| condenser circle | <microscopy> The image of the aperture iris diaphragm of the substage condenser as seen in the back focal plane of the objective. (05 Aug 1998) |
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| population diffusion coefficient | <cell biology> Coefficient that describes the tendency of a population of motile cells to diffuse through the environment. Its use presupposes that the cells move in a random walk. (18 Nov 1997) |
| Haller's circle | <anatomy, nerve> A network of branches of the short ciliary arteries on the sclera around the point of entrance of the optic nerve. Synonym: circulus vasculosus nervi optici, circulus arteriosus halleri, circulus zinnii, Haller's circle, Zinn's corona, Zinn's vascular circle. (05 Mar 2000) |
| semi-closed circle | A circuit for administration of an inhalation anaesthetic in which partial rebreathing with carbon dioxide absorption is combined with loss from the circuit of a portion of respired gases through valves. (05 Mar 2000) |
| Huguier's circle | <anatomy> Anastomosis around the isthmus of the uterus (junction of the cervix with the body) between the right and left uterine arteries. (05 Mar 2000) |
| neoclassical diffusion | <radiobiology> In a magnetised plasma, _classical_ diffusion refers to transport of particles due to Coulomb collisions, taking the spiral orbits in the magnetic field into account. In a toroidal magnetic field, the actual rate of diffusive transport is much higher due to slow changes in the positions of the centres of the spirals, known as banana orbits. This faster transport is called _neo-classical_. With very few exceptions the transport in toroidal devices is observed to be 10-100 times larger still, presumably due to small-scale turbulence. The observed transport is called _anomalous_ (although it actually is the normal state). (09 Oct 1997) |
| nicked circle | <molecular biology> During extraction of plasmid DNA from the bacterial cell, onestrand of the DNA becomes nicked. This relaxes the torsional strain needed to maintain supercoiling, producing the familiar form of plasmid. (09 Oct 1997) |
| defensive circle | An obsolete term for the addition of a secondary affection that limits or arrests the progress of the primary affection, as thought to occur when pneumothorax supervenes on pulmonary tuberculosis, the former having a therapeutic effect on the latter. (05 Mar 2000) |
| diffusion | The process of becoming diffused or widely spread, the spontaneous movement of molecules or other particles in solution, owing to their random thermal motion, to reach a uniform concentration throughout the solvent, a process requiring no addition of energy to the system. (18 Nov 1997) |
| diffusion anoxia | Diffusion hypoxia severe enough to result in the absence of oxygen in alveolar gas. (05 Mar 2000) |
| diffusion chambers, culture | Devices used in a technique by which cells or tissues are grown in vitro or, by implantation, in vivo within chambers permeable to diffusion of solutes across the chamber walls. The chambers are used for studies of drug effects, osmotic responses, cytogenic and immunologic phenomena, metabolism, etc., and include tissue cages. (12 Dec 1998) |
| diffusion coefficient | For the translational diffusion of solutes, diffusion is described by Fick's First Law, that states that the amount of a substance crossing a given area is proportional to the spatial gradient of concentration and the diffusion constant (D), that is related to molecular size and shape. A useful derived relationship is that the mean square distance moved by molecules in time t is 6Dt. (18 Nov 1997) |
| diffusion constant | For the translational diffusion of solutes, diffusion is described by Fick's First Law, that states that the amount of a substance crossing a given area is proportional to the spatial gradient of concentration and the diffusion constant (D), that is related to molecular size and shape. A useful derived relationship is that the mean square distance moved by molecules in time t is 6Dt. (18 Nov 1997) |
| diffusion hypoxia | Abrupt transient decrease in alveolar oxygen tension when room air is inhaled at the conclusion of a nitrous oxide anaesthesia, because nitrous oxide diffusing out of the blood dilutes the alveolar oxygen. (05 Mar 2000) |
| diffusion limitation | The boundary layer hypothesis, that the proliferation of cells in culture is limited by the rate at which some essential component (almost certainly a growth factor) diffuses from the bulk medium into the layer immediately adjacent to the plasma membrane. By spreading out, a cell obtains a supra threshold level of the factor and can divide, if unable to spread (because of crowding or poor adhesion) then the cell will remain in the G0 stage of the cell cycle. (18 Nov 1997) |