全文获取类型
收费全文 | 716433篇 |
免费 | 70249篇 |
国内免费 | 520篇 |
专业分类
787202篇 |
出版年
2018年 | 6969篇 |
2017年 | 6777篇 |
2016年 | 9351篇 |
2015年 | 11435篇 |
2014年 | 13781篇 |
2013年 | 19980篇 |
2012年 | 22076篇 |
2011年 | 22931篇 |
2010年 | 15528篇 |
2009年 | 14156篇 |
2008年 | 20102篇 |
2007年 | 20700篇 |
2006年 | 19494篇 |
2005年 | 18690篇 |
2004年 | 18386篇 |
2003年 | 17665篇 |
2002年 | 17096篇 |
2001年 | 36010篇 |
2000年 | 35850篇 |
1999年 | 27845篇 |
1998年 | 8680篇 |
1997年 | 9257篇 |
1996年 | 8691篇 |
1995年 | 8048篇 |
1994年 | 7717篇 |
1993年 | 7645篇 |
1992年 | 21960篇 |
1991年 | 21089篇 |
1990年 | 20582篇 |
1989年 | 20039篇 |
1988年 | 18411篇 |
1987年 | 17244篇 |
1986年 | 15927篇 |
1985年 | 15971篇 |
1984年 | 12992篇 |
1983年 | 11010篇 |
1982年 | 8038篇 |
1981年 | 7316篇 |
1980年 | 6865篇 |
1979年 | 12039篇 |
1978年 | 9243篇 |
1977年 | 8520篇 |
1976年 | 7880篇 |
1975年 | 8786篇 |
1974年 | 9466篇 |
1973年 | 9341篇 |
1972年 | 8405篇 |
1971年 | 7686篇 |
1970年 | 6733篇 |
1969年 | 6261篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
1. The anoas are two species of dwarf buffalo, the lowland anoa Bubalus depressicornis and mountain anoa Bubalus quarlesi that are endemic to the island of Sulawesi, Indonesia. The classification of the subgenus Anoa within Bubalus is upheld by assessment of recent genetic and morphological research. The classification of anoas into two species is still debated, but with the absence of significant opposing evidence, this position is adopted here. 2. Information about the distribution of the two species is presented that adds to but largely supports existing reports. However, it is still uncertain whether the two putative species are sympatric or parapatric in their distribution. A review of anoa distribution from historical reports and recent field data (1990s to 2002) highlights their decline throughout Sulawesi, especially in the southern and north-eastern peninsulas. The decline has been attributed to local hunting for meat and habitat loss. Most populations are rapidly becoming fragmented, suggesting that the conservation of viable populations may eventually require management of metapopulations. 3. There is an urgent requirement for conservation efforts to: (i) protect anoas from hunting; (ii) prevent habitat loss in key sites; (iii) complete genetic studies to better determine the number of anoa taxa and Management Units and assess their distribution; and (iv) determine the status of the remaining anoa populations. 相似文献
72.
In this study we present evidence to suggest that gastroduodenal mucosal defects may occur in gastric fistula dogs actively immunized with PGE2-thyroglobulin conjugate. One of four PGE2-immunized dogs developed a chronic pyloroduodenal ulcer with penetration into the pancreas and the other three had endoscopic evidence of gastric and/or duodenal erosions. In contrast, no gastroduodenal mucosal defects were seen in control dogs immunized with thyroglobulin alone. Occurrence of gastroduodenal ulcers or erosions was temporally related to formation of specific antibody to PGE2 suggesting that PGE2 antibody may be responsible for lesion formation. An increase in gastric acid secretion was not observed in PGE2-immunized dogs. Thus, it is likely that mucosal defects occur as a result of an impairment of PGE2-mediated mucosal defense mechanisms. Since gastroduodenal lesions can be visualized by endoscopy, the dog may prove to be useful in studying the role of endogenous PG in ulcer diseases. 相似文献
73.
