全文获取类型
收费全文 | 50231篇 |
免费 | 4589篇 |
国内免费 | 31篇 |
专业分类
54851篇 |
出版年
2022年 | 365篇 |
2021年 | 761篇 |
2020年 | 454篇 |
2019年 | 570篇 |
2018年 | 706篇 |
2017年 | 649篇 |
2016年 | 1046篇 |
2015年 | 1758篇 |
2014年 | 1944篇 |
2013年 | 2553篇 |
2012年 | 3237篇 |
2011年 | 3289篇 |
2010年 | 2140篇 |
2009年 | 1909篇 |
2008年 | 2721篇 |
2007年 | 2828篇 |
2006年 | 2761篇 |
2005年 | 2641篇 |
2004年 | 2545篇 |
2003年 | 2408篇 |
2002年 | 2309篇 |
2001年 | 548篇 |
2000年 | 456篇 |
1999年 | 619篇 |
1998年 | 735篇 |
1997年 | 548篇 |
1996年 | 469篇 |
1995年 | 434篇 |
1994年 | 415篇 |
1993年 | 440篇 |
1992年 | 481篇 |
1991年 | 420篇 |
1990年 | 408篇 |
1989年 | 398篇 |
1988年 | 371篇 |
1987年 | 332篇 |
1986年 | 352篇 |
1985年 | 379篇 |
1984年 | 457篇 |
1983年 | 374篇 |
1982年 | 478篇 |
1981年 | 494篇 |
1980年 | 394篇 |
1979年 | 323篇 |
1978年 | 338篇 |
1977年 | 294篇 |
1976年 | 330篇 |
1975年 | 225篇 |
1974年 | 277篇 |
1973年 | 276篇 |
排序方式: 共有10000条查询结果,搜索用时 21 毫秒
41.
42.
43.
Breast microcysts are considered to be a normal findings in the adult female breast without any increased risk of developing carcinomatous change. Breast cysts fluid contains steroid but not studies have been reported on the ability of breast microcysts to metabolise steroid hormones. It was, therefore, the aim of this study to identify the metabolites formed on incubation of radiolabelled testosterone with microcysts. In all instances dihydrotestosterone and androstenedione were formed. Oestrogens were not identified. Tis study, therefore, provides evidence for th presence of 5-alpha-reductase and 17-oxidoreductase enzyme systems in breast microcysts. 相似文献
44.
James A. Wanner 《American journal of physical anthropology》1977,47(1):99-102
An analysis of the anterior patellar groove of the human femur shows considerable variation in its medial and lateral spects. The groove itself, measured by the angle it encloses, shows considerably less variation than its individual components. The suggested functional relationship between bicondylar angle and lateral elevation of the patellar groove does not obtain for this sample. 相似文献
45.
46.
OBJECTIVE: To estimate the prevalence and distribution of elevated blood pressure (BP) among Canadian adults and to determine the level of control, treatment, awareness and prevalence of other risk factors among adults with high BP. DESIGN: Population-based cross-sectional surveys. SETTING: Nine Canadian provinces, from 1986 to 1990. PARTICIPANTS: A probability sample of 26,293 men and women aged 18 to 74 years was selected from the health insurance registers in each province. For 20,582 subjects, BP was measured at least twice. Nurses administered a standard questionnaire and recorded two BP measurements using a standardized technique. Two further BP readings, anthropometric measurements and a blood specimen for lipid analysis were obtained from those subjects who attended a clinic. OUTCOME MEASURES: Mean values of systolic and diastolic BP, prevalence of elevated BP using different criteria, and prevalence of smoking, elevated blood cholesterol, body mass index, physical activity and presence of diabetes by high BP status are reported. MAIN RESULTS: Sixteen percent of men and 13% of women had diastolic BP of 90 mm Hg or greater or were on treatment (or both). About 26% of these subjects were unaware of their hypertension, 42% were being treated and their condition controlled, 16% were treated and not controlled, and 16% were neither treated nor controlled. Use of non-pharmacologic treatment of high BP with or without medication was low (22%). Hypertensive subjects showed a higher prevalence of elevated total cholesterol, high body mass index, diabetes and sedentary lifestyle than normotensive subjects. Most people with elevated BP were in the 90 to 95 mm Hg range for diastolic pressure and 140 to 160 mm Hg range for systolic pressure. Prevalence of high isolated systolic BP sharply increased in men (40%) and women (49%) 65 to 74 years old. CONCLUSIONS: The relatively low level of control of elevated BP calls for population and individual strategies, stressing a non-pharmacologic approach and addressing isolated systolic hypertension in the elderly. 相似文献
47.
48.
49.
The Escherichia coli dnaW mutation is an allele of the adk gene 总被引:3,自引:0,他引:3
Joan M. Henson Aleksandra Blinkowa James R. Walker 《Molecular & general genetics : MGG》1982,186(4):488-492
Summary A dnaW mutant, isolated on the basis of inability to effect conjugal DNA transfer at high temperature, has been shown by complementation and enzyme assay to be defective in the adk (adenylate kinase; EC 2.7.4.3) locus. The adk mutant, known to have reduced ATP concentration at the nonpermissive temperature (Cousin and Belaich 1966), was used to demonstrate a donor energy requirement for stable aggregate formation and for chromosome transfer in conjugation. 相似文献
50.