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   526167篇
  免费   49846篇
  国内免费   227篇
  2018年   5495篇
  2017年   5264篇
  2016年   7009篇
  2015年   8134篇
  2014年   10068篇
  2013年   14175篇
  2012年   16538篇
  2011年   16990篇
  2010年   11779篇
  2009年   10376篇
  2008年   15074篇
  2007年   15752篇
  2006年   14832篇
  2005年   14119篇
  2004年   13925篇
  2003年   13471篇
  2002年   13449篇
  2001年   25778篇
  2000年   25740篇
  1999年   19879篇
  1998年   6021篇
  1997年   6156篇
  1996年   5817篇
  1995年   5420篇
  1994年   5294篇
  1993年   5082篇
  1992年   16017篇
  1991年   15881篇
  1990年   15411篇
  1989年   14989篇
  1988年   13970篇
  1987年   13017篇
  1986年   11876篇
  1985年   12008篇
  1984年   9569篇
  1983年   8192篇
  1982年   5859篇
  1981年   5088篇
  1980年   4862篇
  1979年   9029篇
  1978年   6822篇
  1977年   6178篇
  1976年   5808篇
  1975年   6799篇
  1974年   7236篇
  1973年   7125篇
  1972年   6500篇
  1971年   5837篇
  1970年   5155篇
  1969年   4890篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
221.
The infection of Planorbarius banaticus with sporocysts of Cotylurus cornutus is accompanied by an increase in aldolase activity of molluscs' haemolymph of 1.2 fold. In solutions of nitroammofoska (0.1, 1 and 10 mg/1) the activity of this ferment in infected individuals increases much higher than in non-infected ones. This results in fast carbohydrate expenditures by molluscs, intermediate hosts of trematodes, and their death from exhaustion.  相似文献   
222.
Infravesical obstruction in 300 male patients was determined by prostatic sclerosis (PS). PS is a polyetiological disease developing more frequently as a result of prostatitis, autoimmune, allergic and congestive factors. Sclerosis and shriveling of the gland cause topographo-anatomic changes in the infravesical area, well revealed during a combined roentgeno-urological study. Direct signs of PS are the following: stenosis, deformity and shortening of the prostatic part of the urethra, a high position of the lower outline of the bladder resembling braces over the public symphysis; indirect signs: change of the bladder shape, megacyst, trabecular mucosa and diverticulosis, congestion in the upper urinary tracts, uni- or bilateral urethero-hydronephrosis.  相似文献   
223.
The effects of corticotropin-releasing hormone (CRH) injected into the dorsal neostriatum on the open-field and shuttle-box behavior were studied in rats with high (Koltushi high avoidance, KHA) and low (Koltushi low avoidance, KLA) capability for avoidance learning. The effects of this hormone on the behavior of these rat strains were different. In KLA rats with passive strategy of behavior the CRH injection led to a rapid locomotor activation in the open field, while the rats with active behavioral strategy (KHA) reacted to the injection by a significant decrease in locomotion and change for the passive mode of behavior. The same CRH effects on locomotion were obtained in the shuttle-box experiments. Moreover, in the KLA rats the neurohormone injection resulted in an improvement of avoidance learning in contrast to the KHA rats, in which CRH substantially impaired avoidance learning. The obtained evidence is discussed in terms of the important role of striatal CRH in the choice of behavioral strategy in stress.  相似文献   
224.
1. Although many studies have focussed on the effects of catchment land use on lotic systems, the importance of broad (catchment) and fine (segment/reach) scale effects on stream assemblages remain poorly understood. 2. Nine biological metrics for macrophytes (498 sites), benthic macroinvertebrates (491) and fish (478) of lowland and mountain streams in four ecoregions of France and Germany were related to catchment and riparian buffer land use using partial Redundancy Analysis and Boosted Regression Trees (BRTs). 3. Lotic fauna was better correlated (mean max., r = 0.450) than flora (r = 0.277) to both scales of land use: the strongest correlations were noted for mountain streams. BRTs revealed strong non‐linear relationships between mountain assemblage metrics and land use. Correlations increased with increasing buffer lengths, suggesting the importance of near‐stream land use on biotic assemblages. 4. Several metrics changed markedly between 10–20% (mountain ecoregions) and 40–45% (lowland) of arable land use, irrespective of the buffer size. At mountain sites with >10% catchment arable land use, metric values differed between sites with <30% and sites with >30% forest in the near‐stream riparian area. 5. These findings support the role of riparian land use in catchment management; however, differences between mountain and lowland ecoregions support the need for ecoregion‐specific management.  相似文献   
225.
X-IRRADIATION of mammalian cells in culture yields a survival curve of the threshold type (for review see ref. 1). It isjnter-esting to ask how one can enhance the radiation response by small changes of the physical environment of the cells, as can be done chemically, for example, by incorporation of 5-bromo-deoxyuridine into DNA1,2. Elevation of the temperature is a likely prospect for enhancement of radiosensitivity for the following reasons. It is known that proteins are heat labile and that temperature sensitive mutants of bacteria and phage can be obtained for many different enzymes3 which are operative at 37° C but not at 42° or 43°C. For example4, DNA polymerase is reversibly temperature sensitive; it is rendered inoperative above 42°C, but will be functional again when the temperature is lowered. It is not unreasonable to expect that temperature sensitive mutations for many enzymes occur frequently and that the use of temperatures somewhat higher than the normal range at which the cells grow might disclose sensitivities for specific enzymes in normal cells of higher organisms.  相似文献   
226.
227.
228.
229.
230.
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