S. K. Sudakov 《Neurophysiology》1987,19(5):439-443
Response in pyramidal neurons belonging to the sensorimotor cortex (37% of total nerve cells investigated in this zone) which were identified by stimulating the bulbar pyramids, were investigated during experiments on unrestrained rabbits. Pyramidal neurons having connections with the lateral hypothalamus were activated during operation of feeding behavior, while activity was inhibited in those unconnected with the lateral hypothalamus. Microiontophoretic application of a protein synthesis inhibitor to pyramidal neurons caused their ability to respond to ascending activating influences from the lateral hypothalamus to disappear. When pentagastrin was applied to these neurons following the protein synthesis blocker, they recovered their ability to participate in hypothalamic feeding reaction. It is suggested that synthesis and release of a gastrin-like peptide into the perineuronal space is required for sensorimotor cortex pyramidal neurons to participate in the organization of feeding behavior.P. K. Anokhin Institute of Normal Physiology, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 19, No. 5, 601–606, September–October, 1987. 相似文献
74.
A frozen stabilate was produced from Theileria cervi sporozoites in salivary glands of adult Amblyomma americanum. The stabilate was inoculated into three fallow deer (Cervus dama) and two white-tailed deer (Odocoileus virginianus). Following inoculation, the white-tailed deer developed parasitemias as determined by blood smear examination at 11 and 13 days postexposure. Repeat examination of blood from the three fallow deer for 30 days postexposure failed to reveal observable piro-plasms. These findings indicate that fallow deer are not as susceptible to the Theileria cervi found in white-tailed deer from North America. Thus, there are some questions regarding the taxonomic position of this organism. 相似文献
75.
Atp-activated ionic permeability in smooth muscle cells isolated from the guinea pig urinary bladder
Smooth muscle cells from the guinea pig urinary bladder were investigated by voltage clamping at the plasma membrane and using an intracellular perfusion technique. Applying adenosine triphosphate (ATP) at a concentration greater than 3 × 10–8 M and at a membrane potential of –100 to –30 mV produced a rise in fast inward transmembrane current. A similar effect was exerted by adenosine diphosphate (ADP) and -, -, and ,-methylene ATP. Application of guanosine triphosphate, inosine triphosphate, adenosine monophosphate (AMP), and adenosine failed to activate this current. It was found that AMP blocks ATP receptors competitively. No pharmacological differences were found between the latter ATP receptors and those of rat sensory neurons. The ATP receptors were rapidly desensitized and recovered their sensitivity to agonists extremely slowly. Speed of desensitization was reduced by a decrease in ATP concentration.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 19, No. 1, pp. 95–100, January–February, 1987. 相似文献
76.
Monosynaptic pathway responsible for generation of bursting activity in theHelix pomatia RPal neuron
The connection between a visceral ganglia interneuron initiating bursting pacemaker activity in the RPal neuron and the RPal neuron itself was investigated inHelix pomatia. Stimulating the interneuron either initiated or intensified bursting activity in the RPal neuron, depending on initial electrical activity in this cell. Replacing calcium with magnesium ions in the extracellular fluid and adding CdCl2 to this fluid reversibly inhibited the effect of interneuronal stimulation on the RPal neuron. The latter effect was unaffected by increasing the concentration of extracellular Ca2+ 10 to 70 mM. Intracellular injection of both Cs+ and TEA into the interneuron produced an increase in the duration of its action potentials and rendered the link connecting the neurons more effective. It is deduced that a monosynaptic chemical connection exists between the interneuron and the RPal neuron for which a peptide compound serves as transmitter.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 19, No. 1, pp. 20–28, January–February, 1987. 相似文献
77.
