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   38139篇
  免费   66篇
  国内免费   100篇
系统科学   175篇
丛书文集   757篇
教育与普及   102篇
理论与方法论   221篇
现状及发展   16539篇
研究方法   1569篇
综合类   18432篇
自然研究   510篇
  2013年   201篇
  2012年   501篇
  2011年   1048篇
  2010年   211篇
  2008年   641篇
  2007年   667篇
  2006年   697篇
  2005年   707篇
  2004年   652篇
  2003年   680篇
  2002年   689篇
  2001年   1193篇
  2000年   1112篇
  1999年   755篇
  1994年   340篇
  1992年   720篇
  1991年   572篇
  1990年   625篇
  1989年   598篇
  1988年   583篇
  1987年   621篇
  1986年   630篇
  1985年   774篇
  1984年   620篇
  1983年   510篇
  1982年   445篇
  1981年   450篇
  1980年   551篇
  1979年   1223篇
  1978年   1044篇
  1977年   992篇
  1976年   720篇
  1975年   771篇
  1974年   1069篇
  1973年   914篇
  1972年   966篇
  1971年   1195篇
  1970年   1538篇
  1969年   1168篇
  1968年   1116篇
  1967年   1135篇
  1966年   982篇
  1965年   721篇
  1959年   417篇
  1958年   622篇
  1957年   500篇
  1956年   412篇
  1955年   355篇
  1954年   403篇
  1948年   236篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
191.
Summary The free radical oxidation of 1 and 6-tetrahydrocannabinol has been examined by spin trapping techniques and intermediates that would lead to cannabinol have been trapped. The 1st step in the oxidation of 1-THC involves the removal of 3-H, while for 6-THC, either 2-H or 5-H. Intermediates were isolated which could be pyrolysed to the diene and cannabinol.  相似文献   
192.
Summary Complete removal of villous as well as upper-crypt epithelium was achieved in vivo by vibration of an intestinal segment using tetraphenylboron sodium (TPB) as a disassociation agent.Acknowledgments. The authors thank Hicol Industrieterrein De Bosschen for supplying TPB. Research was supported by MR-II 1.3.14.p.2 grant.  相似文献   
193.
Summary Methanol extracts of the skin of the South African amphibians belonging to the genusHeleophryne (H. purcelli purcelli, H. purcelli depressa, H. purcelli orientalis, H. natalensis) contain large amounts (20–500 g/g fresh tissue) of Hyp3-bradykinin.Supported by grants from the Consiglio Nazionale delle Ricerche, Italy, and from the Ministry of Education, Japan.  相似文献   
194.
Summary 4-Isothiocyanato-4-nitrodiphenylamine possessed anthelmintic activity againstNippostrongylus dubius andHymenolepis nana in mice andH. diminuta in rats as determined by both egg counts and recovery of parasites at autopsy. No activity was detected against the cestode,Spirometra mansonoides, in cats.This investigation was supported in part by grants AI-10512 and AI-09483 from the National Institutes of Health. The authors are indebted to Dr P.P. Weinstein for providing animals infected withSpirometra mansonoides.  相似文献   
195.
Day 25 after insemination is a date of peculiar importance in the maturation of several organs in the Rabbit fetus. From day 25 onward the fetal liver stores increasing amounts of glycogen and the lung stores increasing amounts of lecithins, concomitant with sudden rise in the activity of lung phosphatidic-acid phosphohydrolase. Earlier studies on decapitated fetuses established that glycogen storage in the liver is dependent on a dual hormonal control, comprising a pituitary hormone like growth hormone or prolactin (some placental hormones share the same activity) and corticosteroids (Jost, 1961). Since the variations in endogenous corticosteroids do not seem to herald these liver or lung changes (Mulay et al., 1973), a study was made of growth hormone. Plasma immunoreactive growth hormone--determined with a heterologous Rat system (Kervran et al., 1976)--increases eightfold between days 23 and 25. During the same time plasma prolactin does not change according to McNeily and Friesen, 1978, and to unpublished data obtained with Dr McNeilly. In preliminary assays, Rat growth hormone was seen to increase phosphorylase "a" activity in the lung of 18.5 day-old Rat fetuses, thus anticipating normal development. We suggest that growth hormone plays a role in initiating liver and lung maturation.  相似文献   
196.
Summary Synthetic 3-methylcyclohex-3-en-1-one, or 3,3-MCH, decreased by 77% the flight aggregation ofDendroctonus pseudotsugae (Scolytidae) to sticky traps containing attractant (frontalin, -pinene, and resin). However, windthrown trees were protected by 3,3-MCH much less than is known to occur with the isomer 3,2-MCH.This research was supported in part by NSF grants BMS 72-02522 and DEB 77-11361 and the O.S.U. School of Forestry. We thank the administration of the Siuslaw National Forest for study sites, and Dr L.M. Libbey for GC/MS of 3,3-MCH. Oregon State University Ag. Exp. Sta. Tech. Paper No. 4313.  相似文献   
197.
Summary Microelectrophoretic administration of D--aminoadipate reversibly reduced excitatory responses of cat dorsal horn neurones evoked by iontophoretic glutamate and non-noxious peripheral stimuli but did not influence similar responses evoked by noxious peripheral stimuli or iontophoretic substance P.  相似文献   
198.
Summary The active calcium transport along the small intestine of adult rat was shown to exhibit daily rhythmic variations of considerable amplitude. Both the light-dark cycle and the time of food presentation acted as synchronizers of the rhythm observed.This work has been supported by Polish Academy of Sciences within the project MR II.1.1.6.  相似文献   
199.
G Paz  Z T Homonnai 《Experientia》1979,35(10):1412-1413
Streptozotocin diabetic rats were infertile as a result of decreased Leydig cell function of the testes. The major changes found were: decreased number of Leydig cells and their spontaneous secretion of testosterone. No change in the receptors to LH on the Leydig cells was observed. LH was found to be obligatory for the regulation of Leydig cell function and fertility.  相似文献   
200.
R Foa  M T Kramers  D Catovsky 《Experientia》1979,35(2):269-270
The levels of membrane alkaline phosphatase have been measured on different lymphocyte fractions from human peripheral blood separated on bovine serum albumin discontinuous gradients. The peak in enzyme activity was observed in a non-T-, non-B-cell fraction, rich in "null" lymphocytes; the lowest values were found in the fraction with the highest proportion of T-cells.  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