| second heart sound | The second sound heard on auscultation of the heart; signifies the beginning of diastole and is due to closure of the semilunar valves. Synonym: second sound. (05 Mar 2000) |
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| second incisor | Second maxillary or mandibular permanent or deciduous tooth on either side of the midsagittal plane of the head. Synonym: lateral incisor. (05 Mar 2000) |
| second law of thermodynamics | <physics> All of the chemical and physical processes in a closed system tend to drive that system toward maximum entropy. Clausius statement: No device can operate in a cycle and allow heat to transfer from a colder body to a hotter body unless other compensating activities are taking place. Kelvin-Planck statement: no device can operate in a constant-temperature cycle and convert the heat it receives into work, unless other other compensating activities are taking place. (09 Oct 1997) |
| second line chemotherapy | <oncology> Chemotherapy given for a tumour which has already failed to respond to a first chemotherapy regimen (called first line). (16 Dec 1997) |
| second-look operation | Exploratory celiotomy within a year after apparently curative resection of intra-abdominal cancer, in patients with no sign or symptom of recurrence, to resect an occult tumour if present. (05 Mar 2000) |
| second meiotic division | The second of two consecutive divisions of the nucleus of an eukaryotic cell during the process of meiosis. It includes the following stages of meiosis: prophase II, metaphase II, anaphase II, and telophase II. (09 Oct 1997) |
| second messenger | <molecular biology> In many hormone sensitive systems the systemic hormone does not enter the target cell but binds to a receptor and indirectly affects the production of another molecule within the cell, this diffuses intracellularly to the target enzymes or intracellular receptor to produce the response. This intracellular mediator is called the second messenger. Examples include cyclic AMP, cyclic GMP, IP3 and diacylglycerol. (18 Nov 1997) |
| second messenger systems | Systems in which an intracellular signal is generated in response to an intercellular primary messenger such as a hormone or neurotransmitter. They are intermediate signals in cellular processes such as metabolism, secretion, contraction, phototransduction, and cell growth. Examples of second messenger systems are the adenyl cyclase-cyclic AMP system, the phosphatidylinositol diphosphate-inositol triphosphate system, and the cyclic GMP system. (12 Dec 1998) |
| second molar | Seventh permanent or fifth deciduous tooth in the maxilla and mandible on either side of the midsagittal plane of the head following the arch form. (05 Mar 2000) |
| second-order conditioning | The use of a previously successfully conditioned stimulus as the unconditioned stimulus for further conditioning. (05 Mar 2000) |
| second-order kinetics | A term describing the reaction rate of a chemical reaction in which the rate is proportional to the product of the concentrations (in moles) of two of the reactants (also called bimolecular kinetics), or to the square of the molar concentration of the reactant if there is only one. Such a reaction might have an equation like rate = k[A][B] or rate = k[A]2, where k is the reaction rate constant, [A] is the concentration of reactant A, and [B] is the concentration of reactant B. (09 Oct 1997) |
| second parallel pelvic plane | The plane extending from the middle of the posterior surface of the pubic symphysis to the junction of the second and third sacral vertebrae, and laterally passing through the ischial bones over the middle of the acetabulum. Synonym: second parallel pelvic plane, wide plane. (05 Mar 2000) |
| second set rejection | An accelerated rejection of a transplant that occurs when an individual has been previously sensitised to the graft. (05 Mar 2000) |
| second sight | 1. <ophthalmology> Improved near vision in the aged as a result of increased refractivity of the nucleus of the lens causing myopia. Synonym: senile lenticular myopia. 2. The power of discerning what is not visible to the physical eye, or of foreseeing future events. Such as are of a disastrous kind, the capacity of a seer or prophetic vision. Source: Websters Dictionary (01 Mar 1998) |
| second signaling system | <psychology> Pavlovian term for speech in which words are considered to be the second signals capable of producing conditioned responses. (05 Mar 2000) |