| critical pH | The pH range, about 5.5, at which saliva ceases to be saturated with respect to calcium and phosphate, and below which tooth mineral will dissolve. (05 Mar 2000) |
|---|---|
| critical point | A point at which two phases become identical; thus, at a given critical temperature and critical pressure, the liquid and gaseous state of a particular substance can no longer be differentiated. (05 Mar 2000) |
| critical point drying | <procedure> A method for preparing specimens for the scanning electron microscope that avoids the problems of shrinkage caused by normal drying procedures. Water in the specimen is replaced by an intermediate fluid, for example liquid carbon dioxide, avoiding setting up a liquid/gas interface and then the second fluid is allowed to vaporise by raising the temperature above the critical point, the temperature at which the liquid state no longer occurs. (18 Nov 1997) |
| critical pressure | The minimum pressure required to liquefy a gas at the critical temperature. (05 Mar 2000) |
| critical rate | A heart rate at which aberration or incomplete block will occur; a result of shortening of cycle length so that it barely includes the refractory period. (05 Mar 2000) |
| critical temperature | The temperature of a gas above which it is no longer possible by use of any pressure, however great, to convert it into a liquid. (05 Mar 2000) |
| critical velocity | <biology> The maximum speed a device or organism can sustain over a specified distance or length of time, or the maximum velocity against which device or organism can sustain a position over a specified length of time. <marine biology> Velocity through which a fish will not swim, creating a velocity barrier. (09 Oct 1997) |
| Sudeck's critical point | Region in the colon between the supply of the sigmoid arteries and that of the superior rectal artery. (05 Mar 2000) |
| illumination, critical | <microscopy> The formation of an image of the light source in the object field. The Nelsonian method in which the light source is imaged in the plane of the specimen. A ribbon filament or arc lamp is required to give uniform illumination, the lamp must be focusable, the filament position must be adjustable in all directions. The use of an achromatic condenser is advised. Synonym: Nelson illumination. (13 Jan 1998) |
| acromial angle | The prominent angle at the junction of the posterior and lateral borders of the acromion. Synonym: angulus acromialis. (05 Mar 2000) |
| acute angle | Any angle less than 90 |
| acute angle closure glaucoma | <ophthalmology> An increase in pressure within the anterior chamber of the eye. There are two forms of glaucoma: acute angle closure and open angle glaucoma. (27 Sep 1997) |
| adjacent angle | An angle with a line in common with another angle. (05 Mar 2000) |
| alpha angle | The angle between the visual and optic axes as they cross at the nodal point of the eye, the angle between the visual line and the major axis of the corneal ellipse. (05 Mar 2000) |
| alveolar angle | The angle between the horizontal plane and a line connecting the base of the nasal spine and the middle point of the projection of the alveolus of the maxilla. (05 Mar 2000) |