidecussation of the optic nerves at the chiasm is proved by the homonymous hemianopia seen in brain lesions.
- Young-Helmholtz t. of color vision a t. that there are three color-perceiving elements in the retina : red, green, and blue. Perception of other colors arises from the combined stimulation of these elements; deficiency or absence of any one of these elements results in inability to perceive that color and a misperception of any other color of which it forms a part. SYN: Helmholtz t. of color vision.

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the·o·ry 'thē-ə-rē, 'thi(-ə)r-ē n, pl -ries
1) the general or abstract principles of a body of fact, a science, or an art <the \theory and practice of medicine>
2) a plausible or scientifically acceptable general principle or body of principles offered to explain natural phenomena <a \theory of organic evolution> see ATOMIC THEORY, CELL THEORY, GERM THEORY
3) a working hypothesis that is considered probable based on experimental evidence or factual or conceptual analysis and is accepted as a basis for experimentation
the·o·ret·i·cal .thē-ə-'ret-i-kəl, .thi(ə)r-'et- also the·o·ret·ic -ik adj
the·o·ret·i·cal·ly -i-k(ə-)lē adv

* * *

the·o·ry (theґə-re) (thērґe) [Gr. theōria speculation as opposed to practice] 1. the doctrine or the principles underlying an art as distinguished from the practice of that particular art. 2. a formulated hypothesis, or, loosely speaking, any hypothesis or opinion not based upon actual knowledge.

Medical dictionary. 2011.

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