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   1278752篇
  免费   109672篇
  国内免费   1459篇
  1389883篇
  2021年   17832篇
  2020年   12621篇
  2019年   16186篇
  2018年   17340篇
  2017年   16093篇
  2016年   27847篇
  2015年   42300篇
  2014年   50331篇
  2013年   76572篇
  2012年   34412篇
  2011年   22244篇
  2010年   42582篇
  2009年   44327篇
  2008年   21671篇
  2007年   19188篇
  2006年   25017篇
  2005年   26004篇
  2004年   25365篇
  2003年   23017篇
  2002年   21169篇
  2001年   29054篇
  2000年   26112篇
  1999年   27387篇
  1998年   24876篇
  1997年   24703篇
  1996年   24331篇
  1995年   22353篇
  1994年   22302篇
  1993年   21295篇
  1992年   24236篇
  1991年   22825篇
  1990年   21398篇
  1989年   22547篇
  1988年   20544篇
  1987年   19551篇
  1986年   18579篇
  1985年   20617篇
  1984年   20895篇
  1983年   18513篇
  1982年   19644篇
  1981年   19061篇
  1980年   17749篇
  1979年   17038篇
  1978年   16406篇
  1977年   16062篇
  1976年   15274篇
  1975年   14581篇
  1974年   15025篇
  1973年   15385篇
  1972年   12886篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
191.
192.
The mechanisms of hemodynamic responses to orthostatic stresses and orthostatic stability (OS) of cosmonauts were studied before and after short-and long-term spaceflights (SFs) using orthostatic tests, as well as before, during, and after SFs using ultrasonic methods in tests with exposure to lower body negative pressure (LBNP). The capacitance and distensibility of the veins of the lower extremities were studied using occlusive air plethysmography before, during, and after SFs of different durations. A stay in microgravity has been proved to result in detraining of, mainly, the vascular mechanisms of compensating orthostatic perturbations. It has been established that the decrease in OS under the influence of microgravity is determined by a reduction of the vasoconstrictive ability of large blood vessels of the lower extremities; an increase in venous distensibility and capacitance of the legs; and an impairment of blood flow regulation, which leads to a cerebral blood flow deficit in orthostatic stresses, and of the initial individual OS before the flight. The results of preflight studies of hemodynamics by ultrasonic methods at LBNP and the data of orthostatic tests before SFs make it possible to predict the degree of decrease of OS after an SF proceeding in the normal mode. At the same time, the data of ultrasonic blood flow examination provide more a accurate estimation of OS and make it possible to assess the physiological reserves of hemodynamic regulation and to reveal the loss of regulation capacity even in cases where integrated indices (heart rate and blood pressure) are within the normal ranges.  相似文献   
193.
In 1990 and 1994, two males of Golden Eagles were found with small lead particles in their stomachs. Both individuals died soon after they had been taken sitting on the soil in bad condition. There are some findings of dead eagles in the same area which showed no sign of having been injured. The two cases of presumed lead poisoning coincide with intense efforts to reduce the number of ChamoixRupicapra rupicapra by hunting. The bullets used contain a nucleus of lead and eagles were seen taking the entrails of the shot chamoix.  相似文献   
194.
195.
196.
Agapophytinae subf.n. is a highly diverse lineage of Australasian Therevidae, comprising eight described and two new genera: Agapophytus Guérin‐Méneville, Acupalpa Kröber, Acraspisa Kröber, Belonalys Kröber, Bonjeania Irwin & Lyneborg, Parapsilocephala Kröber, Acatopygia Kröber, Laxotela Winterton & Irwin, Pipinnipons gen.n. and Patanothrix gen.n. A genus‐level cladistic analysis of the subfamily was undertaken using sixty‐eight adult morphological characters and c. 1000 base pairs of the elongation factor‐1α (EF‐1α) protein coding gene. The morphological data partition produced three most parsimonious cladograms, whereas the molecular data partition gave a single most parsimonious cladogram, which did not match any of the cladograms found in the morphological analysis. The level of congruence between the data partitions was determined using the partition homogeneity test (HTF) and Wilcoxon signed ranks test. Despite being significantly incongruent in at least one of the incongruence tests, the partitions were combined in a simultaneous analysis. The combined data yielded a single cladogram that was better supported than that of the individual partitions analysed separately. The relative contributions of the data partitions to support for individual nodes on the combined cladogram were investigated using Partitioned Bremer Support. The level of support for many nodes on the combined cladogram was non‐additive and often greater than the sum of support for the respective nodes on individual partitions. This synergistic interaction between incongruent data partitions indicates a common phylogenetic signal in both partitions. It also suggests that criteria for partition combination based solely on incongruence may be misleading. The phylogenetic relationships of the genera are discussed using the combined data. A key to genera of Agapophytinae is presented, with genera diagnosed and figured. Two new genera are described: Patanothrix with a new species (Pat. skevingtoni) and Pat. wilsoni (Mann) transferred from Parapsilocephala, and Pipinnipons with a new species (Pip. kroeberi). Pipinnipons fascipennis (Kröber) is transferred from Squamopygia Kröber and Pip. imitans (Mann) is transferred from Agapophytus. Agapophytus bicolor (Kröber) is transferred from Parapsilocephala. Agapophytus varipennis Mann is synonymised with Aga. queenslandi Kröber and Aga. flavicornis Mann is synonymised with Aga. pallidicornis (Kröber).  相似文献   
197.
198.
199.
200.
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