首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   12932篇
  免费   981篇
  国内免费   2篇
  13915篇
  2023年   56篇
  2022年   59篇
  2021年   149篇
  2020年   123篇
  2019年   137篇
  2018年   229篇
  2017年   279篇
  2016年   328篇
  2015年   427篇
  2014年   499篇
  2013年   680篇
  2012年   997篇
  2011年   1057篇
  2010年   566篇
  2009年   426篇
  2008年   786篇
  2007年   863篇
  2006年   745篇
  2005年   654篇
  2004年   607篇
  2003年   645篇
  2002年   578篇
  2001年   308篇
  2000年   348篇
  1999年   170篇
  1998年   131篇
  1997年   105篇
  1996年   123篇
  1995年   99篇
  1994年   110篇
  1993年   98篇
  1992年   83篇
  1991年   88篇
  1990年   87篇
  1989年   78篇
  1988年   68篇
  1987年   59篇
  1986年   76篇
  1985年   76篇
  1984年   88篇
  1983年   83篇
  1982年   79篇
  1981年   83篇
  1980年   76篇
  1979年   64篇
  1978年   65篇
  1977年   62篇
  1976年   45篇
  1974年   29篇
  1973年   35篇
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
21.
Summary The increased risk of nonocular cancer seen consistently in studies of survivors of retinoblastoma may be caused in part by the presence of a retinoblastoma gene that also predisposes to other cancers. It has been claimed that this gene also increases the risk for cancer among unaffected relatives of genetic retinoblastoma probands. We report here a population-based study of the risk of nonocular cancer in parents and siblings of persons notified to the Danish Cancer Registry with retinoblastoma during 1943–84. No excess was observed among first degree relatives of 61 genetic retinoblastoma probands, whereas a slight (10%) excess was seen among the parents of 115 nongenetic probands. The latter was the result of significant excesses of malignant melanoma (4 observed, 0.4 expected), multiple myeloma (2 observed, 0.2 expected) and osteogenic sarcoma (1 observed, 0.03 expected). The observed risk pattern cannot be explained by the presence of the retinoblastoma gene.  相似文献   
22.
Summary Serotonin-like immunoreactivity was investigated in the pineal complex of the golden hamster by use of the indirect immunohistochemical technique. The superficial and deep portions of the pineal gland, and also the pineal stalk exhibited an intense cellular immunoreaction for serotonin. In addition, perivascular serotonin-immunoreactive nerve fibers were observed. Some serotonin-immunoreactive processes of the pinealocytes terminated on the surface of the ventricular lumen in the pineal and suprapineal recesses, indicating a receptive or secretory function of these cells. Several serotonin-immunoreactive processes connected the deep pineal with the habenular area. One week after bilateral removal of both superior cervical ganglia the serotonin immunoreaction of the entire pineal complex was greatly decreased. However, some cells in the pineal complex, of which several exhibited a neuron-like morphology, remained intensively stained after ganglionectomy. This indicates that the indoleamine content of some cells in the pineal complex of the golden hamster is independent of the sympathetic innervation.Supported by a Grant from the Italian Society for Veterinary Sciences  相似文献   
23.
Several aspects of energy metabolism (glucose utilization, lactate production,14CO2 production from labeled glucose, glutamate or pyruvate, oxygen consumption and contents of ATP and phosphocreatine) were measured in cerebellar granule cells (glutamatergic) in primary cultures and compared with corresponding data for cerebral cortical neurons (mainly GABA-ergic) and astrocytes. Cerebellar granule cells and astrocytes were metabolically more active than cerebral cortical neurons. Glutamate which is utilized as a major metabolic fuel as astrocytes and, to a lesser extent, in cerebral cortical neurons, was virtually not oxidized in cerebellar granule cells.Special Issue dedicated to Prof. Holger Hydén.  相似文献   
24.
