全文获取类型
收费全文 | 4539篇 |
免费 | 641篇 |
专业分类
5180篇 |
出版年
2021年 | 72篇 |
2018年 | 41篇 |
2017年 | 45篇 |
2016年 | 62篇 |
2015年 | 121篇 |
2014年 | 162篇 |
2013年 | 176篇 |
2012年 | 258篇 |
2011年 | 257篇 |
2010年 | 149篇 |
2009年 | 139篇 |
2008年 | 197篇 |
2007年 | 207篇 |
2006年 | 177篇 |
2005年 | 202篇 |
2004年 | 186篇 |
2003年 | 183篇 |
2002年 | 176篇 |
2001年 | 163篇 |
2000年 | 128篇 |
1999年 | 106篇 |
1998年 | 70篇 |
1997年 | 61篇 |
1996年 | 39篇 |
1995年 | 39篇 |
1994年 | 37篇 |
1993年 | 51篇 |
1992年 | 84篇 |
1991年 | 108篇 |
1990年 | 89篇 |
1989年 | 91篇 |
1988年 | 79篇 |
1987年 | 63篇 |
1986年 | 86篇 |
1985年 | 68篇 |
1984年 | 62篇 |
1983年 | 62篇 |
1982年 | 42篇 |
1981年 | 41篇 |
1980年 | 45篇 |
1979年 | 58篇 |
1978年 | 52篇 |
1977年 | 40篇 |
1976年 | 50篇 |
1975年 | 48篇 |
1974年 | 43篇 |
1973年 | 51篇 |
1972年 | 58篇 |
1969年 | 35篇 |
1968年 | 34篇 |
排序方式: 共有5180条查询结果,搜索用时 15 毫秒
11.
12.
13.
14.
The apical segments of the mandibular condylar cartilage of newborn ICR mice, containing the intact zones of progenitor cells along with a few rows of chondroblasts were initially prelabelled in vitro with [3H]thymidine and were subsequently chased and cultured for as long as eight days. Such explants underwent a process of tissue regeneration, as after three days in culture they reconstituted the original structure of the organ, thus resembling the in vivo appearance of neonatal mandibular condylar cartilage. Cellular proliferation with subsequent differentiation in the regenerating tissue was followed by means of quantitative autoradiography. Immediately after labelling, the autoradiography-positive grains were confined exclusively to progenitor cells. The latter revealed a substantial ability to proliferate in vitro, a fact that was manifested by a progressive increase in the labelling index along the course of the culture. The latter process was followed by cellular differentiation thereby obtaining hypertrophic chondrocytes. The increase in the rate of labelling index and in the total number of [3H]thymidine-labelled cells was significantly correlated with the overall growth of the regenerating explants. 相似文献
15.
16.
Adding noise to a visual image makes object recognition more effortful and has a widespread effect on human electrophysiological responses. However, visual cortical processes directly involved in handling the stimulus noise have yet to be identified and dissociated from the modulation of the neural responses due to the deteriorated structural information and increased stimulus uncertainty in the case of noisy images. Here we show that the impairment of face gender categorization performance in the case of noisy images in amblyopic patients correlates with amblyopic deficits measured in the noise-induced modulation of the P1/P2 components of single-trial event-related potentials (ERP). On the other hand, the N170 ERP component is similarly affected by the presence of noise in the two eyes and its modulation does not predict the behavioral deficit. These results have revealed that the efficient processing of noisy images depends on the engagement of additional processing resources both at the early, feature-specific as well as later, object-level stages of visual cortical processing reflected in the P1 and P2 ERP components, respectively. Our findings also suggest that noise-induced modulation of the N170 component might reflect diminished face-selective neuronal responses to face images with deteriorated structural information. 相似文献
17.
18.
The Yanomama Indians of Southern Vanezuela and Northern Brazil are one of the largest, relatively unacculturated tribes of the tropical rain forest. Over a period of eight years data have been collected from a considerable portion of their territory on estimated age, sex ratio, fertility rates (as determined by physical examination and urine tests), and infant death rates. Although it has been impossible to collect direct data on infanticide, this subject can be approached indirectly through distortions of the sex ratio and anecdotal information. Some historical data are also available as a basis for estimating tribal expansion in the past 100 years. With this material it has been possible to construct Life Tables for the Yanomama, and to explore the results of various perturbations of the input parameters. Data are also presented on patterns of mating and reproduction: number of spouses, mean and variance in number of surviving children, frequency of “extra-marital conceptions” based on the results of extensive blood group typings, and consanguinity rates as determined by observation and computer simulation. Although we do not present the Yanomama as typical, these data are seen as providing a basis for more realistic population models than have existed in the past. In addition, the data provide a basis for relatively precise estimates of such demographic measures as Fisher's Reproductive Value, Crow's Index of Total Selection, and Weiss' Index of Growth Regulation. 相似文献
19.
20.