全文获取类型
收费全文 | 1212篇 |
免费 | 112篇 |
国内免费 | 1篇 |
专业分类
1325篇 |
出版年
2023年 | 8篇 |
2022年 | 11篇 |
2021年 | 35篇 |
2020年 | 26篇 |
2019年 | 22篇 |
2018年 | 16篇 |
2017年 | 18篇 |
2016年 | 35篇 |
2015年 | 65篇 |
2014年 | 57篇 |
2013年 | 62篇 |
2012年 | 77篇 |
2011年 | 74篇 |
2010年 | 40篇 |
2009年 | 28篇 |
2008年 | 54篇 |
2007年 | 41篇 |
2006年 | 44篇 |
2005年 | 46篇 |
2004年 | 42篇 |
2003年 | 40篇 |
2002年 | 31篇 |
2001年 | 32篇 |
2000年 | 26篇 |
1999年 | 35篇 |
1998年 | 18篇 |
1997年 | 15篇 |
1996年 | 11篇 |
1995年 | 8篇 |
1994年 | 7篇 |
1993年 | 9篇 |
1992年 | 21篇 |
1991年 | 25篇 |
1990年 | 20篇 |
1989年 | 16篇 |
1988年 | 14篇 |
1987年 | 13篇 |
1986年 | 14篇 |
1985年 | 7篇 |
1984年 | 12篇 |
1983年 | 7篇 |
1982年 | 9篇 |
1979年 | 10篇 |
1978年 | 6篇 |
1977年 | 11篇 |
1972年 | 6篇 |
1969年 | 6篇 |
1967年 | 6篇 |
1966年 | 9篇 |
1960年 | 5篇 |
排序方式: 共有1325条查询结果,搜索用时 15 毫秒
1.
Carbon and nitrogen stable isotope analyses have improved our understanding of food webs and movement patterns of aquatic
organisms. These techniques have recently been applied to diet studies of elasmobranch fishes, but isotope turnover rates
and isotope diet–tissue discrimination are still poorly understood for this group. We performed a diet switch experiment on
captive sandbar sharks (Carcharhinus plumbeus) as a model shark species to determine tissue turnover rates for liver, whole blood, and white muscle. In a second experiment,
we subjected captive coastal skates (Leucoraja spp.) to serial salinity reductions to measure possible impacts of tissue urea content on nitrogen stable isotope values.
We extracted urea from spiny dogfish (Squalus acanthias) white muscle to test for effects on nitrogen stable isotopes. Isotope turnover was slow for shark tissues and similar to
previously published estimates for stingrays and teleost fishes with low growth rates. Muscle isotope data would likely fail
to capture seasonal migrations or diet switches in sharks, while liver and whole blood would more closely reflect shorter
term movement or shifts in diet. Nitrogen stable isotope values of skate blood and skate and dogfish white muscle were not
affected by tissue urea content, suggesting that available diet–tissue discrimination estimates for teleost fishes with similar
physiologies would provide accurate estimates for elasmobranchs. 相似文献
2.
OBJECTIVE--To examine the relation between the use of aspirin and non-steroidal anti-inflammatory drugs and the presence of asymptomatic colorectal adenomas. DESIGN--Case-control study of subjects participating in a randomised controlled trial of faecal occult blood screening for colorectal cancer. Data on analgesics and other drugs were obtained from a questionnaire which was mainly concerned with diet and was administered by an interviewer. SETTING--Nottingham. SUBJECTS--147 patients with positive results in faecal occult blood tests who were found to have colorectal adenomas (cases), 153 age and sex matched control subjects with negative results in such tests (negative controls), and 176 control subjects with positive results in the tests who were found not to have colorectal adenomas (positive controls). MAIN OUTCOME MEASURES--Relative risk of developing colorectal adenomas according to frequency and duration of use of analgesics. RESULTS--Cases reported taking less aspirin and non-steroidal anti-inflammatory drugs than the negative controls, with the estimated relative risk for any use being 0.49 (95% confidence interval 0.3 to 0.8). The inverse association was less strong when cases were compared with the positive controls (0.66 (0.4 to 1.1)). The association was specific for aspirin and non-steroidal anti-inflammatory drugs there being no association with paracetamol or other drugs. Prescribed use of non-steroidal anti-inflammatory drugs for longer than five years was associated with the lowest risk (0.21 (0.1 to 0.8)), although the numbers reporting prolonged prescribed use were small. CONCLUSIONS--These findings support the hypothesis that aspirin and non-steroidal anti-inflammatory drug use protects against the development of colorectal neoplasia. 相似文献
3.
