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   44063篇
  免费   3949篇
  国内免费   33篇
  2022年   284篇
  2021年   606篇
  2020年   409篇
  2019年   498篇
  2018年   654篇
  2017年   570篇
  2016年   1059篇
  2015年   1795篇
  2014年   1916篇
  2013年   2503篇
  2012年   3051篇
  2011年   3120篇
  2010年   1978篇
  2009年   1698篇
  2008年   2566篇
  2007年   2557篇
  2006年   2337篇
  2005年   2388篇
  2004年   2326篇
  2003年   2220篇
  2002年   2156篇
  2001年   533篇
  2000年   433篇
  1999年   526篇
  1998年   614篇
  1997年   478篇
  1996年   428篇
  1995年   406篇
  1994年   364篇
  1993年   355篇
  1992年   396篇
  1991年   341篇
  1990年   281篇
  1989年   291篇
  1988年   292篇
  1987年   260篇
  1986年   245篇
  1985年   309篇
  1984年   330篇
  1983年   265篇
  1982年   354篇
  1981年   286篇
  1980年   267篇
  1979年   199篇
  1978年   230篇
  1977年   214篇
  1976年   185篇
  1975年   167篇
  1974年   201篇
  1973年   200篇
排序方式: 共有10000条查询结果,搜索用时 140 毫秒
81.
82.
83.
A hand-reared raccoon (Procyon lotor) kit had severe diarrhea and died within 24 hr. Gross and histopathologic findings were compatible with a diagnosis of viral enteritis. The immunoperoxidase test revealed rotavirus group A antigen in the intestinal mucosa. This is the first record of rotaviral enteritis in a raccoon.  相似文献   
84.
85.
86.
Azospirillum strains isolated from the roots and rhizosphere of some plants growing in West Bengal were subjected to qualitative and quantitative evaluation for poly-3-hydroxybutyrate (PHB) production. Out of the total 49 isolates, 13 (26%) were confirmed as PHB producers according to staining and chemical assay methods. The majority of these strains belonged toAzospirillum brasilense butA. amazonense andA. lipoferum were also present. When grown in the presence of NH4Cl in the medium, the PHB content of the strains ranged from 1 to 14% of cell dry mass. The identity of the PHB extracted fromAzospirillum strain 24P-N-72 was confirmed by the characteristic UV and IR absorption peaks at 235 nm and 1730 cm−1, respectively.  相似文献   
87.
Non-B, non-T cells from spleen and bone marrow cells produce IL-4 in response to cross-linkage of high affinity receptors for Fc epsilon R or Fc gamma RII, and to treatment with calcium ionophores. Cells bearing high affinity Fc epsilon R constituted 1 to 2% of non-B, non-T cells of spleen and of total bone marrow cells from naive donors. In mice whose immune systems had been polyclonally activated by injection with anti-IgD antibodies or had been infected with Nippostrongylus brasiliensis larvae, the frequency of Fc epsilon R+ cells in splenic non-B, non-T cells was also 1 to 2% but in bone marrow from anti-IgD-injected mice donors the frequency was approximately 5%. Cell sorting experiments revealed that all of the capacity to produce IL-4 in response to immobilized IgE or IgG2a or to ionomycin was found in the Fc epsilon R+ fraction. Among the Fc epsilon R+ spleen cells from naive donors, the frequency of IL-4-producing cells was 1/20 to 1/40 whereas in mice that had been injected with anti-IgD or infected with N. brasiliensis, the frequency of IL-4 producing cells in the Fc epsilon R+ population was approximately 1/5.  相似文献   
88.
Examining how pathogens cross species boundaries, spread within species, and persist within their hosts is an essential part of understanding the factors that underpin the evolution of virulence and host resistance. Here, we review current knowledge about the genetic diversity, molecular epidemiology, prevalence, and pathogenicity of simian immunodeficiency viruses (SIVs). SIVs have crossed species boundaries from simian hosts to humans on at least 12 separate occasions, one of which led to the global HIV–AIDS crisis. Though SIVs infect a wide range of primates, scientists have only recently begun to describe the natural history of SIV infection in their natural hosts. Several new studies reveal how both viral and host factors are responsible for the transmission to, and adaptation in, new hosts. These studies also suggest that the spread of the virus may be affected by host-specific traits, including social structure, mating system and demographic history. These studies challenge the traditional view that SIV is relatively benign in its natural host, and instead suggest that a highly dynamic relationship exists between SIV and its simian hosts.  相似文献   
89.
A perceived distinctive feature of cryoablation is the stability (cryoadherence) of the catheter tip during cold temperatures at the desired location, even during tachycardia. We report the case report of a young patient with a parahisian accessory pathway where stability of the ablation catheter was not achieved despite using the cryocatheter with a steerable sheath. Ultimately, stability at the desired location was achieved robotically by means of Hansen system (Hansen Medical, Mountain View, CA, USA).  相似文献   
90.
For over a century microbiologists and immunologist have categorized microorganisms as pathogenic or non-pathogenic species or genera. This definition, clearly relevant at the strain and species level for most bacteria, where differences in virulence between strains of a particular species are well known, has never been probed at the strain level in fungal species. Here, we tested the immune reactivity and the pathogenic potential of a collection of strains from Aspergillus spp, a fungus that is generally considered pathogenic in immuno-compromised hosts. Our results show a wide strain-dependent variation of the immune response elicited indicating that different isolates possess diverse virulence and infectivity. Thus, the definition of markers of inflammation or pathogenicity cannot be generalized. The profound understanding of the molecular mechanisms subtending the different immune responses will result solely from the comparative study of strains with extremely diverse properties.  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