全文获取类型
收费全文 | 28122篇 |
免费 | 2620篇 |
国内免费 | 254篇 |
专业分类
30996篇 |
出版年
2023年 | 151篇 |
2022年 | 331篇 |
2021年 | 656篇 |
2020年 | 377篇 |
2019年 | 456篇 |
2018年 | 586篇 |
2017年 | 487篇 |
2016年 | 872篇 |
2015年 | 1396篇 |
2014年 | 1414篇 |
2013年 | 1747篇 |
2012年 | 2139篇 |
2011年 | 1943篇 |
2010年 | 1248篇 |
2009年 | 1215篇 |
2008年 | 1567篇 |
2007年 | 1517篇 |
2006年 | 1366篇 |
2005年 | 1329篇 |
2004年 | 1302篇 |
2003年 | 1232篇 |
2002年 | 1166篇 |
2001年 | 544篇 |
2000年 | 426篇 |
1999年 | 471篇 |
1998年 | 339篇 |
1997年 | 241篇 |
1996年 | 222篇 |
1995年 | 251篇 |
1994年 | 208篇 |
1993年 | 200篇 |
1992年 | 274篇 |
1991年 | 278篇 |
1990年 | 219篇 |
1989年 | 226篇 |
1988年 | 204篇 |
1987年 | 181篇 |
1986年 | 172篇 |
1985年 | 201篇 |
1984年 | 190篇 |
1983年 | 138篇 |
1982年 | 135篇 |
1981年 | 121篇 |
1980年 | 123篇 |
1979年 | 112篇 |
1978年 | 93篇 |
1977年 | 98篇 |
1975年 | 75篇 |
1974年 | 79篇 |
1973年 | 69篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Dr. J. Anne Brown 《Cell and tissue research》1988,253(2):377-381
Summary The vascular pathways associated with the glomerulus of the Atlantic hagfish, Myxine glutinosa have been studied by scanning electron microscopy of corroded resin casts of the vasculature. Although the overall pattern of the renal vasculature did not differ from earlier reports, a previously unreported vascular pathway which arose from the renal artery and bypassed the glomerular capillaries in 28% of glomeruli was clearly demonstrated. Glomerular bypass shunts either ran to join the loose capillary network around Bowman's capsule and thereby drain into the network of vessels associated with the mesonephric duct (ureter), or ran directly into the ureteral system of vessels and subsequently into the posterior cardinal veins. Glomerular bypass shunts which theoretically permit renal arterial blood to bypass the process of filtration may play a role in the regulation of body fluid volume. 相似文献
92.
Predicting genotypes at loci for autosomal recessive disorders using linked genetic markers: application to Wilson's disease 总被引:4,自引:0,他引:4
Lindsay A. Farrer Batsheva Bonne-Tamir Moshe Frydman Abraham Magazanik Kenneth K. Kidd Anne M. Bowcock Luigi L. Cavalli-Sforza 《Human genetics》1988,79(2):109-117
Summary Recently, the Wilson's disease locus (WND) has been mapped to the long arm of chromosome 13. We have analyzed segregation of serveral chromosome 13 markers flanking the WND locus and used multipoint linkage analysis to determine the most likely WND genotype of each of 57 unaffected individuals in 5 Wilson's disease families. Approximately 46% of these could be classified as carrier (heterozygote), homozygous normal, or homozygous affected (not yet symptomatic) with a probability of at least 90%, while 77% could be classified with a probability of at least 80%. Our results demonstrate that even though there is a significant decrease on average in serum copper concentration in Wilson's disease heterozygotes compared to normal homozygotes, other sources of variation in serum copper concentration are much greater and preclude use of serum copper to detect heterozygotes for Wilson's disease. Subsequent analyses showed that a familial component, independent of WND genotype, is the major factor accounting for variation in ceruloplasmin levels among unaffected individuals; age is another factor accounting for more variation in copper levels among unaffected individuals than WND genotype. 相似文献
93.
94.
The effect of beta-estradiol, progesterone and testosterone on the production of human leukocyte derived interferons 总被引:2,自引:0,他引:2
N Le S Yousefi N Vaziri G Carandang J Ocariz T Cesario 《Journal of biological regulators and homeostatic agents》1988,2(4):199-204
Sex hormones including estrogens, progesterone and testosterones are known to have adverse effects on the immune system and particularly on the proliferative response. Since cytokine production is known to be dissociable from the proliferation of lymphocytes and since other steroid hormones profoundly affect cytokine production, we felt it would be important to know the effect of sex steroids on the production of interferons (IFN), particularly since the latter are known to be key substances in the immune response. We have shown estradiol can slightly reduce gamma IFN yields with certain inducers (Con A, SEA) but only in pharmacologic concentrations. Similarly, progesterone had a modest effect in the same concentrations but only when Con A was the inducer. Testosterone did not effect IFN titers at any concentration. None of the sex steroids affected alpha IFN production and none of them influenced the bioactivity of either IFN species. In all cases these hormones diminished proliferative responses as has been previously noted. 相似文献
95.
