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   46094篇
  免费   87篇
  国内免费   148篇
系统科学   455篇
丛书文集   1295篇
教育与普及   84篇
理论与方法论   147篇
现状及发展   20421篇
研究方法   1667篇
综合类   21482篇
自然研究   778篇
  2013年   310篇
  2012年   594篇
  2011年   1367篇
  2008年   716篇
  2007年   848篇
  2006年   800篇
  2005年   841篇
  2004年   907篇
  2003年   794篇
  2002年   771篇
  2001年   1293篇
  2000年   1246篇
  1999年   814篇
  1992年   843篇
  1991年   653篇
  1990年   693篇
  1989年   632篇
  1988年   630篇
  1987年   663篇
  1986年   676篇
  1985年   905篇
  1984年   672篇
  1983年   550篇
  1982年   481篇
  1981年   514篇
  1980年   644篇
  1979年   1424篇
  1978年   1111篇
  1977年   1073篇
  1976年   881篇
  1975年   947篇
  1974年   1318篇
  1973年   1119篇
  1972年   1188篇
  1971年   1351篇
  1970年   1842篇
  1969年   1440篇
  1968年   1271篇
  1967年   1307篇
  1966年   1207篇
  1965年   904篇
  1964年   281篇
  1959年   506篇
  1958年   877篇
  1957年   634篇
  1956年   526篇
  1955年   471篇
  1954年   508篇
  1953年   262篇
  1948年   359篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
881.
Summary One of two cases of acquired immune deficiency syndrome-related persistent generalized lymphadenopathy revealed a profoundly altered pattern of dendritic reticulum cells as demonstrated by immunoreactive acid cysteine proteinase inhibitor. The alterations could be related to totally or partially destructed lymphoid secondary follicles.Acknowledgments. This work has been partially supported by the grant from the Sigrid Juselius Foundation, Finland.  相似文献   
882.
The phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) mimicked the effects of isoprenaline on the force of contraction, the cAMP content and the slow Ca++ inward current (Isi) in isolated guinea pig papillary muscles. The results support the hypothesis that phosphodiesterase inhibitors and beta-adrenoceptor agonists exert their positive inotropic effects by increasing Isi via the common mediator cAMP.  相似文献   
883.
28 anticancer agents have been surveyed for Epstein-Barr virus (EBV) activating potency. Two vinca alkaloids with antitubulin activity, vinblastine (VLB) and vincristine (VCR), enhanced the expression of EBV early antigens (EA) in a latently infected human lymphoblastoid cell line (Raji), when used in combination with n-butyrate. Other antitubulin agents, colchicine, colcemid, and podophyllotoxin, had the same effect, although their effects were less than that of the potent tumor promoter, 12-O-tetradecanoyl phorbol-13-acetate (TPA).  相似文献   
884.
885.
    
Conclusion The insulin receptor is an integral protein of the plasma membrane of the cell. It is composed of two subunits: an subunit, which binds the hormone, and a subunit which is a tyrosine specific protein kinase capable of undergoing autophosphorylation. These independent subunits are synthesized by way of a higher molecular weight single chain precursor and thus are the product of a single gene29, 49, 85 localized to chromosome 1929, 91. Assuming that the insulin receptor is synthesized in the same fashion as other integral membrane glycoproteins, then the nucleus, the rough endoplasmic reticulum, and the Golgi apparatus are involved in its biosynthesis. Further, there must be some form of transport of the mature receptor subunits to the plasma membrane where they are inserted.By contrast, the endocytotic route involves coated pits, coated vesicles, large clear vesicles or endosomes, multivesicular bodies and other lysosomal forms. In addition, it is possible that some other as yet unidentified organelle is involved in recycling (fig. 8). At the present time, with respect to the insulin receptor, the biosynthetic pathway and the endocytotic pathway appear to be separate. Further, it does not appear that either pathway, i. e. synthesis or endocytosis, exerts a regulatory function over the other.  相似文献   
886.
Summary The suitability of urethane anesthesia for physiopharmacological investigations is reviewed. Total dose administered and route of administration are recognized as factors having a great influence on both resting parameters and biological responses to drugs. A peculiar characteristic of urethane is represented by its ability to induce a surgical plane of anesthesia without affecting neurotransmission in various subcortical areas and the peripheral nervous system. This makes urethane a suitable general anesthetic for studying neural function in both central and peripheral nervous systems and accounts for the preservation of a number of reflex responses in urethane-anesthetized animals.  相似文献   
887.
888.
In ovariectomized rats the desensitization of the LH cells in vivo, which develops during constant rate infusion of LHRH, 1) does not depend on a concomitant depletion of the pituitary LH stores, 2) proceeds normally when the hypothalamo-pituitary connection has been severed and 3) is a process in which LH itself is not involved.  相似文献   
889.
Summary The location of gene expression by hybridization histochemistry is being applied in many areas of research and diagnosis. The aim of this technique is to detect specific mRNA in cells and tissues by hybridization with a complementary DNA or RNA probe. Requirements for optimal specificity, sensitivity, resolution and speed of detection may not all be encompassed in one simple technique suitable for all applications, thus appropriate procedures should be selected for specific objectives. With reference to published procedures and our own extensive experience, we have evaluated fixatives, probes, labels and other aspects of the technique critical to the preservation and hybridization in situ of mRNA and detection and quantition of hybrids.  相似文献   
890.
    
Summary The comparative study of some steroids in castrated rats has shown 19-norandrostenolone undecylate to stimulate the incorporation of proline-14C in bone and the association of this substance with other steroids reducing its hormonal action to produce an increase in its effect on bone collagen formation.  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