4.
Novobiocin-resistance sequences from the novobiocin-producing strain Streptomyces niveus 总被引:1,自引:0,他引:1
Two distinct DNA sequences expressing novobiocin resistance in Streptomyces lividans were cloned from the novobiocin-producing species Streptomyces niveus. Clone pGL101 (5kb) conferred resistance to 50 micrograms ml-1 novobiocin, whereas clones pGL102 and pGL103, which carry the same 6.5kb insert but in opposite orientations, expressed resistance to 150 micrograms ml-1. The cloned inserts from pGL101 and pGL103 failed to hybridize with each other or with the cloned novobiocin-resistant gyrB sequence from Streptomyces sphaeroides. Both probes hybridized strongly with DNA from the novobiocin-producing species S. niveus and S. sphaeroides but no hybridization (pGL103) or very weak hybridization (pGL101) was detected with DNA from the non-producing species S. lividans, Streptomyces griseus and Streptomyces antibioticus. S. niveus contains at least three novobiocin-resistance determinants with the pGL101 and pGL103 cloned sequences specific for novobiocin-producing strains of Streptomyces. 相似文献
5.
Cholesterol and cholesteryl ester concentrations and cholesteryl ester fatty acid substituents have been measured during the first 10 weeks of life in tissues of normal and dystrophic mice. In normal Swiss and 129ReJ(+/?) mice the concentrations of both cholesterol and cholesteryl esters remain essentially constant in liver, increase in brain and fall sharply in both thigh (mixed fiber type muscles) and chest muscles (predominantly slow oxidative muscles) over this period. In all cases the concentration of free cholesterol exceeds that of esterified cholesterol. In dystrophic mice, similar patterns are found in brain and liver. In both thigh and chest muscles, however, the developmental pattern is significantly different. After an initial decrease the concentrations of cholesterol and cholesteryl esters increase rapidly with the largest increase occurring in the concentration of cholesteryl esters which by 10 weeks of age exceeds the concentration of cholesterol in chest muscle. During the same period the pattern of esterified fatty acids changes gradually in dystrophic tissues towards an increasing ratio of unsaturated/saturated fatty acids. By 10 weeks of age this ratio is significantly higher in dystrophic tissues than normal in all tissues tested. 相似文献
6.
7.
The immunocytochemical demonstration of Toxoplasma antigen in the brains of congenitally infected mice 总被引:1,自引:0,他引:1
J Hay D I Graham G N Dutton S Logan 《Zeitschrift für Parasitenkunde (Berlin, Germany)》1986,72(5):609-615
The peroxidase anti-peroxidase immunocytochemical staining method was used to identify Toxoplasma antigen in paraffin embedded sections of the brains of 22 mice congenitally infected with the parasite. Intact Toxoplasma tissue cysts were readily demonstrated in the brain in all cases. In 4 of the 22 infected mice there was evidence of rupture of the cyst wall and/or presence of extra-cystic Toxoplasma antigen. Further support for the extra-cystic location of Toxoplasma antigen was obtained by electron microscopy of reprocessed tissue which revealed endozoites in the area immediately surrounding a ruptured cyst. The possible implications of these findings in relation to the pathogenesis of congenital toxoplasmic meningo-encephalitis are discussed. 相似文献
8.
9.
10.