全文获取类型
收费全文 | 23158篇 |
免费 | 2710篇 |
国内免费 | 5篇 |
出版年
2021年 | 291篇 |
2020年 | 224篇 |
2019年 | 287篇 |
2018年 | 308篇 |
2017年 | 305篇 |
2016年 | 510篇 |
2015年 | 835篇 |
2014年 | 844篇 |
2013年 | 1043篇 |
2012年 | 1407篇 |
2011年 | 1380篇 |
2010年 | 780篇 |
2009年 | 750篇 |
2008年 | 1113篇 |
2007年 | 1098篇 |
2006年 | 1077篇 |
2005年 | 1025篇 |
2004年 | 990篇 |
2003年 | 909篇 |
2002年 | 866篇 |
2001年 | 515篇 |
2000年 | 490篇 |
1999年 | 468篇 |
1998年 | 266篇 |
1997年 | 282篇 |
1996年 | 254篇 |
1995年 | 216篇 |
1994年 | 245篇 |
1993年 | 191篇 |
1992年 | 401篇 |
1991年 | 354篇 |
1990年 | 335篇 |
1989年 | 296篇 |
1988年 | 307篇 |
1987年 | 305篇 |
1986年 | 308篇 |
1985年 | 309篇 |
1984年 | 280篇 |
1983年 | 223篇 |
1982年 | 212篇 |
1981年 | 160篇 |
1980年 | 169篇 |
1979年 | 218篇 |
1978年 | 225篇 |
1977年 | 167篇 |
1976年 | 179篇 |
1975年 | 157篇 |
1974年 | 181篇 |
1973年 | 191篇 |
1972年 | 146篇 |
排序方式: 共有10000条查询结果,搜索用时 156 毫秒
21.
Slash pine needles and cortex oleoresin have been found to contain a new major diterpene constituent, imbricataloic acid. The closely related imbricatoloic acid, previously reported only in Araucaria imbricata, was found to be present in small amounts in slash pine needle extract. Spectral data are given for an unidentified diterpene alcohol isolated from the cortex oleoresin. 相似文献
22.
23.
Ronald J. Roberts H. J. Ball A. L. S. Munro W. M. Shearer 《Journal of fish biology》1971,3(2):221-224
Fourteen fresh run salmon Satmo salar L. with early extant lesions of ulcerative dermal necrosis (UDN) were kept in separate tanks and treated with zinc free malachite green. Ten of the fish were held at 10° C and 4 at 2° C. The treatment precluded infection with Saprolegnia fungus and allowed natural resolution of the lesions. There was a marked difference in rate of healing between warm and cold water conditions.
Histological examination of healing lesions at different stages showed that there was a primary invasion of cuboidal epithelium over the collagen scar followed by a phase of disorganized proliferation which eventually organized itself into normal epithelium. Melanocytes were very obvious in the dermis of healing lesions. 相似文献
Histological examination of healing lesions at different stages showed that there was a primary invasion of cuboidal epithelium over the collagen scar followed by a phase of disorganized proliferation which eventually organized itself into normal epithelium. Melanocytes were very obvious in the dermis of healing lesions. 相似文献
24.
25.
Stairway climbing provides a ubiquitous and inconspicuous method of burning calories. While typically two strategies are employed for climbing stairs, climbing one stair step per stride or two steps per stride, research to date has not clarified if there are any differences in energy expenditure between them. Fourteen participants took part in two stair climbing trials whereby measures of heart rate were used to estimate energy expenditure during stairway ascent at speeds chosen by the participants. The relationship between rate of oxygen consumption () and heart rate was calibrated for each participant using an inclined treadmill. The trials involved climbing up and down a 14.05 m high stairway, either ascending one step per stride or ascending two stair steps per stride. Single-step climbing used 8.5±0.1 kcal min−1, whereas double step climbing used 9.2±0.1 kcal min−1. These estimations are similar to equivalent measures in all previous studies, which have all directly measured The present study findings indicate that (1) treadmill-calibrated heart rate recordings can be used as a valid alternative to respirometry to ascertain rate of energy expenditure during stair climbing; (2) two step climbing invokes a higher rate of energy expenditure; however, one step climbing is energetically more expensive in total over the entirety of a stairway. Therefore to expend the maximum number of calories when climbing a set of stairs the single-step strategy is better. 相似文献
26.
Peter Lewis 《Journal of bioethical inquiry》2007,4(1):51-55
The display of personal photographs in hospital is a common practice that has yet to be rigorously examined. The photographs
displayed are subject to interpretation by the viewer and may lead to misunderstandings or miscommunication if clarification
of meaning is not sought. This paper explores a range of possible meanings that the display of photographs in hospital may
hold, based on a case study of a 15 year old boy hospitalised with a life threatening illness. Further research is needed
into the actual meanings attributed to the display of photographs in hospital by patients and family members. 相似文献
27.
28.
Eliot C. Bush Anne E. Clark Chris M. DeBoever Lillian E. Haynes Sidra Hussain Singer Ma Matthew M. McDermott Adam M. Novak John S. Wentworth 《PloS one》2012,7(11)
A significant proportion of enzymes display cooperativity in binding ligand molecules, and such effects have an important impact on metabolic regulation. This is easiest to understand in the case of positive cooperativity. Sharp responses to changes in metabolite concentrations can allow organisms to better respond to environmental changes and maintain metabolic homeostasis. However, despite the fact that negative cooperativity is almost as common as positive, it has been harder to imagine what advantages it provides. Here we use computational models to explore the utility of negative cooperativity in one particular context: that of an inhibitor binding to an enzyme. We identify several factors which may contribute, and show that acting together they can make negative cooperativity advantageous. 相似文献
29.
D. F. Roberts 《International Journal of Anthropology》1996,11(2-4):185-191
As a conclusion, this paper reviews briefly the content of the volume. The wealth of demographic data has not been adequately
exploited in anthropology; this is why this publication is valuable in showing attempts to apply demographic data in a variety
of anthropological problems. This symposium has explored many interesting points which we recall here. Yet it has also opened
up a whole range of further questions on the material presented as well as in this broad field. Several directions of research
could be developed, for instance, testing among human populations, over long periods, the ecological thoughts of ecosystems
evolving as a cascade of instabilities, rather than a succession of equilibrium states. Let us also recall the pervasive nature
of demographic facts in topics such as the energy cycle or the genetic structure and evolution of human populations. 相似文献
30.
Alba Luengo Zhaoqi Li Dan Y. Gui Lucas B. Sullivan Maria Zagorulya Brian T. Do Raphael Ferreira Adi Naamati Ahmed Ali Caroline A. Lewis Craig J. Thomas Stefani Spranger Nicholas J. Matheson Matthew G. Vander Heiden 《Molecular cell》2021,81(4):691-707.e6
- Download : Download high-res image (172KB)
- Download : Download full-size image