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   13541篇
  免费   1470篇
  国内免费   2篇
  2021年   159篇
  2020年   110篇
  2019年   121篇
  2018年   136篇
  2017年   143篇
  2016年   227篇
  2015年   362篇
  2014年   464篇
  2013年   603篇
  2012年   646篇
  2011年   637篇
  2010年   430篇
  2009年   334篇
  2008年   513篇
  2007年   577篇
  2006年   505篇
  2005年   513篇
  2004年   489篇
  2003年   431篇
  2002年   471篇
  2001年   413篇
  2000年   398篇
  1999年   341篇
  1998年   175篇
  1997年   151篇
  1996年   143篇
  1995年   127篇
  1994年   150篇
  1993年   132篇
  1992年   241篇
  1991年   240篇
  1990年   243篇
  1989年   220篇
  1988年   226篇
  1987年   245篇
  1986年   184篇
  1985年   208篇
  1984年   186篇
  1983年   180篇
  1982年   148篇
  1981年   133篇
  1980年   126篇
  1979年   178篇
  1978年   170篇
  1977年   129篇
  1976年   130篇
  1975年   139篇
  1974年   146篇
  1973年   124篇
  1972年   149篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
41.
DNA polymerase beta levels were measured in 4 cell lines of normal human skin fibroblasts and in 5 cell lines of skin fibroblasts from patients with ataxia telangiectasia, an autosomal recessive disease exhibiting marked X-ray sensitivity. The enzyme specific activities for the normal lines were similar and the mean value was 2-fold lower than the mean value for the ataxia lines. With both kinds of cells, the enzyme level did not change as the cultures progressed from logarithmic to stationary phase of growth. Thus, this putative DNA repair enzyme appears to be 'constitutive' in human skin fibroblast lines, and a modest elevation of beta-polymerase activity is associated with ataxia telangiectasia. These results are discussed in the context to current views about DNA-repair enzymes in X-ray-sensitive cultured mammalian cells.  相似文献   
42.
Summary The feasibility of using plant cell culture to measure toxicity was determined by investigating the toxicological effects of three chemical compounds, allyl alcohol, propargylglycine, and cadmium chloride, on cell cultures ofCatharanthus roseus G. Don (Madagascar periwinkle). Suspension cultures ofC. roseus were maintained in modified B5 medium and transferred every 5 d. Five-day-old cell cultures were exposed to various concentrations (10,3,1,0.3,0.1,0.03,0.01,0.003,0.001,0.0003,0.0001, 0.00003, and 0.0 mM) of the toxicants in both acute and chronic toxicity tests. In the acute test, cells were exposed to the toxicant for 24 h, washed three times with sterile medium, and plated in petri plates with an equal volume of 1.4% agar medium. Cells in the chronic test were plated with an equal volume of 1.4% agar medium containing various concentrations of the toxicant. Cells were incubated 28 d at 30°C in the dark. The colonies were counted and the results plotted as percent survival versus toxicant concentration. The results indicate, at the concentrations tested, thatC. roseus assay may be feasible in that it fulfills the criteria for a practical assay (e.g., rapid, simple, quantifiable, and reproducible). This work was submitted to the faculty of Miami University in partial fulfillment of the requirements for the degree of Master of Environmental Science, Institute of Environmental Sciences.  相似文献   
43.
44.
Thin-layer chromatography (TLC) was used to separate components in the basic and tar fractions of solvent refined coal (SRC-I) process solvent (PS) to obtain materials suitable for biological and chemical analysis. Those fractions eluted from TLC plates which were mutagenically active in the Ames/Salmonella assay were analyzed by gas chromatographic mass spectrometry (GCMS) for polycyclic azaarenes, polyaromatic primary amines (PAA) and carbazoles. In all materials tested, a strong correlation was observed between the concentration of PAAs in a given TLC region and the mutagenicity of that region in the Ames assay system. Conversely, azaarenes having 2--4 fused rings and carbazoles were present in both mutagenic and non-mutagenic TLC eluates. No PAAs were detected in mutagenically inactive TLC eluates. In comparison to the mutagenic tar fractions, the PS basic fraction contained relatively larger concentrations of 2- and 3-ringed components such as aminonaphthalenes and aminoanthracenes or aminophenanthrenes. The tar fractions, which were essentially devoid of aminonaphthalenes, had a higher average molecular weight and contained relatively higher concentrations of aminopyrenes.  相似文献   
45.
