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   162252篇
  免费   16911篇
  国内免费   546篇
  179709篇
  2021年   1084篇
  2018年   1800篇
  2017年   1751篇
  2016年   2468篇
  2015年   3387篇
  2014年   3938篇
  2013年   5026篇
  2012年   6029篇
  2011年   5772篇
  2010年   3704篇
  2009年   3230篇
  2008年   4824篇
  2007年   4827篇
  2006年   4762篇
  2005年   4423篇
  2004年   4302篇
  2003年   4181篇
  2002年   4161篇
  2001年   11106篇
  2000年   11092篇
  1999年   8435篇
  1998年   2098篇
  1997年   2180篇
  1996年   1947篇
  1995年   1753篇
  1994年   1669篇
  1993年   1580篇
  1992年   5563篇
  1991年   5273篇
  1990年   4706篇
  1989年   4690篇
  1988年   4246篇
  1987年   3657篇
  1986年   3334篇
  1985年   3200篇
  1984年   2439篇
  1983年   2158篇
  1982年   1603篇
  1981年   1352篇
  1980年   1191篇
  1979年   2243篇
  1978年   1718篇
  1977年   1501篇
  1976年   1286篇
  1975年   1511篇
  1974年   1517篇
  1973年   1484篇
  1972年   1387篇
  1971年   1271篇
  1970年   1077篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
761.
Asterionic sutural patterns in Plio-Pleistocene hominid crania have never been examined in detail. We present an analysis of this anatomical region in Australopithecus and Homo and relate different sutural patterns to functional changes in the masticatory apparatus. The great apes and A. afarensis share the common adult higher primate sutural pattern referred to as the "asterionic notch," which develops in response to the hypertrophy of posterior temporalis muscle fibers and the consequent formation of compound temporal/nuchal crests. This sutural configuration also appears to be present on the early Homo cranium KNM-ER 1805. In contrast, adult male A. boisei crania exhibit a unique pattern where the temporal squama overlaps the parietal which, in turn, overlaps the par mastoidea and the upper scale of the occipital bone. We relate this arrangement to the need to reinforce the rear of a thin-walled braincase against the net tensile forces exerted by the temporalis and nuchal muscles. The common juvenile hominoid edge-to-edge asterionic articulation is maintained in adult A. africanus, A. robustus, female A. boisei, and most Homo crania. We discuss the latter pattern in regard to anterior temporalis hypertrophy in A. africanus, A. robustus, and A. boisei and to craniofacial paedomorphosis in Homo.  相似文献   
762.
Cytologic examination of the pleural effusion from a patient with acute leukemia, leukocytosis and bleeding revealed the presence of many leukemic cells, "lymphocytes" and erythrocytes. The significance of these cellular changes was investigated by simultaneous study of blood and effusion leukocytes by morphologic, cytochemical and immunochemical methods. Both the leukemic blasts and the "lymphocytes" in the effusion and the blood were found to be neoplastic and contained antigens characteristic of both myeloid cells (OKM-1) and lymphoblasts (C-ALLA, common acute lymphoblastic leukemia antigen). These results, when analyzed in the context of the clinical findings, were indicative of acute leukemia with pleural involvement. Such a clinically oriented approach may further enhance the potential of cytodiagnosis in patients with serous effusions.  相似文献   
763.
Distributional and morphological features, especially characteristics of the ramification of serotonin-containing supraependymal fibers (SEF), were studied in the ventricular systems of mammals (mouse, rat, guinea pig, rabbit, cat, dog, monkey) by means of a modified peroxidase antiperoxidase technique, using antiserotonin antiserum prepared in our laboratory. SEF were present in all ventricular systems, except on the third ventricle floor and in the choroid plexus. The density of SEF was higher in the smaller species. In the rat, light- and scanning electron microscopical SEF were almost completely abolished 1 week after intraventricular administration of 5,6-dihydroxytryptamine. Ramification of SEF was complicated; the SEF formed a true network with frequent anastomosing. In the ventricular system of rats rendered hydrocephalic by kaolin administration, the mode of axonal branching in the supraependymal plexus could best be analyzed by the scanning electron microscope because the meshes of the plexus were spread out.  相似文献   
764.
765.
766.
The ultrastructure of the luminal epithelium of the hedgehog uterus is described on the basis of material taken from 11 animals in three different hormonal situations: castrated, active and hibernating animals. The whole uterine epithelium is composed of microvillous cells. Its appearance is very similar in ovariectomized and hibernating animals. It differs from that observed in active animals where the epithelium is taller, microvilli are more numerous and longer, and where nuclei and cytoplasm display a very active ultrastructural appearance. The results now available indicate that ultrastructural changes occurring within the cells are certainly correlated with plasma sex steroid hormone concentrations. The present paper also reports the regular occurrence of nuclear bodies in uterine cells.  相似文献   
767.
