全文获取类型
收费全文 | 494417篇 |
免费 | 49915篇 |
国内免费 | 494篇 |
专业分类
544826篇 |
出版年
2018年 | 4565篇 |
2016年 | 6135篇 |
2015年 | 7570篇 |
2014年 | 8937篇 |
2013年 | 13258篇 |
2012年 | 14327篇 |
2011年 | 15220篇 |
2010年 | 10233篇 |
2009年 | 9299篇 |
2008年 | 13556篇 |
2007年 | 13829篇 |
2006年 | 13189篇 |
2005年 | 12655篇 |
2004年 | 12461篇 |
2003年 | 12124篇 |
2002年 | 11777篇 |
2001年 | 22142篇 |
2000年 | 22184篇 |
1999年 | 17283篇 |
1998年 | 5731篇 |
1997年 | 6042篇 |
1996年 | 5875篇 |
1995年 | 5312篇 |
1994年 | 5249篇 |
1993年 | 5142篇 |
1992年 | 14217篇 |
1991年 | 14002篇 |
1990年 | 13735篇 |
1989年 | 13500篇 |
1988年 | 12525篇 |
1987年 | 11932篇 |
1986年 | 10801篇 |
1985年 | 11270篇 |
1984年 | 9145篇 |
1983年 | 7825篇 |
1982年 | 5773篇 |
1981年 | 5381篇 |
1980年 | 4979篇 |
1979年 | 8627篇 |
1978年 | 6629篇 |
1977年 | 6318篇 |
1976年 | 5978篇 |
1975年 | 6754篇 |
1974年 | 7317篇 |
1973年 | 7275篇 |
1972年 | 6420篇 |
1971年 | 6026篇 |
1970年 | 5389篇 |
1969年 | 5093篇 |
1968年 | 4768篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
An unusual case of neurofibrosarcoma of the cranio-orbital-maxillary region was resected by a combined neurosurgical and plastic surgical team. The resulting defect was reconstructed in one stage with a free rectus abdominis myocutaneous flap, obviating the need for subsequent prosthetic obturation of the maxillary defect. This reconstructive technique expedited the patient's convalescent period, allowing him to return to work 3 weeks following surgery. Wound healing was uneventful, and the cosmetic result was acceptable to the patient. 相似文献
992.
S V Litvinov I V Remennik I N Kriukova 《Biulleten' eksperimental'no? biologii i meditsiny》1984,97(5):600-603
Indirect radioimmunoassay and immunoperoxidase studies provided further evidence of human serum reactivity with murine mammary tumor virus (MMTV) structural proteins. Examination of over 400 human sera from breast cancer (BC) patients and controls has shown that the incidence of antibodies which react with MMTV structural proteins was significantly higher in BC patients (50%) than in patients with carcinomas of other organs (3%) or normal women (3%). However, the percentage of subjects immune to MMTV was higher in pregnant women (10%) as compared with normal subjects. All the women with BC of the first clinical stage had antibodies to MMTV. The percentage of immune donors was lower among patients with BC of the later stages (IIa, 90%, IIb, 76%, III, 27%, IV, 0%). Thus, a good agreement has been demonstrated between the first stages of BC and expression of antibodies to MMTV. 相似文献
993.
Murine retrovirus-induced spongiform encephalomyelopathy: host and viral factors which determine the length of the incubation period. 下载免费PDF全文
A molecular clone of wild mouse ecotropic retrovirus CasBrE (clone 15-1) causes a spongiform neurodegenerative disease with a long incubation period, greater than or equal to 6 months. This virus infects the central nervous system (CNS) at low levels. In contrast, a chimeric virus, FrCasE, containing env and 3' pol sequences of 15-1 in a Friend murine leukemia virus background, infects the CNS at high levels and causes a rapid neurodegenerative disease with an incubation period of only 16 days. With both viruses, the induction of neurologic disease is dependent on inoculation during the perinatal period. Since the length of the incubation period of this disease appears to be a function of the relative level of CNS infection, we have attempted to identify the viral and host factors which determine the relative level of virus infection of the CNS. It was previously shown that the CNS is susceptible to infection only during the perinatal period (M. Czub, S. Czub, F. J. McAtee, and J. L. Portis, J. Virol. 65:2539-2544, 1991). Here we have found that the susceptibility of the CNS wanes progressively or gradually as a function of the age of the host, this age-dependent resistance being complete by 12 to 14 days of age. Utilizing a group of chimeric viruses, we found that the relative level of CNS infection achieved after inoculation of mice at 1 day of age was a function of the kinetics of virus replication and spread in peripheral organs. Viruses which reached peak viremia titers early (5 to 7 days of age) infected the CNS at high levels, and viruses which reached peak titers later infected the CNS at lower levels. Among the group of viruses examined in the current study, the kinetics of peripheral virus replication and spread appeared to be influenced primarily by sequences within the R-U5-5' leader region of the viral genome. These results suggested that the relative level of CNS infection was determined very early in life and appeared to be a function of a dynamic balance between the kinetics of virus replication in the periphery and a progressively developing restriction of virus replication in the CNS. 相似文献
994.
