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   326734篇
  免费   38061篇
  国内免费   948篇
  2018年   3179篇
  2016年   4158篇
  2015年   5715篇
  2014年   6715篇
  2013年   9386篇
  2012年   10439篇
  2011年   10493篇
  2010年   7107篇
  2009年   6300篇
  2008年   9052篇
  2007年   9409篇
  2006年   8762篇
  2005年   8391篇
  2004年   8416篇
  2003年   8141篇
  2002年   7973篇
  2001年   14431篇
  2000年   14439篇
  1999年   11570篇
  1998年   3974篇
  1997年   4056篇
  1996年   3858篇
  1995年   3865篇
  1994年   3890篇
  1993年   3733篇
  1992年   9850篇
  1991年   9439篇
  1990年   9243篇
  1989年   8982篇
  1988年   8373篇
  1987年   8006篇
  1986年   7242篇
  1985年   7332篇
  1984年   6056篇
  1983年   5309篇
  1982年   4182篇
  1981年   3694篇
  1980年   3515篇
  1979年   6096篇
  1978年   4546篇
  1977年   4271篇
  1976年   3965篇
  1975年   4295篇
  1974年   4653篇
  1973年   4712篇
  1972年   4353篇
  1971年   4067篇
  1970年   3438篇
  1969年   3335篇
  1968年   2965篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
OBJECTIVE--To identify risk factors for all cause mortality according to glucose tolerance status. DESIGN--Cohort study with an average 15.6 years'' follow up. SETTING--Paris, France. SUBJECTS--7166 working men aged 44-55 in 1968-72 in the Paris prospective study cohort, with non-insulin dependent diabetes or known result of two hour 75 g oral glucose tolerance test. MAIN OUTCOME MEASURES--Risk factors for death from all causes. RESULTS--128 men were known to be diabetic, 180 had diabetes diagnosed, and 697 had impaired glucose tolerance diagnosed. Compared with normoglycaemic men the relative risks of death in these groups were 2.0 (95% confidence interval 1.4 to 3.0), 2.7 (2.0 to 3.6), and 1.6 (1.3 to 2.0) respectively. Obesity, smoking, high blood pressure, and high non-esterified fatty acid concentration were risk factors for death in all subjects and were unaffected by glucose tolerance. The risks for fasting and two hour insulin concentrations and mean corpuscular volume were two times higher in known diabetic men than in men not known to be diabetic. Central obesity was significant only in men not known to be diabetic (1.6 (1.4 to 1.9)). In known diabetic men a two hour glucose concentration higher than 11.1 mmol/l carried a relative risk of death of 3.8 (1.4 to 9.4). CONCLUSIONS--Diabetic men have similar risk factors for early mortality to other men but are at higher risk from hyperinsulinaemia, hyperglycaemia, and high mean corpuscular volume.  相似文献   
12.
13.
14.
15.
An investigation on the influence of lead toxicity on some of the hepatic enzymes was studied in rats both after a shorter interval of 15 d and after longer intervals of 60 and 90 d. Three different doses of lead as 5, 10, and 50 mg/kg body wt were administered orally on every alternate day. Whereas significant inhibition of succinic dehydrogenase was seen following lead poisoning, the activity acid and alkaline phosphatase increased with lead intoxication. The histoarchitecture of the liver was grossly intact. Liver accumulated less lead compared to kidney at 60 and 90 d.  相似文献   
16.
17.
18.
The Escherichia coli secA gene, whose translation is responsive to the proficiency of protein export within the cell, is the second gene in a three-gene operon and is flanked by gene X and mutT. By using gene fusion and oligonucleotide-directed mutagenesis techniques, we have localized this translationally regulated site to a region at the end of gene X and the beginning of secA. This region has been shown to bind SecA protein in vitro. These studies open the way for a direct investigation of the mechanism of secA regulation and its coupling to the protein secretion capability of the cell.  相似文献   
19.
In two patients with fatal Reye's Syndrome, total ornithine carbamoyltransferase (OCTase) activity in the liver was 50 and 75% of that found in three control livers. The levels of enzymatic activity would not be expected to have resulted in the 7- and 17-fold elevations in plasma ammonia levels found in the patients. Levels of 47 and 60% of the OCTase activity, however, were found in the cytosolic fraction compared to an average of 7% for control livers. Thus, the amount of enzymatic activity in the mitochondrial fractions was only 20 and 30% of that found in control mitochondrial fractions. This study suggests that, if only mitochondrial OCTase is active in the urea cycle, the decreases in functional enzyme found in Reye's Syndrome may be considerably greater than that reflected in total enzyme assays.  相似文献   
20.
Triterpenoid saponins from Medicago hispida.   总被引:2,自引:0,他引:2  
S B Mahato 《Phytochemistry》1991,30(10):3389-3393
Soyasaponin III has been characterized and the structure of a new triterpenoid saponin, hispidacin, has been elucidated as soyasapogenol B-3-O-alpha-L-rhamnopyranopyranosyl(1----2)- beta-D-glucopyranosyl(1----2)-beta-D-glucuronopyranoside by a combination of fast-atom bombardment mass spectrometry, 13C NMR spectroscopy, and some chemical transformations. Mechanism of transformation of soyasapogenol B to soyasapogenols D, and F has also been rationalized.  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