Linkage relationships in the bovine MHC region. High recombination frequency between class II subregions 总被引:2,自引:2,他引:0
Leif Andersson Anne Lundén Sunna Sigurdardottir Christopher J. Davies Lars Rask 《Immunogenetics》1988,27(4):273-280
Class II genes of the bovine major histocompatibility complex (MHC) have been investigated by Southern blot analysis using human DNA probes. Previous studies revealed the presence of bovine DO
, DQ
, DQ
, DR
and DR
genes, and restriction fragment length polymorphisms for each of these genes were documented. In the present study, the presence of three additional class II genes, designated DZ
, DY
, and DY
, are reported. DZ
was assumed to correspond to the human DZ
gene while the other two were designated DY because their relationship to human class II genes could not be firmly established. The linkage relationships among bovine class II genes and two additional loci, TCP1B and C4, were investigated by family segregation analysis and analysis of linkage disequilibrium. The results clearly indicated that all these loci belong to the same linkage group. This linkage group is divided into two subregions separated by a fairly high recombination frequency. One region includes the C4, DQ
, DQ
, DR
and DR
loci and the other one is composed of the DO
DY
, DY
, and TCPIB loci. No recombinant was observed within any of these subregions and there was a strong or fairly strong linkage disequilibrium between loci within groups. In contrast, as many as five recombinants among three different families were detected in the interval between these subregions giving a recombination frequency estimate of 0.17 ± 0.07. The fairly high recombination frequency observed between class 11 genes in cattle is strikingly different from the corresponding recombination estimates in man and mouse. The finding implies either a much larger molecular distance between some of the bovine class II genes or alternatively the presence of a recombinational hot spot in the bovine class II region. 相似文献
96.
Anne Kantelinen Roland Waldner Marja-Leena Niku-Paavola Matti S. A. Leisola 《Applied microbiology and biotechnology》1988,28(2):193-198
Summary The ligninolytic enzymes ofPhlebia radiata were produced in static conditions earlier developed forPhanerochaete chrysosporium. The production pattern of lignin peroxidases resembled that ofP. chrysosporium. The extracellular proteins ofPhlebia radiata were separated by isoelectric focusing. Four proteins with acidic isoelectric points (4.15) were detected by peroxidase staining. The peroxidases ofP. radiata reacted with antibodies produced against a peroxidase ofPhanerochaete chrysosporium and vice versa. Thus the lignin peroxidases of the two fungi have major similarities despite slight differences in their isoelectric points and molecular weights. Veratryl alcohol was produced by both fungi and degraded to veratraldehyde, two lactones and a quinone by the ligninolytic cultures. 相似文献
97.
Summary The effects of CO2 enrichment on the growth, biomass partitioning, photosynthetic rates, and leaf nitrogen concentration of a grass, Bromus mollis (C3), were investigated at a favorable and a low level of nitrogen availability. Despite increases in root: shoot ratios, leaf nitrogen concentrations were decreased under CO2 enrichment at both nitrogen levels. For the low-nitrogen treatment, this resulted in lower photosynthetic rates measured at 650 l/l for the CO2-enriched plants, compared to photosynthetic rates measured at 350 l/l for the non-enriched plants. At higher nitrogen availability, photosynthetic rates of plants grown and measured at 650 l/l were greater than photosynthetic rates of the non-enriched plants measured at 350 l/l. Water use efficiency, however, was increased in enriched plants at both nitrogen levels. CO2 enrichment stimulated vegetative growth at both high and low nitrogen during most of the vegetative growth phase but, at the end of the experiment, total biomass of the high and low CO2 treatments did not differ for plants grown at low nitrogen availability. While not statistically significant, CO2 tended to stimulate seed production at high nitrogen and to decrease it at low nitrogen. 相似文献
98.
Comparison of diaphragmatic fatigue in newborn and older rabbits 总被引:1,自引:0,他引:1
Le Souef P. N.; England S. J.; Stogryn H. A.; Bryan A. C. 《Journal of applied physiology》1988,65(3):1040-1044
The ability to maintain occlusion pressure (i.e., fatigability) during activation of the diaphragm via phrenic nerve stimulation was compared in newborn (less than 14 days old) and older (greater than 30 days old) rabbits. The younger animals had lower maximum inspiratory pressures (MIP) and markedly greater falls in pressure during sustained diaphragmatic contractions at greater than 40% MIP than did the older animals. Histological analysis showed a paucity of high-oxidative type I fibers in the diaphragms of the young animals. We therefore conclude that the newborn rabbit diaphragm is extremely susceptible to fatigue and that this susceptibility correlates with the distribution of muscle fiber types. 相似文献
99.
100.