全文获取类型
收费全文 | 1674175篇 |
免费 | 152843篇 |
国内免费 | 1464篇 |
专业分类
1828482篇 |
出版年
2021年 | 18271篇 |
2019年 | 16210篇 |
2018年 | 19799篇 |
2017年 | 18638篇 |
2016年 | 29436篇 |
2015年 | 42855篇 |
2014年 | 50983篇 |
2013年 | 77042篇 |
2012年 | 47970篇 |
2011年 | 39901篇 |
2010年 | 48145篇 |
2009年 | 48105篇 |
2008年 | 36536篇 |
2007年 | 35944篇 |
2006年 | 38159篇 |
2005年 | 39579篇 |
2004年 | 38346篇 |
2003年 | 35700篇 |
2002年 | 33706篇 |
2001年 | 51436篇 |
2000年 | 49157篇 |
1999年 | 44616篇 |
1998年 | 27555篇 |
1997年 | 27257篇 |
1996年 | 26440篇 |
1995年 | 24620篇 |
1994年 | 24203篇 |
1993年 | 23545篇 |
1992年 | 37847篇 |
1991年 | 36552篇 |
1990年 | 35007篇 |
1989年 | 35435篇 |
1988年 | 32478篇 |
1987年 | 31208篇 |
1986年 | 29302篇 |
1985年 | 30831篇 |
1984年 | 28619篇 |
1983年 | 25191篇 |
1982年 | 23453篇 |
1981年 | 22279篇 |
1980年 | 20904篇 |
1979年 | 24803篇 |
1978年 | 22189篇 |
1977年 | 20741篇 |
1976年 | 19873篇 |
1975年 | 20310篇 |
1974年 | 21406篇 |
1973年 | 21621篇 |
1972年 | 18839篇 |
1971年 | 17113篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Anton J. Rozsypal 《Biological cybernetics》1985,52(1):15-22
A model for lateral inhibition is presented in the context of the auditory channel. The mechanical analyzing system of the inner ear cannot alone account for the frequency resolution of hearing. Some additional mechanism, possibly lateral inhibition located in the auditory neural network, is needed to achieve the frequency selectivity observed in electrophysiological and psychoacoustical experiments. In a computer simulation study, the shape of an ideal lateral inhibition function was obtained. Such a function is applicable to all sensory modalities. In hearing, this function permits the sharpest possible frequency resolution as it can completely remove the frequency desharpening effect of the mechanical properties of the basilar membrane. In vision, it can compensate for abberations caused by the imperfections of the optical system of the eye.An expanded version of a paper presented at the XIth Intenational Congress on Acoustics, Paris, 1983 相似文献
992.
993.
Luke L. M. Heaton Eduardo López Philip K. Maini Mark D. Fricker Nick S. Jones 《Proceedings. Biological sciences / The Royal Society》2010,277(1698):3265-3274
Cord-forming fungi form extensive networks that continuously adapt to maintain an efficient transport system. As osmotically driven water uptake is often distal from the tips, and aqueous fluids are incompressible, we propose that growth induces mass flows across the mycelium, whether or not there are intrahyphal concentration gradients. We imaged the temporal evolution of networks formed by Phanerochaete velutina, and at each stage calculated the unique set of currents that account for the observed changes in cord volume, while minimizing the work required to overcome viscous drag. Predicted speeds were in reasonable agreement with experimental data, and the pressure gradients needed to produce these flows are small. Furthermore, cords that were predicted to carry fast-moving or large currents were significantly more likely to increase in size than cords with slow-moving or small currents. The incompressibility of the fluids within fungi means there is a rapid global response to local fluid movements. Hence velocity of fluid flow is a local signal that conveys quasi-global information about the role of a cord within the mycelium. We suggest that fluid incompressibility and the coupling of growth and mass flow are critical physical features that enable the development of efficient, adaptive biological transport networks. 相似文献
994.
995.
Summary The conditions required for the production of a polylysine-coated gold (PL-G) complex, which shows optimal sensitivity for the demonstration of tissue anionic sites, expressed under different conditions of pH have been investigated. Problems encountered with this complex have been compared with those found with other methods of conjugation of polylysine to colloidal gold. The performance of a bovine serum albumin (BSA)-stabilized PL-G complex was examined against other PL-G conjugates, including complexes that are commercially available, for the detection of heterogeneous glomerular anionic site populations, expressed at pH 2.5 and pH 7.0. 相似文献
996.
997.
998.
999.
A E Rettie M Boberg A W Rettenmeier T A Baillie 《The Journal of biological chemistry》1988,263(27):13733-13738
The cytochrome P-450-mediated desaturation of valproic acid (VPA) to its hepatotoxic metabolite, 2-n-propyl-4-pentenoic acid (4-ene-VPA), was examined in liver microsomes from rats, mice, rabbits and humans. The highest substrate turnover was found with microsomes from rabbits (44.2 +/- 2.7 pmol of product/nmol P-450/15 min), while lower activities were observed in preparations from human, mouse, and rat liver, in that order. Pretreatment of animals with phenobarbital led to enhanced rates of formation of 4-ene-VPA in vitro and yielded induction ratios for desaturation ranging from 2.5 to 8.4, depending upon the species. Comparative studies in the rat showed that phenobarbital is a more potent inducer of olefin formation than either phenytoin or carbamazepine. The mechanism of the desaturation reaction was studied by inter- and intramolecular deuterium isotope effect experiments, which demonstrated that removal of a hydrogen atom from the subterminal C-4 position of VPA is rate limiting in the formation of both 4-ene- and 4-hydroxy-VPA. Hydroxylation at the neighboring C-5 position, on the other hand, was highly sensitive to deuterium substitution at that site, but not to deuteration at C-4. Based on these findings, it is proposed that 4-ene- and 4-hydroxy-VPA are products of a common P-450-dependent metabolic pathway, in which a carbon-centered free radical at C-4 serves as the key intermediate. 5-Hydroxy-VPA, in contrast, derives from an independent hydroxylation reaction. 相似文献
1000.
Enumeration of denitrifying microbial populations in turf 总被引:2,自引:0,他引:2
Summary Denitrifer populations of a silt and silt loam soil under a Kentucky bluegrass turf were enumerated using the most probable
number (MPN) procedure. The influence of soil texture, soil depth, soil moisture, and additions of nitrate fertilizer on denitrifier
populations were determined.
Saturated soil conditions increased denitrifier populations 87-fold in the silt soil and 121-fold in the silt loam soil. Denitrifier
populations did not differ significantly between soil depths and additions of fertilizer nitrate did not influence populations. 相似文献