全文获取类型
收费全文 | 670746篇 |
免费 | 84762篇 |
国内免费 | 436篇 |
专业分类
755944篇 |
出版年
2019年 | 7088篇 |
2018年 | 9265篇 |
2017年 | 9125篇 |
2016年 | 11528篇 |
2015年 | 14712篇 |
2014年 | 16203篇 |
2013年 | 22000篇 |
2012年 | 22468篇 |
2011年 | 22158篇 |
2010年 | 16642篇 |
2009年 | 14066篇 |
2008年 | 18954篇 |
2007年 | 19330篇 |
2006年 | 18079篇 |
2005年 | 17545篇 |
2004年 | 17266篇 |
2003年 | 16604篇 |
2002年 | 16487篇 |
2001年 | 30352篇 |
2000年 | 30753篇 |
1999年 | 24180篇 |
1998年 | 8014篇 |
1997年 | 8280篇 |
1996年 | 7889篇 |
1995年 | 7447篇 |
1994年 | 7445篇 |
1993年 | 7175篇 |
1992年 | 20071篇 |
1991年 | 19310篇 |
1990年 | 18719篇 |
1989年 | 18131篇 |
1988年 | 16903篇 |
1987年 | 15894篇 |
1986年 | 14544篇 |
1985年 | 14586篇 |
1984年 | 11919篇 |
1983年 | 10423篇 |
1982年 | 8032篇 |
1981年 | 7141篇 |
1980年 | 6655篇 |
1979年 | 11704篇 |
1978年 | 8817篇 |
1977年 | 8127篇 |
1976年 | 7451篇 |
1975年 | 8374篇 |
1974年 | 8864篇 |
1973年 | 8843篇 |
1972年 | 8195篇 |
1971年 | 7390篇 |
1970年 | 6430篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
2.
3.
4.
A patient with chronic anemia is presented who radiologically showed prominent rugae of the stomach. Angiography demonstrated an arteriovenous malformation with a large feeding artery and prominent draining veins. 相似文献
5.
6.
Juan Viruel Pilar Catalán José Gabriel Segarra‐Moragues 《Journal of Biogeography》2012,39(6):1073-1085
Aim The Chilean endemic Dioscorea biloba (Dioscoreaceae) is a dioecious geophyte that shows a remarkable 600 km north–south disjunction in the peripheral arid area of the Atacama Desert. Its restricted present‐day distribution and probable Neogene origin indicate that its populations have a history linked to that of the Atacama Desert, making this an ideal model species with which to investigate the biogeography of the region. Location Chile, Atacama Desert and peripheral arid area. Methods Two hundred and seventy‐five individuals from nine populations were genotyped for seven nuclear microsatellite loci, and plastid trnL–F and trnT–L sequences were obtained for a representative subset of these. Analyses included the estimation of genetic diversity and population structure through clustering, Bayesian and analysis of molecular variance analyses, and statistical parsimony networks of chloroplast haplotypes. Isolation by distance was tested against alternative dispersal hypotheses. Results Microsatellite markers revealed moderate to high levels of genetic diversity within populations, with those from the southern Limarí Valley showing the highest values and northern populations showing less exclusive alleles. Bayesian analysis of microsatellite data identified three genetic groups that corresponded to geographical ranges. Chloroplast phylogeography revealed no haplotypes shared between northern and southern ranges, and little haplotype sharing between the two neighbouring southern valleys. Dispersal models suggested the presence of extinct hypothetical populations between the southern and northern ranges. Main conclusions Our results are consistent with prolonged isolation of the northern and southern groups, mediated by the life‐history traits of the species. Significant isolation was revealed at both large and moderate distances as gene flow was not evident even between neighbouring valleys. Bayesian analyses of microsatellite and chloroplast haplotype diversity identified the southern area of Limarí as the probable area of origin of the species. Our data do not support recent dispersal of D. biloba from the southern range into Antofagasta, but indicate the fragmentation of an earlier wider range, concomitant with the Pliocene–Pleistocene climatic oscillations, with subsequent extinctions of the Atacama Desert populations and the divergence of the peripheral ones as a consequence of genetic drift. 相似文献
7.
8.
Changes in the protein and steroid hormones of follicular fluid, aspirated from different follicles of sheep and human ovaries, have been measured and correlated with the size of the follicles. As the fluid contains a number of proteins, steroids have been measured directly and after ether extraction. The follicular fluid concentrations of progesterone and 17 beta-oestradiol measured directly in the fluid increased with the size of the follicles. The levels of free testosterone remained constant in all sizes of follicles, while those of bound hormone showed a 10- to 15-fold increase over the free testosterone concentrations in both the sheep and human follicular fluid. A decrease in the levels of bound testosterone in the fluid of large follicles (LFFL) coincided with the increase in bound 17 beta-oestradiol, suggesting the possible conversion of bound testosterone to oestrogen as the follicle attained maturity. The ratio of follicle-stimulating hormone (FSH) to luteinizing hormone (LH) varied in the fluid obtained from different size follicles, being 1:7 in small (SFFL), 1.3.5 in medium (MFFL) and 1:2.3 in large (LFFL) follicles of sheep ovaries. The LH content of follicular fluid of different size follicles appeared to be the same, with LFFL showing a minor increase over SFFL. In the human, the fluid from medium follicles contained very little LH compared to LFFL. These differences in the pattern of LH levels present in the fluid from different size follicles between human and sheep ovaries presumably reflect species variations in the entry of LH into the follicles. 相似文献
9.
10.
In vivo EMG biofeedback was found to be an effective pedagogical tool for removing unwanted left-hand tension in nine violin and viola players. Improvement occurred rapidly and persisted throughout a 5-month follow-up period. Further studies will be necessary to assess the effect of biofeedback independent of placebo effects. The brevity of the method and the magnitude of improvement warrant further investigation. 相似文献