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   105789篇
  免费   1321篇
  国内免费   886篇
  107996篇
  2023年   69篇
  2022年   157篇
  2021年   271篇
  2020年   160篇
  2019年   193篇
  2018年   12042篇
  2017年   10870篇
  2016年   7823篇
  2015年   1242篇
  2014年   1036篇
  2013年   1205篇
  2012年   5425篇
  2011年   13871篇
  2010年   12599篇
  2009年   8787篇
  2008年   10589篇
  2007年   12199篇
  2006年   1027篇
  2005年   1253篇
  2004年   1625篇
  2003年   1648篇
  2002年   1393篇
  2001年   362篇
  2000年   243篇
  1999年   129篇
  1998年   152篇
  1997年   110篇
  1996年   86篇
  1995年   71篇
  1994年   77篇
  1993年   100篇
  1992年   73篇
  1991年   85篇
  1990年   50篇
  1989年   33篇
  1988年   41篇
  1987年   36篇
  1986年   24篇
  1985年   16篇
  1984年   31篇
  1983年   38篇
  1981年   17篇
  1980年   15篇
  1978年   11篇
  1975年   13篇
  1972年   249篇
  1971年   285篇
  1965年   14篇
  1962年   24篇
  1944年   12篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
As revealed in earlier studies, the antinocifensive effect of morphine is brought about, among other things, with involvement of serotoninergic transmission mechanisms. In this context the role of the serotoninergic raphe-hippocampus system has been studied in this paper. Topical microinjections of serotonin into the dorsal hippocampus increased morphine analgesia in a dose-dependent fashion, while application into the striatum had no effect. Morphine injections into the median raphe nucleus in relatively low doses exert an antinocifensive effect which is inhibitable by methysergide. Lysergic acid diethylamide administered into the median raphe nucleus also abolished the effect of morphine in a dose-dependent manner. The results in connection with literature data lend support to the presumed integrative function of the serotoninergic raphe-hippocampus system in the mechanism of antinocifensive action of morphine.  相似文献   
52.
As compared to cutaneous leishmaniasis, vaccination against visceral leishmaniasis (VL) has received limited attention. In this study, we demonstrate for the first time that an UDP-Galactose: N-acetylglucosamine β 1–4 galactosyltransferase (GenBank Accession No. EF159943) expressing attenuated LD clonal population (A-LD) is able to confer protection against the experimental challenge with the virulent LD AG83 parasite. A-LD was also effective in established leishmania infection. The vaccinated animals showed both cell mediated (in vitro T-cell proliferation, and DTH response) and humoral responses (Th1 type). These results demonstrate the potential of the attenuated clones as an immunotherapeutic and immunoprophylactic agent against visceral leishmaniasis.  相似文献   
53.
54.
Shallow-water vegetated estuarine habitats, notably seagrass, mangrove and saltmarsh, are known to be important habitats for many species of small or juvenile fish in temperate Australia. However, the movement of fish between these habitats is poorly understood, and yet critical to the management of the estuarine fisheries resource. We installed a series of buoyant pop nets in adjacent stands of seagrass, mangrove and saltmarsh in order to determine how relative abundance of fishes varied through lunar cycles. Nets were released in all habitats at the peak of the monthly spring tide for 12 months, and in the seagrass habitat at the peak of the neap tide also. The assemblage of fish in each habitat differed during the spring tides. The seagrass assemblage differed between spring and neap tide, with the neap tide assemblage showing greater abundances of fish, particularly those species which visited the adjacent habitats when inundated during spring tides. The result supports the hypothesis that fish move from the seagrass to the adjacent mangrove and saltmarsh during spring tides, taking advantage of high abundances of zooplankton, and use seagrass as a refuge during lower tides. The restoration and preservation of mangrove and saltmarsh utility as fish habitat may in some situations be linked to the proximity of available seagrass.  相似文献   
55.
56.
Pathogens causing bovine respiratory tract disease in Finland were investigated. Eighteen cattle herds with bovine respiratory disease were included. Five diseased calves from each farm were chosen for closer examination and tracheobronchial lavage. Blood samples were taken from the calves at the time of the investigation and from 86 calves 3–4 weeks later. In addition, 6–10 blood samples from animals of different ages were collected from each herd, resulting in 169 samples. Serum samples were tested for antibodies to bovine parainfluenza virus-3 (PIV-3), bovine respiratory syncytial virus (BRSV), bovine coronavirus (BCV), bovine adenovirus-3 (BAV-3) and bovine adenovirus-7 (BAV-7). About one third of the samples were also tested for antibodies to bovine virus diarrhoea virus (BVDV) with negative results. Bacteria were cultured from lavage fluid and in vitro susceptibility to selected antimicrobials was tested. According to serological findings, PIV-3, BAV-7, BAV-3, BCV and BRSV are common pathogens in Finnish cattle with respiratory problems. A titre rise especially for BAV-7 and BAV-3, the dual growth of Mycoplasma dispar and Pasteurella multocida, were typical findings in diseased calves. Pasteurella sp. strains showed no resistance to tested antimicrobials. Mycoplasma bovis and Mannheimia haemolytica were not found.  相似文献   
57.
The effects of invasive macrophytes, water level fluctuations and predation on freshwater unionids Pyganodon grandis and Utterbackia imbecillis were studied in three small impoundments in Northeastern Texas in 2003–2005. Mussel density was sampled with quadrats. Mortality, associated with the water level fluctuations and predation, was estimated by collecting dead shells on the shore at about two month intervals. In two ponds, horizontal distribution of unionids was limited by dense beds of invasive and noxious macrophytes (mainly Eurasian watermilfoil Myriophyllum spicatum and American lotus Nelumbo lutea): mussel densities were significantly lower in these macrophyte beds (P < 0.001). In the third pond with the lowest density of macrophytes (stonewort Chara sp.), unionids were distributed more evenly, and the average unionid biomass was the highest among all ponds studied. Vertical distribution of unionids in all ponds was likely limited by low oxygen at depth >2 m. The total amount of shells found on the shore per year varied from 0.1% to 28% of the total population in the pond and was negatively correlated with water level (r = −0.72 to −0.81, P < 0.005). Mammalian predators consumed up to 19% of the total unionid population and predation was facilitated by water level fluctuations. Handling editor: K. Martens  相似文献   
58.
59.

Background  

It has long been known that rates of synonymous substitutions are unusually low in mitochondrial genes of flowering and other land plants. Although two dramatic exceptions to this pattern have recently been reported, it is unclear how often major increases in substitution rates occur during plant mitochondrial evolution and what the overall magnitude of substitution rate variation is across plants.  相似文献   
60.
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