全文获取类型
收费全文 | 567473篇 |
免费 | 56565篇 |
国内免费 | 1087篇 |
出版年
2018年 | 17283篇 |
2017年 | 16168篇 |
2016年 | 14206篇 |
2015年 | 9269篇 |
2014年 | 9870篇 |
2013年 | 13612篇 |
2012年 | 20574篇 |
2011年 | 30495篇 |
2010年 | 24234篇 |
2009年 | 19792篇 |
2008年 | 25107篇 |
2007年 | 27607篇 |
2006年 | 12853篇 |
2005年 | 13856篇 |
2004年 | 13327篇 |
2003年 | 12957篇 |
2002年 | 12120篇 |
2001年 | 21973篇 |
2000年 | 22039篇 |
1999年 | 17445篇 |
1998年 | 6275篇 |
1997年 | 6515篇 |
1996年 | 6332篇 |
1995年 | 5718篇 |
1994年 | 5773篇 |
1993年 | 5618篇 |
1992年 | 13693篇 |
1991年 | 13025篇 |
1990年 | 12662篇 |
1989年 | 12532篇 |
1988年 | 11186篇 |
1987年 | 10808篇 |
1986年 | 9877篇 |
1985年 | 9659篇 |
1984年 | 8206篇 |
1983年 | 7115篇 |
1982年 | 5539篇 |
1981年 | 5015篇 |
1980年 | 4663篇 |
1979年 | 7693篇 |
1978年 | 5927篇 |
1977年 | 5433篇 |
1976年 | 5069篇 |
1975年 | 5381篇 |
1974年 | 5816篇 |
1973年 | 5659篇 |
1972年 | 5695篇 |
1971年 | 5309篇 |
1970年 | 3933篇 |
1969年 | 3855篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
A series of measurements of blood pressure in normotensive and hypertensive subjects showed that measurements made with a sphygmomanometer with the arm dependent by the side were consistently higher than those made with the arm horizontal at heart level. The mean difference in a group of 90 hypertensive outpatients was 11/12 mm Hg. Failure to appreciate the importance of arm position may lead to erroneous measurements of blood pressure. This has important implications for clinical practice and research. 相似文献
82.
83.
Genes for the biosynthesis of spinosyns: applications for yield improvement in Saccharopolyspora spinosa 总被引:2,自引:0,他引:2
K Madduri C Waldron P Matsushima M C Broughton K Crawford D J Merlo R H Baltz 《Journal of industrial microbiology & biotechnology》2001,27(6):399-402
Spinosyns A and D are the active ingredients in an insect control agent produced by fermentation of Saccharopolyspora spinosa. Spinosyns are macrolides with a 21-carbon, tetracyclic lactone backbone to which the deoxysugars forosamine and tri-O-methylrhamnose are attached. The spinosyn biosynthesis genes, except for the rhamnose genes, are located in a cluster that
spans 74 kb of the S. spinosa genome. DNA sequence analysis, targeted gene disruptions and bioconversion studies identified five large genes encoding type
I polyketide synthase subunits, and 14 genes involved in sugar biosynthesis, sugar attachment to the polyketide or cross-bridging
of the polyketide. Four rhamnose biosynthetic genes, two of which are also necessary for forosamine biosynthesis, are located
outside the spinosyn gene cluster. Duplication of the spinosyn genes linked to the polyketide synthase genes stimulated the
final step in the biosynthesis — the conversion of the forosamine-less pseudoaglycones to endproducts. Duplication of genes
involved in the early steps of deoxysugar biosynthesis increased spinosyn yield significantly. Journal of Industrial Microbiology & Biotechnology (2001) 27, 399–402.
Received 31 May 2001/ Accepted in revised form 09 July 2001 相似文献
84.
85.
The heart rate and respiratory patterns in hypoxia are not well documented in unanaesthetized intact newborn animals. We studied heart rate and respiratory patterns during quiet sleep in 17% inspired O2 in 31 unanaesthetized newborns of five species: lamb, piglet, puppy, kitten, and rabbit. There was no significant change in mean heart rate and respiratory rate with hypoxia for any species. Brief apneas greater than 5 s were frequent (5-8/h), both in 21 and 17% O2 only in lambs and puppies. No sustained periodic breathing was induced by hypoxia. Thus, mild hypoxia has little steady-state effect on heart rate and respiratory rate and pattern in these unanaesthetized newborns. These findings are compatible with depressed chemoreceptor threshold, but indicate a remarkably mature respiratory pattern in full-term newborns of these species. 相似文献
86.
Mechanisms of changes in human hemodynamics under the conditions of microgravity and prognosis of postflight orthostatic stability 总被引:1,自引:0,他引:1
The mechanisms of hemodynamic responses to orthostatic stresses and orthostatic stability (OS) of cosmonauts were studied before and after short-and long-term spaceflights (SFs) using orthostatic tests, as well as before, during, and after SFs using ultrasonic methods in tests with exposure to lower body negative pressure (LBNP). The capacitance and distensibility of the veins of the lower extremities were studied using occlusive air plethysmography before, during, and after SFs of different durations. A stay in microgravity has been proved to result in detraining of, mainly, the vascular mechanisms of compensating orthostatic perturbations. It has been established that the decrease in OS under the influence of microgravity is determined by a reduction of the vasoconstrictive ability of large blood vessels of the lower extremities; an increase in venous distensibility and capacitance of the legs; and an impairment of blood flow regulation, which leads to a cerebral blood flow deficit in orthostatic stresses, and of the initial individual OS before the flight. The results of preflight studies of hemodynamics by ultrasonic methods at LBNP and the data of orthostatic tests before SFs make it possible to predict the degree of decrease of OS after an SF proceeding in the normal mode. At the same time, the data of ultrasonic blood flow examination provide more a accurate estimation of OS and make it possible to assess the physiological reserves of hemodynamic regulation and to reveal the loss of regulation capacity even in cases where integrated indices (heart rate and blood pressure) are within the normal ranges. 相似文献
87.
88.
89.
90.
Synaptonemal complexes and meiosis in myxomycetes 总被引:4,自引:0,他引:4
Synaptonemal complexes (SC) have been observed in spores 18–24 hr past cleavage in natural fruitings of Physarum cinereum, P. bogoriense, Hemitrichia stipitata, Tubifera ferruginosa, and Arcyria incarnata. Laboratory fruitings of Arcyria cinerea, Stemonitis herbatica, and a homothallic isolate of Physarum pusillum also have SC's present in spores during the same postcleavage period. The presence of these paired chromosomes of meiotic prophase in spores of species collected in nature and in a diversity of taxa suggests that the usual position of meiosis in Myxomycetes is inside the postcleavage spore. Criteria are proposed for evaluating the validity of the SC as an indicator of meiosis. 相似文献