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   443182篇
  免费   51453篇
  国内免费   719篇
  2018年   4165篇
  2017年   4008篇
  2016年   5718篇
  2015年   8255篇
  2014年   9604篇
  2013年   12944篇
  2012年   15337篇
  2011年   15794篇
  2010年   10458篇
  2009年   9491篇
  2008年   13845篇
  2007年   14292篇
  2006年   13428篇
  2005年   12804篇
  2004年   12864篇
  2003年   12006篇
  2002年   11653篇
  2001年   18893篇
  2000年   18832篇
  1999年   14947篇
  1998年   5418篇
  1997年   5410篇
  1996年   5134篇
  1995年   5140篇
  1994年   4846篇
  1993年   4849篇
  1992年   12472篇
  1991年   12434篇
  1990年   12186篇
  1989年   11662篇
  1988年   10825篇
  1987年   10200篇
  1986年   9727篇
  1985年   9498篇
  1984年   7875篇
  1983年   6822篇
  1982年   5135篇
  1981年   4622篇
  1980年   4309篇
  1979年   7344篇
  1978年   5961篇
  1977年   5288篇
  1976年   4958篇
  1975年   5768篇
  1974年   6300篇
  1973年   6114篇
  1972年   5457篇
  1971年   5073篇
  1970年   4273篇
  1969年   4200篇
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
91.
To gain insight into female-to-male HIV sexual transmission and how male circumcision protects against this mode of transmission, we visualized HIV-1 interactions with foreskin and penile tissues in ex vivo tissue culture and in vivo rhesus macaque models utilizing epifluorescent microscopy. 12 foreskin and 14 cadaveric penile specimens were cultured with R5-tropic photoactivatable (PA)-GFP HIV-1 for 4 or 24 hours. Tissue cryosections were immunofluorescently imaged for epithelial and immune cell markers. Images were analyzed for total virions, proportion of penetrators, depth of virion penetration, as well as immune cell counts and depths in the tissue. We visualized individual PA virions breaching penile epithelial surfaces in the explant and macaque model. Using kernel density estimated probabilities of localizing a virion or immune cell at certain tissue depths revealed that interactions between virions and cells were more likely to occur in the inner foreskin or glans penis (from local or cadaveric donors, respectively). Using statistical models to account for repeated measures and zero-inflated datasets, we found no difference in total virions visualized at 4 hours between inner and outer foreskins from local donors. At 24 hours, there were more virions in inner as compared to outer foreskin (0.0495 +/− 0.0154 and 0.0171 +/− 0.0038 virions/image, p = 0.001). In the cadaveric specimens, we observed more virions in inner foreskin (0.0507 +/− 0.0079 virions/image) than glans tissue (0.0167 +/− 0.0033 virions/image, p<0.001), but a greater proportion was seen penetrating uncircumcised glans tissue (0.0458 +/− 0.0188 vs. 0.0151 +/− 0.0100 virions/image, p = 0.099) and to significantly greater mean depths (29.162 +/− 3.908 vs. 12.466 +/− 2.985 μm). Our in vivo macaque model confirmed that virions can breach penile squamous epithelia in a living model. In summary, these results suggest that the inner foreskin and glans epithelia may be important sites for HIV transmission in uncircumcised men.  相似文献   
92.
93.
In crustaceans, grooming behaviors decrease fouling by removing debris from the exoskeleton and body structures; these grooming behaviors improve respiration, sensory reception, movement, and reproduction. Setal morphologies of the following grooming appendages in the decapod crustacean spider crab Libinia dubia are examined including the first pereiopod (cheliped), first, second, and third maxillipeds (mouthparts), and first, second, and third epipods (internal extensions of the maxillipeds). The objective of this study was to describe setal morphologies of these grooming appendages and to elucidate possible functions and efficiencies of setal structures. Spider crabs are hypothesized to have elaborate setal morphologies, mainly for cleaning specialized decorating setae as well as for cleaning inside the gill chamber, which has a higher likelihood of becoming fouled compared to other decapods such as shrimps. Fourteen setal types are documented and included several varieties of serrate and pappose setae as well as simple setae, cuspidate setae, papposerrate setae, and canoe setae. Maxillipodal epipods in the gill chamber are free of fouling, suggesting the setation on the third maxilliped protopod has an efficient functional morphology in removing debris before water enters the gill chamber. Serrate setae may function for detangling and separating structures whereas pappose setae may function for fine detailed grooming. The cheliped is the only grooming appendage that can reach decorating setae and it contains only pappose setae; thus decorating setae is not likely groomed in a manner that would greatly decrease fouling. J. Morphol. 277:1045–1061, 2016. © 2016 Wiley Periodicals, Inc.  相似文献   
94.
95.
Growing of winter rape (Brassica napus L.) plants for 7–8 days in the presence of a sulfonylurea herbicide Magnum (methsulfuron methyl; 200 and 500 mg/L) exerted an organ-specific influence on the seedlings: suppressed hypocotyl and root growth and increased the accumulation of fresh weight of cotyledonary leaves. Exogenous 5-aminolevulinic acid (ALA; 0.1, 1.0, and 10 mg/L) partially negated the adverse effect of Magnum on the length and fresh weight of roots and hypocotyls and stimulated the development of cotyledons. The herbicide suppressed the activity of ascorbate peroxidase (APX). The addition of ALA to the 200 mg/L herbicide solution caused a steady activation of APX as compared with the effect of herbicide alone. In plants grown on Magnum solutions, the activity of glutathione reductase (GR) rose. Positive effect of exogenous ALA was only observed when 500 mg/L herbicide was used (Magnum-500 type of treatment). In plants grown on the Magnum solution, the content of reduced and oxidized forms of glutathione rose. In contrasty, exogenous ALA reduced the total content of glutathione but in this case the ratio between its reduced and oxidized forms rose. In the presence of the herbicide, the content of anthocyans considerably decreased and upon the addition of exogenous ALA their levels rose. In rape plants treated with Magnum, the ability to generate superoxide anion radical was essentially the same as in control plants; however, the content of hydrogen peroxide (H2O2) therein rose. The addition of ALA to the herbicide reduced the ability to generate superoxide anion radical and the level of H2O2. The activity of acetolactate synthase remained on the control level when 200 mg/L Magnum was used, decreased in the plants of Magnum-500 treatment, and rose upon the addition of exogenous ALA to this type of treatment. It was concluded that ALA had a positive effect on the development of winter rape resistance to Magnum via stimulation of growth processes, activation of APX and GR, predominant elevation of the content of reduced glutathione and anthocyans, and partial recovery of acetolactate synthase activity, which on the whole caused a decrease in the H2O2 level and in the ability of plants to generate superoxide anion radical.  相似文献   
96.
