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   20621篇
  免费   1727篇
  国内免费   5篇
  22353篇
  2023年   88篇
  2022年   234篇
  2021年   441篇
  2020年   237篇
  2019年   310篇
  2018年   427篇
  2017年   363篇
  2016年   645篇
  2015年   1045篇
  2014年   1192篇
  2013年   1436篇
  2012年   1781篇
  2011年   1674篇
  2010年   1085篇
  2009年   959篇
  2008年   1284篇
  2007年   1320篇
  2006年   1131篇
  2005年   1084篇
  2004年   1054篇
  2003年   966篇
  2002年   908篇
  2001年   231篇
  2000年   175篇
  1999年   168篇
  1998年   236篇
  1997年   167篇
  1996年   141篇
  1995年   132篇
  1994年   115篇
  1993年   113篇
  1992年   93篇
  1991年   78篇
  1990年   79篇
  1989年   75篇
  1988年   57篇
  1987年   69篇
  1986年   46篇
  1985年   60篇
  1984年   47篇
  1983年   68篇
  1982年   44篇
  1981年   35篇
  1980年   38篇
  1979年   35篇
  1978年   47篇
  1977年   37篇
  1976年   37篇
  1975年   31篇
  1974年   33篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Summary The distribution of Corynebacterium parvum labeled with 131iodine or 99mtechnetium was studied in 17 patients with bronchogenic carcinoma. The labeled bacteria were given intravenously or intrapleurally and monitored by whole-body gamma tracking and samples of blood and urine. Even though the rate of physical decay is quite different for 131iodine and 99mtechnetium, the tracking time of labeled bacteria was limited to 24 h after injection for both radioactive isotopes. Technetium labeling was preferred because of greater imaging resolution and less radiation dose to the patient. Following intravenous administration, labeled C. parvum was found predominantly in the liver and spleen, and in a lesser amount in the lung. Radioactivity was confined to the pleural cavity after intrapleural injection. These results suggest the combined intravenous and intrapleural route of adjuvant immunosupportive agents such as C. parvum for operable lung cancer patients.  相似文献   
992.
993.
The possibility of using the enzyme (R)-Oxynitrilase in a biphasic lyotropic liquid crystal/dibutylether system has been demonstrated. This reaction system is applicable for the continuous production of (R)-benzaldehydecyanohydrin in a fixed bed reactor. The optical purity was between 94 and 96% ee and independent of the flow rate. The space time yield was maximal (2650 g/(1*d)) at a flow rate of 1.6 ml/min.  相似文献   
994.
In liver homogenate the biosynthesis ofN-acetylneuraminic acid usingN-acetylglucosamine as precursor can be followed stepwise by applying different chromatographic procedures. In this cell-free system 16 metal ions (Zn2+, Mn2+, La3+, Co2+, Cu2+, Hg2+, VO 3 , Pb2+, Ce3+, Cd2+, Fe2+, Fe3+, Al3+, Sn2+, Cs+ and Li+) and the selenium compounds, selenium(IV) oxide and sodium selenite, have been checked with respect to their ability to influence a single or possible several steps of the biosynthesis ofN-acetylneuraminic acid. It could be shown that the following enzymes are sensitive to these metal ions (usually applied at a concentration of 1 mmoll–1):N-acetylglucosamine kinase (inhibited by Zn2+ and vandate), UDP-N-acetylglucosamine-2-epimerase (inhibited by zn2+, Co2+, Cu2+, Hg2+, VO 3 , Pb2+, Cd2+, Fe3+, Cs+, Li+, selenium(IV) oxide and selenite), andN-acetylmannosamine kinase (inhibited by Zn2+, Cu2+, Cd2+, and Co2+). Dose dependent measurements have shown that Zn2+, Cu2+ and selenite are more efficient inhibitors of UDP-N-acetylglucosamine-2-epimerase than vanadate. As for theN-acetylmannosamine kinase inhibition, a decreasing inhibitory effect exists in the following order Zn2+, Cd2+, Co2+ and Cu2+. In contrast, La3+, Al3+ and Mn2+ (1 mmoll–1) did not interfere with the biosynthesis ofN-acetylneuraminic acid. Thus, the conclusion that the inhibitory effect of the metal ions investigated cannot be regarded as simply unspecific is justified.Dedicated to Professor Theodor Günther on the occasion of his 60th birthday  相似文献   
995.
996.
