首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   492305篇
  免费   58628篇
  国内免费   299篇
  2021年   4022篇
  2018年   4809篇
  2017年   4367篇
  2016年   6877篇
  2015年   10359篇
  2014年   11687篇
  2013年   15675篇
  2012年   18512篇
  2011年   18505篇
  2010年   12117篇
  2009年   11208篇
  2008年   15917篇
  2007年   16255篇
  2006年   14612篇
  2005年   14220篇
  2004年   13821篇
  2003年   12988篇
  2002年   12271篇
  2001年   22585篇
  2000年   22656篇
  1999年   18140篇
  1998年   6569篇
  1997年   6743篇
  1996年   6507篇
  1995年   5878篇
  1994年   5931篇
  1993年   5718篇
  1992年   13901篇
  1991年   13188篇
  1990年   12853篇
  1989年   12676篇
  1988年   11299篇
  1987年   10924篇
  1986年   9976篇
  1985年   9750篇
  1984年   8252篇
  1983年   7146篇
  1982年   5576篇
  1981年   5055篇
  1980年   4693篇
  1979年   7743篇
  1978年   5971篇
  1977年   5477篇
  1976年   5107篇
  1975年   5403篇
  1974年   5865篇
  1973年   5705篇
  1972年   5133篇
  1971年   4768篇
  1970年   3947篇
排序方式: 共有10000条查询结果,搜索用时 109 毫秒
791.
792.
793.
During 1975--77 Hemophilus vaginalis bacteremia occurred post partum in eight previously healthy women. Seven had been admitted for delivery at term and one because of threatened abortion. Six underwent cesarean section. Post-partum pyrexia and neutrophilia were the main features. All the patients recovered uneventfully while receiving antibiotics. H. vaginalis is an infrequent agent of bacteremia; it affects predominantly women after obstetric trauma.  相似文献   
794.
Water hyacinth (Eichhornia crassipes), duckweed (Spirodela sp. andLemna sp.), water pennywort (Hydrocotyle ranunculoides), and kudzu (Pueraria lobata) were anaerobically fermented using an anaerobic filter technique that reduced the total digestion time from 90 d to an average of 23 d and produced 0.14-0.22 m3 CH4/kg (dry weight) (2.3-3.6 ft3/lb) from mature filters for the 3 aquatic species. Kudzu required an average digestion time of 33 d and produced an average of 0.21 m3 CH4/kg (dry weight) (3.4 ft3/lb). The anaerobic filter provided a large surface area for the anaerobic bacteria to establish and maintain an optimal balance of facultative, acid-forming, and methane-producing bacteria. Consequently the efficiency of the process was greatly improved over prior batch fermentations.  相似文献   
795.
Ester components in the surface wax from diapausing tobacco hornworm pupae, Manduca sexta L., were separated by thin layer chromatography and gas-liquid chromatography, and characterized by infrared spectroscopy and gas-liquid chromatography-mass spectrometry. Three groups of esters were identified as natural derivatives of acetic acid, acetoacetic acid, and 3-hydroxybutyric acid. The major ester fraction was identified as a mixture of C26 (10%), C27 (5%), and C28 (85%) oxoalcohol esters of acetoacetic acid. The major homolog consisted of equal amounts of 11-oxooctacosanyl 3-oxobutanoate and 12-oxooctacosanyl 3-oxobutanoate. Lesser amounts of 11- and 12-oxooctacosanyl and n-octacosanyl esters of acetic and 3-hydroxybutyric acids were also identified. The chain length distributions of these C26, C27, and C28 oxoalcohol and n-primary alcohol ester moieties, as well as the isomeric ratios for the 11- and 12-oxoalcohol isomers, were similar to the oxoaldehydes and unesterified oxoalcohols previously identified by Buckner et al (Buckner, J. S., Nelson, D. R., Haak, H., and Pomonis, J. G. (1984) J. Biol. Chem. 259, 8452-8470) as lipid components of the surface wax of M. sexta pupae.  相似文献   
796.