The neural basal lamina in hindbrain regions of exencephalic loop-tail (Lp/Lp) mice and of their normal (+/+; Lp/+) littermates was analyzed histochemically at the electron microscopic level by means of enzyme digestion and alcian blue staining with critical electrolyte concentrations (CEC) of MgCl2. At 9 days of gestation, the normal and abnormal embryos showed a similar pattern of alcian blue staining with a CEC of 0.00 M or 0.05 M MgCl2. However, with a CEC of 0.30 M MgCl2, the basal lamina in the abnormals stained more prominently, particularly the lamina rara externa, suggesting the presence of more sulfated glycosaminoglycans (GAG) in the abnormals. Moreover, predigestion of the tissues with Streptomyces hyaluronidase, which removes hyaluronic acid (HA), indicated that the abnormal basal lamina contained relatively less HA than in the normal embryos. By 10 days of gestation the normal basal lamina contained relatively more sulfated GAG and less HA and was thus more similar in appearance to that in the abnormal embryos. This apparently premature shift from HA predominance to sulfated GAG predominance in the abnormal basal lamina may be of significance in the etiology of dysraphism in this mutant.  相似文献   
46.
Toxic shock syndrome after chemical face peel   总被引:1,自引:0,他引:1  
Two cases of toxic shock syndrome following chemical face peel are reported. Toxic shock syndrome is a severe toxin-mediated multisystem disease. The major signs are fever, rash, desquamation, and hypotension. It can occur in males as well as females and is not necessarily related to menstruation. The surgical wound does not usually appear infected. Early recognition is the hallmark of successful treatment. Therapy is symptomatic, with aggressive administration of fluids. Antistaphylococcal agents are used. Prophylactic antibiotics are not necessarily recommended.  相似文献   
47.
Forty-one zoological gardens in seven European countries were visited to investigate activity level in captive environments for great apes. Forty-three groups of gorillas and 68 groups of orangutans were observed. The seven factors quantified for each of the environments were size of the enclosure, usable surface area, frequency of feeding, number of animals, and number of objects (stationary, temporary, and movable). Activity level of each group was measured by instantaneous scan sampling for one hour on two consecutive days. For both species, the factors most highly related to activity level were number of animals, and stationary, temporary, and movable objects. The usefulness of these variables for predicting group activity level was different for the two species, however. Factors important for gorillas were stationary and temporary objects, while stationary and movable objects were significant for orangutans. These findings suggest that objects within environments may be more important for captive apes than the size or construction of the enclosure. Also, the types of objects that need to be included in environments may be related to the natural behavior of the individual species.  相似文献   
48.
Arts medicine has come of age, resulting from 3 important developments over the past decade: improved methods of diagnosis and treatment, an awareness that artists suffer from special problems related to their occupation and lifestyle, and the establishment of health programs emphasizing an interdisciplinary approach to these patients. We focus on the patterns of illness afflicting performing artists, specifically dancers, singers, actors, and instrumental musicians, and explain some of the things a health care team can do in treating these patients. The conditions governing these patients'' lives--early exposure to high expectations of excellence, incessant demands for perfection, long periods of intense practicing, fierce competition, high levels of anxiety associated with performance, and uncertain careers--need to be understood. Levels of disease and disability are remarkably high, but artists often ignore symptoms. We discuss the musculoskeletal, neurologic, vocal, psychological, and other syndromes found among performers and some of the difficulties in treating them. The prevention of injury, conservative management, collaboration with teachers, and a psychotherapeutic approach are desirable. Arts medicine programs for professional consultation exist in several major cities of the United States and abroad. Although research is needed regarding the effectiveness of health care services for performing artists, the scientific literature devoted to this field is growing.  相似文献   
49.
50.
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