Ecologists are increasingly using statistical models to predict animal abundance and occurrence in unsampled locations. The reliability of such predictions depends on a number of factors, including sample size, how far prediction locations are from the observed data, and similarity of predictive covariates in locations where data are gathered to locations where predictions are desired. In this paper, we propose extending Cook’s notion of an independent variable hull (IVH), developed originally for application with linear regression models, to generalized regression models as a way to help assess the potential reliability of predictions in unsampled areas. Predictions occurring inside the generalized independent variable hull (gIVH) can be regarded as interpolations, while predictions occurring outside the gIVH can be regarded as extrapolations worthy of additional investigation or skepticism. We conduct a simulation study to demonstrate the usefulness of this metric for limiting the scope of spatial inference when conducting model-based abundance estimation from survey counts. In this case, limiting inference to the gIVH substantially reduces bias, especially when survey designs are spatially imbalanced. We also demonstrate the utility of the gIVH in diagnosing problematic extrapolations when estimating the relative abundance of ribbon seals in the Bering Sea as a function of predictive covariates. We suggest that ecologists routinely use diagnostics such as the gIVH to help gauge the reliability of predictions from statistical models (such as generalized linear, generalized additive, and spatio-temporal regression models).  相似文献   
97.
Italy counts several sheep breeds, arisen over centuries as a consequence of ancient and recent genetic and demographic events. To finely reconstruct genetic structure and relationships between Italian sheep, 496 subjects from 19 breeds were typed at 50K single nucleotide polymorphism loci. A subset of foreign breeds from the Sheep HapMap dataset was also included in the analyses. Genetic distances (as visualized either in a network or in a multidimensional scaling analysis of identical by state distances) closely reflected geographic proximity between breeds, with a clear north–south gradient, likely because of high levels of past gene flow and admixture all along the peninsula. Sardinian breeds diverged more from other breeds, a probable consequence of the combined effect of ancient sporadic introgression of feral mouflon and long‐lasting genetic isolation from continental sheep populations. The study allowed the detection of previously undocumented episodes of recent introgression (Delle Langhe into the endangered Altamurana breed) as well as signatures of known, or claimed, historical introgression (Merino into Sopravissana and Gentile di Puglia; Bergamasca into Fabrianese, Appenninica and, to a lesser extent, Leccese). Arguments that would question, from a genomic point of view, the current breed classification of Bergamasca and Biellese into two separate breeds are presented. Finally, a role for traditional transhumance practices in shaping the genetic makeup of Alpine sheep breeds is proposed. The study represents the first exhaustive analysis of Italian sheep diversity in an European context, and it bridges the gap in the previous HapMap panel between Western Mediterranean and Swiss breeds.  相似文献   
98.
The database systems PaleoTax and GONIAT provide detailed information on literature, taxonomy, morphology and occurrence of fossil invertebrate groups. PaleoTax is designed for Mesozoic corals, GONIAT for Paleozoic ammonoids, but both can be modified to cover other fossil groups. Both systems use dBASE format, but with different database structures. PaleoTax aims at the complete storage of all available objective data, GONIAT provides determinations based upon morphologic criteria, and includes information on geographic distribution and stratigraphic range of every taxon. A combination of both systems could lead to the establishment of a comprehensive paleontological information system useful for research and practical stratigraphic applications.  相似文献   
99.
100.
Regional distribution of blood flow within the diaphragm   总被引:2,自引:0,他引:2  
We investigated the regional distribution of blood flow (Q) within the costal and crural portions of the diaphragm in a total of eight anesthetized supine mongrel dogs. Q was measured with 15-microns microspheres, radiolabeled with three different isotopes, injected into the left ventricle during spontaneous breathing (SB), inspiratory resistive loading (IR), and mechanical ventilation after paralysis (P). At necropsy, the costal and crural portions of each hemidiaphragm were arbitrarily subdivided along a sagittal plane into five to seven and three sections, respectively. During P, there was a dorsoventral Q gradient within the costal part of the diaphragm. During SB there was a fourfold increase in the gradient of Q. Furthermore, during IR, in which mouth pressures of -16 +/- 4 cmH2O were generated, there was a further increase in the gradient of Q. During both SB and IR, Q to the most ventral portion of the costal diaphragm was 26 +/- 6% less than the peak value. In two dogs, studied prone and supine, there was no difference in the Q gradients between the two postures. Over the dorsal 80% of the costal diaphragm there was also a dorsoventral gradient of muscle thickness, such that the most dorsal part was 54 +/- 2% (n = 5) that of the ventral portion. In contrast, there was no consistent gradient of Q or muscle thickness within the crural diaphragm. Our results demonstrate a topographical gravity-independent distribution of Q in the costal, but not the crural, diaphragm.(ABSTRACT TRUNCATED AT 250 WORDS)  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