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   796568篇
  免费   82475篇
  国内免费   250篇
  879293篇
  2018年   7931篇
  2017年   7582篇
  2016年   10637篇
  2015年   13460篇
  2014年   16029篇
  2013年   23139篇
  2012年   25978篇
  2011年   26759篇
  2010年   18279篇
  2009年   16897篇
  2008年   23925篇
  2007年   24885篇
  2006年   23267篇
  2005年   22370篇
  2004年   22144篇
  2003年   21306篇
  2002年   20826篇
  2001年   34734篇
  2000年   34195篇
  1999年   27570篇
  1998年   10181篇
  1997年   10263篇
  1996年   9841篇
  1995年   9062篇
  1994年   8721篇
  1993年   8753篇
  1992年   22334篇
  1991年   21916篇
  1990年   21370篇
  1989年   20812篇
  1988年   19081篇
  1987年   18322篇
  1986年   17091篇
  1985年   16955篇
  1984年   13936篇
  1983年   12184篇
  1982年   9230篇
  1981年   8356篇
  1980年   7755篇
  1979年   12941篇
  1978年   10202篇
  1977年   9191篇
  1976年   8802篇
  1975年   9816篇
  1974年   10485篇
  1973年   10350篇
  1972年   9477篇
  1971年   8456篇
  1970年   7384篇
  1969年   7255篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
SUMMARY: We developed a collaborative pedigree environment called CoPE. This environment includes a Java program for drawing pedigrees and a standardized system for pedigree storage. Unlike other existing pedigree programs, this software is particularly intended for epidemiologists in the sense that it allows customized automatic drawing of large numbers of pedigrees and remote and distributed consultation of pedigrees. AVAILABILITY: At http://www.infobiogen.fr/services/CoPE  相似文献   
992.
Some linear truncated analogs of endothelin-1 display potent agonistic activity at the ET(B) receptor, especially when the side chain of Trp21 is N-formylated. Then, the three-dimensional arrangements of six structurally reduced linear analogs, three formylated and three nonformylated, have been investigated by high resolution NMR spectroscopy and molecular modeling, in order to pinpoint the conformational features related to the biological activity. Two-dimensional double-quantum-filtered correlation spectroscopy (DQFCOSY), total correlation spectroscopy (TOCSY) and nuclear Overhauser enhancement spectroscopy (NOESY) were recorded and analyzed for each molecule. Interspatial distance constraints were derived from the intensity of the NOESY connectivities. The formation of hydrogen bonding was monitored from the temperature dependence of the NH chemical shifts. Molecular models calculated by means of distance geometry, simulated annealing and energy minimization, using the NMR constraints, strongly suggested a global elongated structure for the formylated analogs exhibiting biological activity, and a folded arrangement for the unformylated derivatives. Homology comparisons allowed the identification of a beta-turn-like folding of the C-terminal segment Asp18-Trp21 as a probable key-factor for activity.  相似文献   
993.
This study investigated the ability of pretreatment variables from three different domains (social-demographic, psychological, and psychophysiological) to predict posttreatment mean arterial pressure (MAP) for 59 unmedicated males with mild hypertension who were participating in a cross-cultural (USA-USSR) comparison of autogenic training and thermal biofeedback to a self-relaxation control. The overall multiple regression equation consisted of two variables and indicated that higher diastolic blood pressures during a cold pressor task were predictive of greater MAP reductions while higher scores on the Irritability subscale of the Buss-Durkee Hostility Scale were predictive of less MAP reductions. Suggestions for future research in this area are provided.  相似文献   
994.
995.
996.
The involvement of gibberellins in the control of flowering of sunflower was studied by direct application of GA3 to the apex of the plants, analysis of the endogenous levels of gibberellin-like substances at different plant ages, and indirectly by the application of paclobutrazol, an inhibitor of gibberellin synthesis. GA3 speeded-up flower initiation and floral apex development. The time of GA3 application was more critical than the amount of GA3 applied. The endogenous levels of gibberellin-like compounds increased significantly by day 15 after sowing. The application of paclobutrazol markedly delayed floral initiation and this effect was also depedent on plant age. Both GA3 and paclobutrazol had their greatest effects between 10 and 20 days after sowing suggesting that an increase in gibberellins in that time period plays a role in floral initiation.  相似文献   
997.
Arginine vasopressin (AVP) concentrations were determined in plasma and in cerebrospinal fluid (CSF) in 8 adult male patients suffering from hydrocephalus of various etiologies, before and after intravenous administration of 10 mg metoclopramide. Metoclopramide was able to increase the plasma (2.6 +/- 0.2 ng/l in basal conditions and 6.1 +/- 0.6 ng/l at 30 min) but not the CSF AVP levels. The results suggest that the neurons which secrete AVP into the CSF may be functionally different from those secreting into the peripheral circulation.  相似文献   
998.
999.

Backgound  

It has been reported that Toll-like receptor 4 (TLR4) deficiency reduces infarct size after myocardial ischemia/reperfusion (MI/R). However, measurement of MI/R injury was limited and did not include cardiac function. In a chronic closed-chest model we assessed whether cardiac function is preserved in TLR4-deficient mice (C3H/HeJ) following MI/R, and whether myocardial and systemic cytokine expression differed compared to wild type (WT).  相似文献   
1000.

Background  

Ethidium homodimer is a cell-membrane impermeant nuclear fluorochrome that has been widely used to identify necrotic cells in culture. Here, we describe a novel technique for evaluating necrosis of epithelial cells in the proximal tubule that involves perfusing ethidium homodimer through the intact rat kidney. As a positive control for inducing necrosis, rats were treated with 3.5, 1.75, 0.87 and 0.43 mg/kg mercuric chloride (Hg2+, intraperitoneal), treatments which have previously been shown to rapidly cause dose-dependent necrosis of the proximal tubule. Twenty-four h after the administration of Hg2+, ethidium homodimer (5 μM) was perfused through the intact left kidney while the animal was anesthetized. The kidney was then removed, placed in embedding medium, frozen and cryosectioned at a thickness of 5 μm. Sections were permeabilized with -20°C methanol and then stained with 4',6-diamidino-2-phenylindole (DAPI) to label total nuclei. Total cell number was determined from the DAPI staining in random microscopic fields and the number of necrotic cells in the same field was determined by ethidium homodimer labeling.  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