In previous studies we have reported on the detection of a strong e.s.r. signal in samples of normal human cervix; the signal is much reduced or absent in samples of invasive cancer of the cervix. In order to identify the species responsible for the strong signal, we have used X-, S- and Q-band e.s.r. spectroscopy. The major signal that is detectable in ground-up samples of cervix preserved at -196 degrees C has features consistent with the presence of a peroxy free radical. Good agreement with the experimental findings was obtained by computer simulation, using values for the g-tensor of gx = 2.002, gy = 2.005 and gz = 2.036. The peroxy radical is produced on grinding the normal cervix samples to a powder under liquid N2, and appears to be formed by modification of a pre-existing oxygen-containing complex. Control experiments eliminated the possibility that the strong signals seen in frozen powders prepared from normal cervix were artefacts only of the grinding procedure. Experiments with rats in vivo and with cervix samples in vitro are consistent with the conclusion that the peroxy radical is formed by disturbing the cyclo-oxygenase system that is involved in prostaglandin synthesis.  相似文献   
797.
Evidence is presented of lethal blood clot formation in fishes. A variety of factors commonly encountered under aquacultural conditions may generate such clots.  相似文献   
798.
The number of orientation, feeding and social calls emitted by pipistrelle batsPipistrellus pipistrellus Schreber, 1774 andP. pygmaeus Leach, 1825 was recorded throughout the night at eight different sites. Social calls were unaffected by weather variables, whereas orientation calls and feeding buzzes were both significantly affected by cloud and temperature conditions. The number of emissions of each call type was significantly different between sites, indicating that the bats used different sites for different activities. Significant positive correlations between all three combinations of call types occurred only during the middle of the night, corresponding with the nadir of flying insects. This suggests that bats were engaged in activities other than feeding at this time, such as territory protection or mate attraction.  相似文献   
799.
Noni (Morinda citrifolia) is increasing in worldwide popularity as a food or dietary supplement with versatile health benefits. The aim of this study was to investigate the effects of Noni fruit on anxiety symptoms in vitro. To this end, a competitive GABAa receptor-binding assay was developed. Our preliminary study indicates that the methanol crude extract of Noni fruit showed significant affinity to the gamma-aminobutyric acid A (GABAa) inhibitory neurotransmitter receptors, and displayed 75% binding inhibition of the agonist radioligand [3H] muscimol at a concentration of 100 microg/ml. Further experiments demonstrated that the MeOH extract, and its BuOH and H2O partitions, exhibited IC50 values of 22.8, 27.2, and 17.1 microg/ml, respectively, in the GABAa-binding assay. Experimental results with Noni fruit indicate the presence of competitive ligand(s), which may bind to the GABAa receptor as an agonist, and thus induce its anxiolytic and sedative effects. The study provides an in vitro rationale for one of Noni's versatile and traditional uses. In addition, an HPLC fingerprint profile of the methanolic extract of Noni fruit has been established for quality control purpose.  相似文献   
800.
The flap endonuclease, FEN1, plays a critical role in DNA replication and repair. Human FEN1 exhibits both a 5' to 3' exonucleolytic and a structure-specific endonucleolytic activity. On primer-template substrates containing an unannealed 5'-tail, or flap structure, FEN1 employs a unique mechanism to cleave at the point of annealing, releasing the 5'-tail intact. FEN1 appears to track along the full length of the flap from the 5'-end to the point of cleavage. Substrates containing structural modifications to the flap have been used to explore the mechanism of tracking. To determine whether the nuclease must recognize a succession of nucleotides on the flap, chemical linkers were used to replace an interior nucleotide. The nuclease could readily traverse this site. The footprint of the nuclease at the time of cleavage does not extend beyond 25 nucleotides on the flap. Eleven-nucleotide branches attached to the flap beyond the footprinted region do not prevent cleavage. Single- or double-thymine dimers also allow cleavage. cis-Platinum adducts outside the protected region are moderately inhibitory. Platinum-modified branch structures are completely inert to cleavage. These results show that some flap modifications can prevent or inhibit tracking, but the tracking mechanism tolerates a variety of flap modifications. FEN1 has a flexible loop structure through which the flap has been proposed to thread. However, efficient cleavage of branched structures is inconsistent with threading the flap through a hole in the protein.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号