Kappel K  Sandøe P 《Bioethics》1992,6(4):297-316
... We can therefore conclude that either we should go for equality; and in that case QALYs are unfair because they haven't got enough of an ageist bias. Or we should accept consequentialism; and in that case QALYs have just the right sort of ageist bias. No plausible case can, however, be made for the claim that QALYs have an unfair bias against old people. Other things being equal we ought when distributing resources essential for survival favour the young. This ethical claim can be supported both by reference to equality (the life-time-view) and by reference to consequentialism (and the premises that resources generally will be more useful when given to young people).  相似文献   
25.
Reindeer calves (n = 632) were slaughtered in November/December (n = 476) or in January (n = 156). Dressed weights and amount of perirenal fat were recorded, and the reproductive organs were collected. A separate group of 130 reindeer calves were weighed at 7 months of age and were followed up with repeated weighings and pregnancy examinations up to 21 months. The onset of puberty and the pregnancy rate were significantly influenced by body weight and the amount of perirenal fat. Approximately 60 g of perirenal fat and 22 kg dressed weight were found at the lower limits for pregnancy. A total of 222 (35%) animals had reached puberty and 126 (20%) were pregnant when examined after slaughter. Animals which conceived during their first autumn showed only a moderate weight gain the following year, and the calf mortality rate in these animals was 47.4%. It was concluded that calf pregnancies are common among the reindeer of Southern Norway and that measures need to be taken to prevent them.  相似文献   
26.
Aplanospores ofHaematococcus pluvialis MUR 145 contained 0.7% carotenoids (dry wt. basis) consisting of β,β-carotene (5% of total carotenoid), echinenone (4%), canthaxanthin (4%), (3S,3′S)-astaxanthin diester (34%), (3S,3′S)-astaxanthin monoester (46%), (3S,3′S)-astaxanthin (1%) and (3R,3′R,6′R)-lutein (6%). The astaxanthin esters were examined by TLC and HPLC and VIS,1H NMR and mass spectra recorded. Their chirality was determined by the camphanate method (Vecchi & Müller, 1979) after anaerobic hydrolysis. The tough cell wall of the aplanospores required enzymatic treatment prior to pigment extraction. The potential use of this microalga as a feed ingredient in aquaculture is discussed briefly.  相似文献   
27.
Population densities and total phosphorus concentrations in samples from different lakes of south-eastern Norway were determined. In addition some transplant experiments with dilute phytoplankton populations were carried out. A laboratory batch culture method was used.The diatoms studied may be divided into three ecological groups based on their cell densities and total phosphorus concentrations in the samples. This classification was supported by the experimental results. Cyclotella spp., Asterionella formosa and Tabellaria fenestrata did not grow or had low growth rates above pH 9. Synedra cf. acus and Fragilaria crontonensis had often high growth rates within the pH 9–10 range, but were not able to grow at pH values above 10. High pH-values had no effect on the growth rate of Oscillatoria. Oscillatoria, Synedra and Stephanodiscus were severely growth-limited in filtered water from oligotrophic lakes. Maximum growth rates of all the populations studied were often obtained after addition of phosphate and chelated iron (FeEDTA) in combination to filtered water samples from oligotrophic/mesotrophic lakes.  相似文献   
28.
29.
30.
Corynebacterium (Eubacterium) suis strains from boars and sows haemagglutinated erythrocytes of different animal species (calf, guinea pig, poultry, pig, and human). The haemaigglutination was man nose resistant (MR) and was neither inhibited by L-fucose nor D-galactose. The hydrophobicity measured by salt aggregation test (0.1–0.9 mol/1 (NH4)2SO4) and the hydrophobic interaction chromatography test (90 % retention in octyl sepharose) together with the haemagglutinating activity, indicated the presence of fimbriae on the bacteria. The haemagglutinating and hydrophobic properties were heat-sensitive (60°C for 10 min) suggestive of the presence of a protein structure. Two types of fimbria-tion were demonstrated by electron microscopy. Fetuin and glyco^ protein inhibited the haemagglutination, whereas porcine mucin was without any effect. These results indicate that branched glycoproteins might be important receptors for these fimbriae. The pathogenic aspects of C. suis are discussed, based on recent acquired knowledge of the effect of other pyelonephritogenic bacteria.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号