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   622082篇
  免费   63249篇
  国内免费   776篇
  2018年   6703篇
  2017年   6344篇
  2016年   8615篇
  2015年   10866篇
  2014年   13161篇
  2013年   18138篇
  2012年   21366篇
  2011年   22200篇
  2010年   15006篇
  2009年   13505篇
  2008年   19569篇
  2007年   20215篇
  2006年   19118篇
  2005年   18066篇
  2004年   18242篇
  2003年   17192篇
  2002年   16820篇
  2001年   25867篇
  2000年   25594篇
  1999年   20236篇
  1998年   7331篇
  1997年   7194篇
  1996年   6813篇
  1995年   6447篇
  1994年   6163篇
  1993年   6122篇
  1992年   16593篇
  1991年   16721篇
  1990年   16447篇
  1989年   15958篇
  1988年   14803篇
  1987年   13995篇
  1986年   13228篇
  1985年   13156篇
  1984年   10794篇
  1983年   9235篇
  1982年   6799篇
  1981年   6026篇
  1980年   5738篇
  1979年   10068篇
  1978年   8066篇
  1977年   7244篇
  1976年   6854篇
  1975年   8024篇
  1974年   8796篇
  1973年   8563篇
  1972年   7686篇
  1971年   7172篇
  1970年   6077篇
  1969年   5998篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
951.
It has been found that fertility alpha 2-microglobulin content in male and female serum does not exceed 20 ng/ml and 40 ng/ml, respectively. A high level of fertility alpha 2-microglobulin was found in the serum in early pregnancy, with its concentration decreased by parturition.  相似文献   
952.
Effect of alkylating agent dipin was studied on hematopoietic (CFU-S) and stromal (CFU-F) progenitor cells. Single administration of dipin (0.06 mg/g) to adult (CBA × C57Bl/6) F1 hybrid mice induced a long-term (2 years) oscillations in the numbers of day 7 CFU-S and day 11 CFU-S in the bone marrow and spleen. Dipin also damaged the hematopoietic stroma as indicated by decreased numbers of CFU-F which remained low for at least a year. The capacity of stromal cells to form ectopic hematopoietic foci was considerably decreased and also remained low for 10 months. The obtained data suggest high dipin sensitivity of the earliest hematopoietic and stromal cells. The dynamics of CFU-S numbers in the hematopoietic organs supports their functioning on the basis of clonal succession (Kay, 1965).__________Translated from Izvestiya Akademii Nauk, Seriya Biologicheskaya, No. 3, 2005, pp. 267–272.Original Russian Text Copyright © 2005 by Domaratskaya, Bueverova, Payushina, Starostin.  相似文献   
953.
954.
955.
The potential impact of frog diseases on basic research is discussedin practical terms.  相似文献   
956.
957.
Acetylcholine, oxotremorine and carbachol, compounds that exhibit muscarinic agonist activity, maximally inhibited basal prolactin secretion from GH3 cells by approx. 50% and intracellular cyclic AMP levels by approx. 20%. Both parameters were inhibited with similar potencies by each agonist. These inhibitory effects were blocked by a muscarinic but not by a nicotinic receptor antagonist. In the presence of VIP or IBMX, which raise intracellular cyclic AMP levels and stimulate hormone release, the degree of muscarinic inhibition was increased, but the potency remained unchanged. Similar changes in the secretory rate of prolactin and growth hormone occurred in these and in cell perifusion experiments. These results suggest that the inhibition of hormone secretion from GH3 cells by muscarinic agonists is mediated by a decrease in intracellular cyclic AMP levels.  相似文献   
958.
Summated electrical activity of the human amygdaloid nucleus was investigated in the neurosurgical clinic by chronically implanted electrodes. It was found that odoriferous stimulation of this structure produced bursts of rapid rhythm (20–30 cps, 30–50 µV). The quasisinusoidal waves of olfactory rhythm consist of sinusoidal components which are more pronounced within the 20–30-Hz frequency range. Spindling of 1–3 sec duration occurs at the end of inhalation and the beginning of exhalation in time with breathing. During monorhinal breathing this activity, whose amplitude depends on degree of olfactory stimulation, can only be recorded ipsilaterally. Room air also activates the amygdaloid nucleus, but less strongly than odoriferous substances: No characteristic odor-dependent differences were discovered in the frequency range of the olfactory rhythm within a 20–30-Hz band.Institute of Physiology, Kiev State University, Kiev. Translated from Neirofiziologiya, Vol. 18, No. 1, pp. 61–69, January–February, 1986.  相似文献   
959.
960.
Major knowledge gaps exist with respect to light-quality regimes in the coastal-zone Strandzha Quercus frainetto (Q.f.) forest region adjoining the southern Bulgarian Black Sea. This paper presents preliminary results that help narrow these gaps. In conjunction with leaf area index (LAI) field campaigns we undertook measurements with an array of 7 broad-band (ca 40 nm) sensors covering the range 0.40–0.94 μm, plus 1 sensor for UVB (0.297 μm peak) and 1 for photosynthetically active radiation (PAR). Measurements focused on inside-forest shade conditions at sites 0 to ca 15 km from the Black Sea and at altitudes up to ca 120 m above sea level. Some of the sites were also studied using a high-resolution spectroradiometer. A sequential measuring strategy was necessary. This involves potentially large uncertainties, here addressed through estimations of the variability around the sinusoidal course of daylight. Light-quality regimes were found to be in general support of earlier studies of deciduous forests. Our data from the broad-band sensors and from the spectroradiometer are mutually supportive. They indicate a stronger red-shift below Q.f. canopies than below canopies in enclaves dominated by Fagus orientalis and Pinus sylvestris. Transmission in the range 0.50–0.55 μm increases beneath the three types of canopies, most pronounced in the Q.f. case. Analysis of relationships between the inside-forest to open-field irradiance ratio and LAI supports the use of Beer’s Law. We found a fairly strong relationship between the red (0.66 μm) to far-red (0.73 μm) irradiance ratios (R/FR) and LAI for the Q.f. forest. In quantitative terms, the result is new for this Q.f. region, and suggests further research to explore whether a two-sensor approach (0.66 and 0.73 μm) might offer possibilities for further low-cost mapping of the spatio-temporal patterns of R/FR and LAI in Strandzha. Such mapping would assist in further studies of the region’s forest biogeochemistry and vitality.  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