全文获取类型
收费全文 | 750450篇 |
免费 | 82673篇 |
国内免费 | 581篇 |
专业分类
833704篇 |
出版年
2018年 | 6926篇 |
2017年 | 6666篇 |
2016年 | 9312篇 |
2015年 | 12156篇 |
2014年 | 14283篇 |
2013年 | 21019篇 |
2012年 | 23119篇 |
2011年 | 23665篇 |
2010年 | 15992篇 |
2009年 | 14688篇 |
2008年 | 20968篇 |
2007年 | 21563篇 |
2006年 | 20364篇 |
2005年 | 19574篇 |
2004年 | 19360篇 |
2003年 | 18512篇 |
2002年 | 17894篇 |
2001年 | 36210篇 |
2000年 | 36212篇 |
1999年 | 28383篇 |
1998年 | 9356篇 |
1997年 | 10099篇 |
1996年 | 9425篇 |
1995年 | 8616篇 |
1994年 | 8371篇 |
1993年 | 8399篇 |
1992年 | 22870篇 |
1991年 | 22180篇 |
1990年 | 21342篇 |
1989年 | 20847篇 |
1988年 | 19159篇 |
1987年 | 17975篇 |
1986年 | 16751篇 |
1985年 | 16722篇 |
1984年 | 13706篇 |
1983年 | 11585篇 |
1982年 | 8769篇 |
1981年 | 7961篇 |
1980年 | 7532篇 |
1979年 | 13016篇 |
1978年 | 9972篇 |
1977年 | 9372篇 |
1976年 | 8511篇 |
1975年 | 9411篇 |
1974年 | 10270篇 |
1973年 | 10011篇 |
1972年 | 9121篇 |
1971年 | 8385篇 |
1970年 | 7315篇 |
1969年 | 6974篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
941.
942.
OBJECTIVE: To determine the vaccination rate among infants discharged from a neonatal intensive care unit (NICU) and factors affecting that rate. DESIGN: Cross-sectional survey conducted when the children were 12 to 18 months of age. SETTING: NICU at the Royal University Hospital, Saskatoon, Sask. PARTICIPANTS: All 395 infants discharged from the NICU between Jan. 1 and June 30, 1992. MAIN OUTCOME MEASURES: Vaccination rate, ethnic background (native or non-native), place of residence (urban or rural), health status (number of days spent in the NICU), reasons for delay in or incomplete vaccinations (those involving parents'' responsibility, infant illness or contraindications). RESULTS: Of the 395 infants, 20 (5.0%) had died and incomplete information was available for 30 (7.6%). Complete data were available for 345 (87.3%). Of the infants for whom data were available, 8 (2.3%) had never been vaccinated and 142 (41.2%) had a delayed vaccination schedule or had not completed their scheduled vaccinations. Only 195 (56.6%) of the infants had received a full vaccination series. Non-native ethnic background was a predictor of completed vaccinations (odds ratio [OR] 5.40, 95% confidence interval [CI] 3.05 to 9.52). In a univariate model, urban area of residence was not a significant predictor of vaccination status, but when ethnic background was controlled for in a multivariate logistic regression analysis, urban area of residence was found to be inversely associated with completed vaccinations (OR 0.34, 95% CI 0.15 to 0.79). The number of days the child had spent in the NICU was not a significant predictor of vaccination status. CONCLUSION: The vaccination rate of infants discharged from the NICU is not optimal. Urban native children appears to be at risk of not being vaccinated. Non-native infants are five times more likely than native infants to have completed all of their scheduled vaccinations. Methods to improve the rate of completed vaccinations, especially for native children, must be sought and tested. 相似文献
943.
944.
A sucrase gene from Leuconostoc mesenteroides was cloned and expressed in Escherichia coli. The cloned enzyme did not show dextransucrase or sucrose phosphorylase activity. HPLC and GC-MS analyses of the sucrase products indicated the presence of fructose and glucose in equimolar amounts. IPTG induction did not increase sucrase activity in E. coli indicating that the cloned gene may be transcribed from its own promoter. To our knowledge, this is the first sucrase cloned from L. mesenteroides that has invertase activity. 相似文献
945.
946.
947.
948.
Jill S. Marty James P. Higham Elizabeth L. Gadsby Caroline Ross 《International journal of primatology》2009,30(6):807-823
Sexual selection has driven the evolution and elaboration of a wide variety of displays and ornaments in male nonhuman primates,
including capes, cheek flanges, and sexual coloration. Among the most sexually dimorphic of all primates is the drill (Mandrillus leucophaeus), the males of which can be 3 times the mass of females, possess large canines, and exhibit extremely bright sexual skin
coloration. However, the function of male coloration in this species has never been examined. Here, we present data on male
color (measured objectively using digital photography), dominance rank, measures of male-female association, and key sexual
behaviors, of adult male drills (n = 17) living in four semi free-ranging enclosures at the Drill Rehabilitation and Breeding Center in Nigeria. We test the
hypothesis that male coloration is a badge of status, indicating dominance rank, and the hypothesis that male coloration attracts
females. We found that male coloration did indicate rank, and that high ranking, strongly colored males were more likely to
associate with adult females, and more specifically with fully tumescent females. These males also engaged in more sexual
activity. However, measures of male-female association and sexual behaviors were not related to male color once rank had been
taken into consideration; i.e., for males of a given rank, females did not prefer those that were more colorful. We discuss
the results in light of what is known about the wild drill social system, in which unfamiliar individuals may come together
in “supergroups,” and in which quality indicators of competitive ability may be particularly important. 相似文献
949.
950.
Human erythrocyte and brain acetylcholinesterase are preferentially inhibited by the P(-)-isomers of C(+/-)P(+/-)-soman. The enzymes inhibited by the P(-)-isomers behave similarly with respect to oxime-induced reactivation and aging. HI-6 is the best reactivator for C(+)P(-)-soman-inhibited acetylcholinesterases. Oxime-induced reactivation of the C(-)P(-)-soman-inhibited acetylcholinesterases is much more difficult to achieve. 相似文献