A lightweight finger printing stand is described which can be adjusted to the proper printing height. Based upon experience printing over 1,100 subjects, 12 advantages of using the stand are suggested. 相似文献
995.
996.
E Cr?cea S Constantinescu N Tofan F C?runtu D Dogaru 《Archives roumaines de pathologie expérimentales et de microbiologie》1989,48(1):13-17
Q fever urban sporadic cases in Romania in the 1981-87 period are reviewed as concerns their incidence, seasonality and epidemiological data. Urban cases represented 76.8% as against 23.2% rural cases from the 134 Q fever sporadic cases detected in this period. Cases were distributed throughout all months with peaks during April (18.4% of cases) and June (19.4%). Infections were not related to contact with livestock or domestic birds or with raw milk drinking. Cases could not be identified as Q fever occupational ones. 36.7% of patients were working in nonalimentary industry and only 12.6% were involved in meat industry, veterinary or human medical practices. New studies concerning unusual sources of urban infection such as dogs, cats or urban street pigeons are emphasized. 相似文献
997.
998.
The spontaneous allele quakingviable (qk
v
) exerts effects on myelination and spermiogenesis. The defects generated by qk
v
were not separated in a multilocus mapping cross that provided a mapping resolution of 0.1 centiMorgans (cM). Furthermore, no distortions suggestive of a large chromosomal anomaly associated with qk
v
were apparent. One plausible interpretation is that the quaking locus contains more than one functional domain, either organized into overlapping genes or expressed by alternative splicing mechanisms. The cloning needed to analyze this locus will be enhanced by the very high resolution of the meiotic mapping cross reported here. The recombinational distances on this qk
v
map were compressed compared with those previously reported in a high-resolution map for qk
1–1, an embryonic lethal allele of quaking induced by ethylnitrosourea. Additional crosses confirmed prior reports that the sex and the genetic background of the heterozygous parent can affect recombinational distances. These joint effects on recombination are strong enough to account for the discrepancy between the two maps. This variability of two-factor map values leads to the preferred multilocus map-building protocol discussed in the accompanying paper. 相似文献
999.
Comparison of uptake kinetics in freshly isolated suspensions and short-term primary cultures of rat hepatocytes 总被引:2,自引:0,他引:2
The apparent kinetics of uptake of various model substrates were examined for hepatocytes in suspension and primary culture up to 72 h. The ability of hepatocytes to take up taurocholate and ouabain was decreased in culture. Vmax for uptake of both substrates diminished rapidly with increasing time in culture. An increase in Km was observed in cultures 6 h after plating, but there was no further change with prolongation of culture time. The decrease of uptake of taurocholate and ouabain during culture may be due to the reduction in the number of transport carriers plus a decrease of affinity of the carrier to substrates. The nonsaturable component of cadmium uptake was much reduced in cultured cells compared with the suspensions. The saturable process was lower in 6 h culture but increased to a level comparable with the fresh cells at longer culture time. No significant change was found in the Km between suspensions and cultures. Uptake of alpha-aminoisobutyric acid was greater in culture while that of 3-O-methyl-D-glucose was relatively stable but about one-half that found in cell suspension. Thus, uptake of two substrates, taurocholate and ouabain, is clearly compromised with increasing time in primary culture, while uptake of the other substrates does not reflect such a dramatic decrease. It is therefore apparent that the cell preparation of choice in uptake studies depends on the substrate and the nature of the experiments. 相似文献
1000.