全文获取类型
收费全文 | 1306411篇 |
免费 | 115995篇 |
国内免费 | 1412篇 |
出版年
2021年 | 17794篇 |
2020年 | 12599篇 |
2019年 | 16158篇 |
2018年 | 17478篇 |
2017年 | 16301篇 |
2016年 | 27941篇 |
2015年 | 42359篇 |
2014年 | 50342篇 |
2013年 | 76632篇 |
2012年 | 35193篇 |
2011年 | 23179篇 |
2010年 | 43093篇 |
2009年 | 44551篇 |
2008年 | 22230篇 |
2007年 | 20212篇 |
2006年 | 25741篇 |
2005年 | 26830篇 |
2004年 | 26257篇 |
2003年 | 23847篇 |
2002年 | 22142篇 |
2001年 | 31282篇 |
2000年 | 28500篇 |
1999年 | 29380篇 |
1998年 | 25165篇 |
1997年 | 24932篇 |
1996年 | 24584篇 |
1995年 | 22746篇 |
1994年 | 22579篇 |
1993年 | 21627篇 |
1992年 | 25944篇 |
1991年 | 24177篇 |
1990年 | 22842篇 |
1989年 | 23750篇 |
1988年 | 21770篇 |
1987年 | 20684篇 |
1986年 | 19582篇 |
1985年 | 21534篇 |
1984年 | 21730篇 |
1983年 | 19287篇 |
1982年 | 20179篇 |
1981年 | 19396篇 |
1980年 | 18146篇 |
1979年 | 18107篇 |
1978年 | 17052篇 |
1977年 | 16543篇 |
1976年 | 15665篇 |
1975年 | 15009篇 |
1974年 | 15419篇 |
1973年 | 15898篇 |
1972年 | 13476篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
151.
OBJECTIVE--To test the hypothesis that farmers are at high risk of hip osteoarthritis and to investigate possible causes for such a hazard. DESIGN--Cross sectional survey. SETTING--Five rural general practices. SUBJECTS--167 male farmers aged 60-76 and 83 controls from mainly sedentary jobs. All those without previous hip replacement underwent radiography of the hip. MAIN OUTCOME MEASURES--Hip replacement for osteoarthritis or radiological evidence of hip osteoarthritis. RESULTS--Prevalence of hip osteoarthritis was higher in farmers than controls and especially in those who had farmed for over 10 years (odds ratio 9.3, 95% confidence interval 1.9 to 44.5). The excess could not be attributed to any one type of farming, and heavy lifting seems the likely explanation. CONCLUSIONS--Manual handling in agriculture should be limited where possible. Consideration should be given to making hip osteoarthritis a prescribed industrial disease in farmers. There may be wider implications for the prevention of hip osteoarthritis in the general population. 相似文献
152.
Synthesis of prenylquinones in chloroplasts 总被引:3,自引:0,他引:3
153.
CONSENSUS CLADOGRAMS AND GENERAL CLASSIFICATIONS 总被引:2,自引:0,他引:2
154.
155.
156.
An earlier report (1) of an adverse effect of high doses of oxytocin on human memory included results of studies on women receiving oxytocin as part of the treatment to induce 2nd trimester therapeutic abortion. These women served as their own controls. We have now been able to study a group of women who have been treated in all ways like the original group, with the exception that they did not receive oxytocin. The results from this external control corroborate the finding that oxytocin affected memory. 相似文献
157.
G. E. Tanyi 《Bulletin of mathematical biology》1982,44(4):501-535
The Thom gradient model of morphogenesis poses the followinga posteriori problem: “From the observed morphology of a given natural process (effect) determine the dynamics of the process (cause)”. In this paper we consider the classicala priori problem: “Given the cause (dynamics) determine the effect (resultant morphology)”.
We find that in biochemical processes the mechanisms for energy activation, energy-matter interaction and energy dissipation
determine the dynamics. Furthermore there exists basic energy mechanisms which drive the equilibrium states through the elementary
catastrophes of Thom. A comparison with current theories shows that our models describe open ecological food chains and their
dynamical systems generalize the equations of organisation posed by M. Eigen.
Work supported by a Research Associateship of the International Centre for Theoretical Physics, P.O.B. 586, Miramare, 34100
Trieste, Italy. 相似文献
158.
WERNER A. MÜLLER 《Differentiation; research in biological diversity》1982,22(1-3):141-150
Several topics and concepts in developmental biology, such as induction of secondary axes (as in amphibians), the rules of distal transformation (valid, e.g., for insect and vertebrate limbs), and the rule one piece regenerates, the other one duplicates (valid also for imaginal discs) originally emerged from pioneering studies in hydroids. In addition, one finds phenomena of intercalation, i.e., the insertion of intervening parts when tissues of distant origin are confronted with each other. However, since not only short-range but also long-range interactions contribute to pattern regulation, one observes more regulative flexibility as compared to insect systems. 相似文献
159.
O. Munk 《Zoomorphology》1984,104(3):180-183
Summary A conus-like structure, the hyaloid conus, located on the optic nerve head of the mesopelagic deep-sea teleost Radiicephalus elongatus is described. The hyaloid conus consists of a tapering sheath of unpigmented, vascularized connective tissue enveloping the proximal part of the hyaloid artery which proceeds from the optic nerve head through the vitreous body to the ventrally located falciform process and lens muscles. The hyaloid artery passes through the hyaloid conus without giving off any branches. The conus vessels encircling the hyaloid artery receive arterial blood from the choroid via small arteries and are drained to the choroid by a single vein. The hyaloid conus is compared with the lacertilian conus papillaris. The function of the hyaloid conus is unknown. Because of its small dimensions relative to those of the eyeball and its few capillaries, it is unlikely that the hyaloid conus is a supplemental nutritive device for the retina. 相似文献
160.
Puerto Rican populations of two species of sea anemones (Bunodosoma cavernata and B. granulifera) which had previously been considered one were assayed electrophoretically for enzymes encoded by 12 loci. The two species shared no common allozymes at 6 of the 12 loci. Genetic distance and identity values based on these allozymes were computed for the Puerto Rican populations and for B. cavernata from Florida and B. granulifera from Panama. The Puerto Rican populations of both species had much higher genetic identities for their geographically distant conspecifics than for each other. These results indicate that the two species are reproductively isolated and should be considered as separate valid species. Average heterozygosities are presented which are the first published for coelenterate species. 相似文献