It has been proposed that Lacerta viridis magnifica Sobolevssky, 1930 represents an extinct species or subspecies of green lizard endemic to the southern Crimea. Using NGS protocols optimized for heavily degraded DNA, we sequenced the complete mitogenome of one of the originally formalin-preserved specimens collected in the late 19th century. A comparison with sequence data of other green lizards revealed that L. v. magnifica is a junior synonym of the northern subspecies of the western green lizard (L. b. bilineata Daudin, 1802), which occurs at least 1,500 km away, beyond the distribution ranges of other green lizards. In medieval times, a Genoese colony existed in the Crimean region where the extinct green lizards occurred. Until the early 20th century, close ties to Italy persisted, and locals of Genoese descent sent their children for education to Italy, where L. b. bilineata occurs. This suggests that the extinct Crimean green lizards have been introduced accidentally or intentionally from Italy. Our study exemplifies the value of historical formalin-preserved museum specimens for clarifying the status of questionable rare or extinct taxa.  相似文献   
997.
998.
Fertilization in the clear egg (1 mm in diameter) of the ctenophore Beroe ovata and, in particular, the positioning and movements of pronuclei, and their relationship to the larval oral-aboral axis have been observed. Fertilization can take place anywhere on the egg surface. The sperm pronucleus remains at its entry site and becomes surrounded by a specialized zone (30–50 μm in diameter) beneath the surface referred to as the sperm pronuclear zone or SPZ and devoid of large cortical granules. Polyspermy has been observed to be frequent; each pronucleus is surrounded by its own SPZ. Only the egg pronucleus migrates with a continuous velocity (averaging 18 μm/min) and moves beneath the surface directly toward the immobile sperm pronucleus. In polyspermic eggs, the egg pronucleus can probe several SPZ, each containing a single sperm nucleus, before it finally enters one SPZ and fuses with the chosen sperm pronucleus. These migrations of the egg pronucleus occur over several millimeters and take hours, but the mechanism underlying the motion or how the egg pronucleus decides which SPZ to enter is not yet known. Under our experimental conditions the mitotic apparatus and the first cleavage plane which defines the oral-aboral axis of the larva (see Reverberi (1971). “Experimental Embryology of Marine and Fresh-Water Invertebrates.” North-Holland, Amsterdam. for review) pass through the point of sperm entry. During fertilization and cleavage, movements of a cortical autofluorescent material are clearly seen. This material is segregated into micromeres as cleavage progresses.  相似文献   
999.
Abstract Pseudomonas aeruginosa possesses a peptidase N activity analogous to those described in Escherichia coli and Salmonella typhimurium . This activity resides in a protein with an M r value of 85 000. Part of this peptidase activity appears to be associated with the cytoplasmic membrane. The K M value for this peptidase bound to the cytoplasmic membrane is in the range of 0.5 mM.  相似文献   
1000.
Previous work has demonstrated that there is a selective increase in extracellular taurine in the brain during acute water intoxication. One aim of the present study was to investigate whether plasma taurine contributes to this increase. To this end, the concentrations of taurine, other amino acids, and ethanolamine (EA) were measured in plasma and CSF of urethane-anesthetized rats injected with 150 ml/kg body weight of distilled water. Blood pressure, blood gases, and pH, as well as plasma and CSF osmolality, were also measured. The CSF level of albumin was quantitated to study the function of the blood-CSF barrier. In separate experiments, hippocampal microdialysis was performed to determine the effects of acute plasma hypoosmolality on extracellular amino acids. Finally, the effect of water injection on hippocampal specific gravity and tissue amino acids was assessed. Blood gases and pH were essentially unchanged after water administration. Mean arterial blood pressure increased to peak levels approximately 50 mm Hg above control. Plasma osmolality decreased rapidly, whereas the depression of CSF osmolality was slower and less pronounced. The average volume of the hippocampus increased by 8%. Water injection was accompanied by a 25-fold elevation of taurine in plasma, whereas phosphoethanolamine (PEA) and EA increased moderately. A small fraction of the increase in plasma taurine might derive from blood cells because dilution of blood in vitro led to doubled plasma levels of the amino acid. Taurine, PEA, and EA increased consistently in CSF and hippocampal microdialysates. Plasma hypoosmolality transiently opened the blood-CSF barrier is reflected by augmented CSF concentrations of albumin.(ABSTRACT TRUNCATED AT 250 WORDS)  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