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   304541篇
  免费   35413篇
  国内免费   156篇
  340110篇
  2018年   2579篇
  2016年   3366篇
  2015年   4451篇
  2014年   5457篇
  2013年   7408篇
  2012年   8502篇
  2011年   8736篇
  2010年   5710篇
  2009年   5545篇
  2008年   7775篇
  2007年   7994篇
  2006年   7839篇
  2005年   7515篇
  2004年   7353篇
  2003年   7170篇
  2002年   7018篇
  2001年   17403篇
  2000年   17543篇
  1999年   13427篇
  1998年   3984篇
  1997年   4233篇
  1996年   3956篇
  1995年   3600篇
  1994年   3545篇
  1993年   3652篇
  1992年   10469篇
  1991年   10378篇
  1990年   9876篇
  1989年   9700篇
  1988年   9043篇
  1987年   8420篇
  1986年   7603篇
  1985年   7474篇
  1984年   5912篇
  1983年   5138篇
  1982年   3700篇
  1981年   3264篇
  1980年   3092篇
  1979年   5400篇
  1978年   4162篇
  1977年   3794篇
  1976年   3336篇
  1975年   3812篇
  1974年   4009篇
  1973年   3928篇
  1972年   3453篇
  1971年   3207篇
  1970年   2842篇
  1969年   2758篇
  1968年   2431篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
993.
The upper limb nerves of 8 human embryos (Carnegie stages 13-21) were studied by reconstruction. In stage 13, upper limb nerves (C5-T1) extended from the spinal cord. In stage 14, these nerves united to form the nascent brachial plexus. In stages 16 and 17, the median nerve, the radial nerve and the ulnar nerve entered into the hand plate. In stages 20 and 21, the upper limb nerves were observed in an orientation and arrangement similar to those in the adult.  相似文献   
994.
To identify the sites important for the different biological activities of human interleukin-1 alpha (hIL-1 alpha), 56 single-amino acid-substituted mutants of hIL-1 alpha were produced in Escherichia coli using site-directed mutagenesis, and were examined for their biological activities such as mouse lymphocyte activating factor activity (LAF activity), cytostatic activity against human melanoma cells A-375 (A375 activity) and prostaglandin E2 (PGE2) inducing activity in human osteosarcoma cells MG-63 (PEI activity). Two amino acid residues, Asp26 and Asp151, were found to be important for these activities. The replacement of Asp26 by Val caused a decrease in LAF and PEI activities by one or two orders of magnitude and a slight decrease in A375 activity. The Tyr or Phe substitution for Asp151 caused decreases in LAF and A375 activities by one or two orders of magnitude and complete loss of PEI activity. The change from Asp151 to Lys or Arg resulted in marked decrease in LAF activity and complete loss of A375 and PEI activities. Since Asp26 and Asp151 are close to each other in the three-dimensional structure, the region involving these amino acids seems to be important for the biological activities of hIL-1 alpha.  相似文献   
995.
Calmodulin activates the ATP-dependent transport of Ca2+. The V0 value for this reaction in the absence of calmodulin is 0.82, that in the presence of 10(-7) M calmodulin is 5 times as high, i. e. 4.5 nmol 45Ca2+/mg protein/min. The Vmax value in the absence of calmodulin is 2.07, that with the activator is 4.33 nmol 45Ca2+/mg protein/min. The corresponding Km values are 0.75 X 10(-6) M and 0.66 X 10(-7) M, respectively, i. e., the affinity of the Ca-pump for Ca2+ increases. The half-maximum Ca-binding activity of calmodulin measured with a help of the fluorescent probe, N-phenyl-1-naphthylamine (PNA), is observed at 5 X 10(-7) M Ca2+. Mg2+ (3 mM) decreases 10-fold the Ca-binding affinity. No significant effect of ATP on the Ca-binding properties of calmodulin was found; the Hill coefficient is suggestive of a positive cooperativity of this reaction. A comparison of dependences of the calmodulin-stimulated component of ATP-dependent transport of Ca2+ in myometrium plasma membranes and of the Ca-binding activity of calmodulin measured with a help of PNA suggests that the effect of calmodulin on the affinity of the Ca-pump for Ca2+ can also be realized when some (but not all) Ca-binding sites in the calmodulin molecule are saturated with Ca2+.  相似文献   
996.
997.
998.
Upper gastrointestinal endoscopy is a valuable diagnostic tool, but for an endoscopy service to be effective it is essential that it is not overloaded with inappropriately referred patients. A joint working party in Britain has considered the available literature on indications for endoscopy, assessed standard practice through a questionnaire, and audited randomly selected cases using an independent panel of experts and an American database system. They used these data to produce guidelines on the appropriate and inappropriate indications for referral for endoscopy, although they emphasise that under certain circumstances there may be reasons to deviate from the advice given. The need for endoscopy is most difficult to judge in patients with dyspepsia, and this aspect is discussed in detail. Early endoscopy will often prove more cost effective than delaying until the indications are clearer.  相似文献   
999.
1000.
The citrus whiteflyDialeurodes citri Ashmead is one of the most important insect pests of citrus groves on the Black Sea coast of the Caucasus and on the Caspian Sea coast of the Talysh Mountains; it is the most important citrus pest of trench cultivation in Central Asia and in glasshouses. The parasitoidEncarsia lahorensis Howard was introduced in 1983 into Russia (Moscow) from Pakistan. It was reared in a laboratory of the All-Russian Plant Quarantine Institute. The parasitoid was released as follows: in 1985 in Adzharia (Batumi); in 1987 in a glasshouse in Russia (near Moscow) and in 1988-1990 in Central Asia - Uzbekistan (Namangan) and other regions. It established well in all places of release. The rate of parasitization ofD. citri byE. lahorensis reached 50–52% in the orchard in Batumi five years after the first release, 56–60% in the glasshouse in Moscow three years after the first release, and 50% in trenches in Namangan two years after the first release. Levels of the pest populations decreased dramatically in the glasshouse and trenches. The efficiency of the additional feeding ofE. lahorensis adults on the host larvae was evaluated to 26 to 60%. The differences between populations of the parasitoid in the three different areas are discussed.  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