The T=4 envelope of Sindbis virus is organized by interactions with a complementary T=3 capsid 总被引:70,自引:0,他引:70
S D Fuller 《Cell》1987,48(6):923-934
The three-dimensional structure of Sindbis virus, an enveloped animal virus, has been determined to a resolution of 35 A by using a common lines procedure to combine cryoelectron micrographs of vitrified particles. The spikes of the virus appear as columnar trimers arranged on a T=4 lattice. The lipid bilayer of the virus envelope is polyhedral and surrounds a smooth T=3 nucleocapsid. Hence, a complete Sindbis virion (molecular weight 46.4 X 10(6)) contains 240 copies of each of the spike proteins and 180 copies of the capsid protein. The arrangement of the spike proteins is complementary to that of the nucleocapsid. Two types of spike-capsid interactions are seen. Spike trimers near the fivefold axes interact tightly with triplets of capsid elements, whereas those on the threefold axes interact more loosely. 相似文献
78.
Additive and independent responses in a single receptor: aspartate and maltose stimuli on the tar protein 总被引:12,自引:0,他引:12
The aspartate and maltose responses of E. coli are mediated through a single membrane receptor, yet the responses are independent and additive. Both stimuli cause methylation of the same 4 glutamic acid residues. More extensive methylation occurs when a cell that has adapted to one stimulus is exposed to the second, or when both stimuli are added simultaneously. The degree of methylation, as well as receptor migration on two-dimensional gels, demonstrates that only one type of protein is involved, rather than two different receptors arising from differential processing of a single gene. A conformational "push-pull" mechanism in which binding of stimulus and covalent modification, producing opposing stresses, can explain these diverse results. 相似文献
79.
Transpososomes: stable protein-DNA complexes involved in the in vitro transposition of bacteriophage Mu DNA 总被引:50,自引:0,他引:50
We report that two types of stable protein-DNA complexes, or transpososomes, are generated in vitro during the Mu DNA strand transfer reaction. The Type 1 complex is an intermediate in the reaction. Its formation requires a supercoiled mini-Mu donor plasmid, Mu A and HU protein, and Mg2+. In the Type 1 complex the two ends of Mu are held together, creating a figure eight-shaped molecule with two independent topological domains; the Mu sequences remain supercoiled while the vector DNA is relaxed because of nicking. In the presence of Mu B protein, ATP, target DNA, and Mg2+, the Type 1 complex is converted into the protein-associated product of the strand transfer reaction. In this Type 2 complex, the target DNA has been joined to the Mu DNA ends held in the synaptic complex at the center of the figure eight. Supercoils are not required for the latter reaction. 相似文献
80.
Short-term movements of chalk-stream invertebrates 总被引:1,自引:0,他引:1
SUMMARY 1. Trays of gravel were implanted for 24 h periods in several chalk streams. The trays were variously modified to monitor colonization from different sources, particularly drift (D), other downstream movements (DSM), upstream movements (USM), and all sources combined (ALL).
2. The abundance of invertebrates in the streams was high (maximum of 717 per 0.04 m2 ) and colonization of the trays was extensive, especially by Gammarus pulex , the predominant species in all but one of the streams (maximum number of individuals of G. pulex per tray: for D, 473; for DSM, 462; tor USM, 286; and tor ALL, 639). In some cases benthic densities were achieved in the trays in the 24 h exposure period, and in most cases any one source of colonization could provide benthic densities in a few days.
3. Patterns of movement differed between species, between size classes of G. pulex , and between sites for particular species. No one source of colonization could be regarded as generally predominant although downstream movements in total (D + DSM) usually far exceeded USM. DSM was usually an important source of colonization.
4. The extent and direction of movement appeared to be determined by local factors and were explained in terms of the high densities of invertebrates and the current and substratum characteristics of each site.
5. Estimates of the relative importance of different sources of colonization varied greatly according to the criteria used in determining proportions. 相似文献
2. The abundance of invertebrates in the streams was high (maximum of 717 per 0.04 m
3. Patterns of movement differed between species, between size classes of G. pulex , and between sites for particular species. No one source of colonization could be regarded as generally predominant although downstream movements in total (D + DSM) usually far exceeded USM. DSM was usually an important source of colonization.
4. The extent and direction of movement appeared to be determined by local factors and were explained in terms of the high densities of invertebrates and the current and substratum characteristics of each site.
5. Estimates of the relative importance of different sources of colonization varied greatly according to the criteria used in determining proportions. 相似文献