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   71702篇
  免费   291篇
  国内免费   598篇
系统科学   1273篇
丛书文集   711篇
教育与普及   305篇
理论与方法论   542篇
现状及发展   37117篇
研究方法   1538篇
综合类   28911篇
自然研究   2194篇
  2013年   829篇
  2012年   813篇
  2011年   2923篇
  2009年   649篇
  2008年   1069篇
  2007年   1217篇
  2006年   1265篇
  2005年   1428篇
  2004年   2589篇
  2003年   2183篇
  2002年   1894篇
  2001年   1973篇
  2000年   1470篇
  1999年   1307篇
  1998年   662篇
  1997年   776篇
  1994年   688篇
  1993年   693篇
  1992年   1230篇
  1991年   1004篇
  1990年   1141篇
  1989年   949篇
  1988年   905篇
  1987年   986篇
  1986年   995篇
  1985年   1189篇
  1984年   991篇
  1983年   850篇
  1982年   910篇
  1981年   946篇
  1980年   1079篇
  1979年   1892篇
  1978年   1676篇
  1977年   1637篇
  1976年   1388篇
  1975年   1440篇
  1974年   1557篇
  1973年   1633篇
  1972年   1727篇
  1971年   1843篇
  1970年   2125篇
  1969年   1789篇
  1968年   1757篇
  1967年   1620篇
  1966年   1390篇
  1965年   1052篇
  1959年   611篇
  1958年   927篇
  1957年   728篇
  1956年   624篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
631.
Summary The binding of coumarin to serum proteins of the rat has been demonstrated. Of the total bound coumarin (37% of injected dose), 36% was bound to the slow and fast 1 globulins, 11% to the post albumins, 10% to globulin and 9% to albumin.  相似文献   
632.
Summary The male antifertility agent 1-amino-3-chloropropan-2-ol (ACP,I) has been shown to be metabolized to-chlorohydrin (III) and metabolites of-chlorohydrin. This accounts for the similar antifertility and renal toxicity effects of both compounds.  相似文献   
633.
Summary Abnormal-8-tetrahydrocannabidiol (ABN-8-THC) failed to elicit central nervous system and cardiovascular effects in laboratory animals. Abnormal-cannabidiol (ABN-CBD) was also devoid of overt behavioral effects but produced marked hypotension with only slight bradycardia in anesthesized dogs.Acknowledgment. This work was supported by NIDA (grant No. DA-00574-01 and DA-00490) and Virginia Heart Society (grant No. RR-05697)  相似文献   
634.
635.
Summary Naja mossambica pallida venom administered i.v. (300 /kg) produces an initial brief fall in blood pressure, due to a direct myocardial depressant effect, and a sustained fall due to central depressant effect.This study was supported by University of Nairobi research grants (670–376). We also thank Mr E. Njogu for photographic assistance.  相似文献   
636.
Summary N-methanesulfonyl 16-phenoxy--tetranor PGE2 is a prostaglandin analog which is markedly more tissue selective than PGE2. This compound is 10–30 times more potent than PGE2 in animal models which are considered relevant to antifertility effects in humans. In pharmacological tests which are believed to be predictive for side effects in humans, the compound has potency either equal to or less than that of PGE2.  相似文献   
637.
Summary Contrary to earlier findings in rats, cerebral lateral ventricular infusions of 1×10–3 M or 2×10–3 M solutions of phloridzin at a rate of 2.5 l/min for 90 min had no significant stimulating effects on food intake and weight gain in hens and cocks. These different responses to intraventricular phloridzin might reflect a difference of sensitivity to the inhibitory action of phloridzin on glucose transport in cerebral cells or certain peculiarities of mechanisms controlling food intake in chickens.  相似文献   
638.
Cell lineage analysis of germ cells of Drosophila melanogaster.   总被引:1,自引:0,他引:1  
M Nissani 《Nature》1977,265(5596):729-731
  相似文献   
639.
P Nevers  H Saedler 《Nature》1977,268(5616):109-115
Transposable genetic elements in prokaryotes and eukaryotes, when inserted at a given locus, can control expression of the locus and cause large scale rearrangements of adjacent DNA sequences. Striking similarities in genetic behaviour between the two groups of elements have led to the proposal of a molecular model of eukaryotic controlling elements, and to suggestions about the part such elements may play in evolution and differentiation.  相似文献   
640.
Leask MJ 《Nature》1977,267(5607):144-145
Migratory birds and homing pigeons can apparently obtain directional information from the Earth's magnetic field. The effect is difficult to detect, and discussion of the possible process of magnetic field detection by birds seems so far to have foundered on the simple fact that the orientational effect of the Earth's magnetic field on a single electron spin associated with a molecule of animal tissue would be of the order 10(-8) eV--almost certainly too small to be detectable biologically. Here I direct attention to a process which would overcome this basic problem, and which also seems to provide an explanation of all the main features of published data. It is a mechanism in principle only, however, and is discussed here in no more detail than is necessary to clarify the basic ideas and to provide a basis for further investigation.  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