Pigs deriving from 150 breeding centres constituting a representative section of elite breeding herds (2496 Swiss Landrace pigs, 587 Swiss Large White pigs) were subjected to blood typing during the period 1981 to 1984. Production traits such as daily gain, feed conversion ratio, lean meat content and meat quality score were available to show the trend in these performance traits since 1978. Field data on the halothane reaction of 14 270 Swiss Landrace (SL) pigs were used to assess the porcine stress syndrome during the period 1978-1983. In SL pigs the frequency of the alleles Ha, PhiB and AdaA decreased significantly, and that of the Hc and PhiA increased during the period of the study. The frequency of the Ha allele dropped from 0.36 in 1981 to 0.20 in 1984, whereas the Hc allele rose from 0.22 to 0.37. In Swiss Large White (SLW) pigs, on the other hand, the frequency of the Ha allele increased constantly from 0.31 to 0.37 during this period. An initial frequency of 17.7% (1978) halothane reactors in SL pigs was lowered to 0.7% (1982) after five years of halothane testing. In SL pigs the meat quality scores improved regularly, whereas in SLW pigs it did not change very much. The percentage of PSE animals in the SL breed was reduced from 32.7% in 1978 to 7.1% in 1983. Because the Hal locus is associated with production traits such as meat quality, linkage disequilibria could explain the observed associations between the H and Phi types and production traits.  相似文献   
768.
Serum antifreeze polypeptides (AFP) from Newfoundland ocean pout have been resolved by ion exchange chromatography and reverse phase high performance liquid chromatography into at least 12 components. The protein sequences of three of the AFP were determined using a combination of protein Edman degradation and cDNA sequencing. The AFP precursor protein encodes for a preprotein of 87 amino acids with no obvious prosequences. Two of the AFP (SP1-A and SP1-C) were separate gene products with minor amino acid sequence differences. The protein structure of SP1-C precursor is MKSVILTGLLFVLLCVDHMTASQSVVAT QLIPINTALTPAMMEGKVTNPIGIPFAEMSQIVGKQVNTPVAKGQTLMPNMVKTYVAGK. The third AFP (SP1-B) is a post-translation modification product of SP1-C. These experiments indicate that the ocean pout AFP are a multigene family with protein structure different from any other known polypeptide antifreezes.  相似文献   
769.
The gene structure of cytochrome P-450b, a major form of phenobarbital-inducible cytochrome P-450 in rat livers was elucidated by sequence analysis of the cloned genomic DNAs and was compared with the previously determined gene structures of cytochrome P-450e, a minor form of phenobarbital-inducible cytochrome P-450 and two forms of 3-methylcholanthrene-inducible cytochrome P-450 (P-450c and -d). The gene for cytochrome P-450b is 23 kilobase pairs (kb) long and is separated into 9 exons by 8 intervening sequences. This gene structure is very similar to that of cytochrome P-450e except for the first intron, the first intron being much longer in cytochrome P-450b gene (approximately 12 kb) than in cytochrome P-450e gene (3.2 kb), but differs greatly from the gene structures of two 3-methylcholanthrene-inducible cytochrome P-450s as pointed out previously (Sogawa, K., Gotoh, O., Kawajiri, K. & Fujii-Kuriyama, Y. (1984) Proc. Natl. Acad. Sci. U.S.A. 81, 5066-5070). The nucleotide sequences in all 9 exons and their flanking regions in introns show very close homology between the two phenobarbital-inducible cytochrome P-450 genes. Forty base substitutions are found in approximately 1900 nucleotides of all exonic sequences, and 15 of them result in 14 amino acid replacements. These base substitutions occur in relatively limited regions of the gene sequences. Most of them are found in exons 6, 7, 8, and 9, most frequently in exon 7 as described previously (Mizukami, Y., Sogawa, K., Suwa, Y., Muramatsu, M. & Fujii-Kuriyama, Y. (1983) Proc. Natl. Acad. Sci. U.S.A. 80, 3958-3962). The close sequence homology between the two phenobarbital-inducible cytochrome P-450 genes is also found to extend to the promoter region with one notable exception. The simple repeated sequences of (CA)n which is present at -254 position in cytochrome P-450e gene is also observed at the equivalent position in cytochrome P-450b gene, but the repetitiveness is greatly reduced in cytochrome P-450b gene ((CA)5 for P-450b versus (CA)19 for P-450e), and this may somehow be related to the difference in the level of cytochrome P-450b and P-450e in the inductive phase of phenobarbital administration.  相似文献   
770.
We have previously demonstrated that 1) BALB/c mice and patients with IgA myeloma developed a marked expansion of T cells with surface IgA-Fc receptors (T alpha cells), 2) the FcR alpha were induced by direct interaction of soluble myeloma IgA with T cells, and 3) the T alpha cells induced in IgA myeloma were Lyt-1-2+, IgA isotype-specific suppressor cells in normal immune responses. These findings established that the host with IgA myeloma responds to the large amounts of Ig produced by the tumor by activating an otherwise normal IgA isotype-specific suppressor circuit. In the present studies, we extend our previous observations and show that T alpha cells can suppress both the growth and secretion of MOPC-315 myeloma tumor cells. Thus, the isotype-specific immunoregulatory circuit activated in myeloma is capable of suppressing tumor cells as well as normal 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