全文获取类型
收费全文 | 36243篇 |
免费 | 3377篇 |
国内免费 | 2916篇 |
专业分类
42536篇 |
出版年
2024年 | 130篇 |
2023年 | 670篇 |
2022年 | 749篇 |
2021年 | 1049篇 |
2020年 | 1481篇 |
2019年 | 1905篇 |
2018年 | 1588篇 |
2017年 | 1391篇 |
2016年 | 1574篇 |
2015年 | 1624篇 |
2014年 | 2025篇 |
2013年 | 2773篇 |
2012年 | 1631篇 |
2011年 | 1883篇 |
2010年 | 1378篇 |
2009年 | 1866篇 |
2008年 | 1883篇 |
2007年 | 1962篇 |
2006年 | 1763篇 |
2005年 | 1518篇 |
2004年 | 1300篇 |
2003年 | 1212篇 |
2002年 | 1178篇 |
2001年 | 923篇 |
2000年 | 837篇 |
1999年 | 674篇 |
1998年 | 683篇 |
1997年 | 553篇 |
1996年 | 459篇 |
1995年 | 495篇 |
1994年 | 374篇 |
1993年 | 319篇 |
1992年 | 310篇 |
1991年 | 225篇 |
1990年 | 231篇 |
1989年 | 186篇 |
1988年 | 107篇 |
1987年 | 98篇 |
1986年 | 89篇 |
1985年 | 119篇 |
1984年 | 232篇 |
1983年 | 111篇 |
1982年 | 168篇 |
1981年 | 146篇 |
1980年 | 118篇 |
1979年 | 127篇 |
1978年 | 109篇 |
1977年 | 101篇 |
1976年 | 63篇 |
1975年 | 54篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Substance use disorders (SUDs) are highly prevalent and exact a large toll on individuals’ health, well-being, and social functioning. Long-lasting changes in brain networks involved in reward, executive function, stress reactivity, mood, and self-awareness underlie the intense drive to consume substances and the inability to control this urge in a person who suffers from addiction (moderate or severe SUD). Biological (including genetics and developmental life stages) and social (including adverse childhood experiences) determinants of health are recognized factors that contribute to vulnerability for or resilience against developing a SUD. Consequently, prevention strategies that target social risk factors can improve outcomes and, when deployed in childhood and adolescence, can decrease the risk for these disorders. SUDs are treatable, and evidence of clinically significant benefit exists for medications (in opioid, nicotine and alcohol use disorders), behavioral therapies (in all SUDs), and neuromodulation (in nicotine use disorder). Treatment of SUDs should be considered within the context of a Chronic Care Model, with the intensity of intervention adjusted to the severity of the disorder and with the concomitant treatment of comorbid psychiatric and physical conditions. Involvement of health care providers in detection and management of SUDs, including referral of severe cases to specialized care, offers sustainable models of care that can be further expanded with the use of telehealth. Despite advances in our understanding and management of SUDs, individuals with these conditions continue to be stigmatized and, in some countries, incarcerated, highlighting the need to dismantle policies that perpetuate their criminalization and instead develop policies to ensure support and access to prevention and treatment. 相似文献
92.
Abstract This paper describes four new species of Coenosia Meigen, 1826, namely C. angustifolia sp.nov., C. obscuriabdominis sp. nov., C. sparagmocerca sp. nov. and C. sponsa sp. nov. Type specimens are deposited in Institute of Entomology, Shenyang Normal University, Shenyang, China. 相似文献
93.
The relationship between birth weight and relative subcutaneous fat distribution at school age was considered in 131 boys and 106 girls 7 to 12 years of age. Relative fat distribution at school age was estimated with the ratio of the subscapular to triceps skinfolds (S/T) for the total sample, and with the ratio of the sum of two trunk (subscapular, midaxillary) to the sum of two extremity (triceps, medial calf) skinfolds (T/E) for subsamples of 102 boys and 63 girls. There were no sex differences in the S/T ratio (mm/mm), boys 0.62 ± 0.15, girls 0.63 ± 0.18; T/E ratio (mm/mm), boys 0.58 ± 0.13, girls 0.59 ± 0.16; and BMI (kg/m2), boys 17.1 ± 2.4, girls 16.9 ± 2.2. Second order partial correlations, controlling for age and the BMI or age and sum of skinfolds, between birth weight and the skinfold ratios are, respectively, ?0.22 and ?0.20 (p<0.01) for S/T and ?0.29 and ?032 (p<0.01) for T/E in girls, and ?0.18 and ?0.17 (p<0.05) for S/T and ?0.06 and ?0.6 for T/E in boys. Though low, the correlations suggest that as birth weight decreases proportionally more subcutaneous fat is accumulated on the trunk than on the extremities, more so in females than in males. Results of stepwise multiple regression analyses indicate that birth weight accounts for from 2% to 8% of the variance in relative subcutaneous fat distribution at school age. 相似文献
94.
J. H. Lawton D. E. Bignell G. F. Bloemers P. Eggleton M. E. Hodda 《Biodiversity and Conservation》1996,5(2):261-273
Theoretically, there are three principal ways in which ecosystem processes might respond to reductions in species richness. These theories are reviewed, and then considered in the context of a study of the diversity of soil nematodes and termites in near-primary forest sites at Mbalmayo, Cameroon, and the contribution made by these two taxa to carbon fluxes (CO2 and CH4) from the forest floor. Nematode abundances average 2.04 × 106 m-2, and termites between 2933 and 6957 m-2. The site is the most species-rich yet investigated for both groups anywhere in the world, so that a very large number of species contribute to carbon fluxes. We speculate about how much redundancy might be built into the functioning of both assemblages, and point out the enormous difficulties of resolving such questions, and of producing such detailed species-inventories. 相似文献
95.
Abstract. The Channel Tunnel workings on the UK side have yielded nearly 4 million m3 of chalk-marl spoil which now forms a 36 ha landscaped reclamation platform. To establish vegetation of amenity and conservation interest on the spoil, seed mixtures of native wild flowers and grasses were sown with Lolium perenne (perennial rye grass) as a nurse species. Potentially, L. perenne is a suitable nurse species for grassland creation on infertile substrates as it provides rapid initial cover and stability, but it is non-persistent and declines in vigour with time, allowing wild flower species sown alongside to expand their cover and spread in the longer term. On very low fertility substrates like chalk marl, an initial application of fertilizer is needed to encourage plant growth. Results are reported of a fertilizer experiment on Channel Tunnel spoil to determine appropriate levels of fertilizer for establishment of species-rich grassland vegetation. An area hydroseeded with L. perenne and wild flowers in autumn 1992 was subjected to factorial treatment of four levels each of N and P in spring 1993. The results the following summer showed significant positive effects of nitrogen and phosphorus on L. perenne biomass and a negative impact of nitrogen on densities of wild flower species, especially legumes, establishing in the L. perenne sward. In general, low fertilizer applications encouraged low productivity and maximal species richness in the vegetation. Conversely high applications encouraged high productivity and competitive exclusion of sown wild flower species. Fertilizer applications must therefore balance encouragement of the stabilising nurse grass sward, while preventing competitive exclusion of wild flowers by the nurse grass. 相似文献
96.
Abstract The present paper deals with a new species Habrophlebiodes zijinensis sp. nov. collected in Nanjing, Jiangsu Povince, China. 相似文献
97.
Summary
Rhamnus alaternus L. is a dioecious, fleshy-fruited shrub, typical of Mediterranean vegetation, which has been recorded from the region since the tropical Tertiary. Reproductive biology of this species has been studied in Southern Italy over a period of 4 years with both field and laboratory experiments. In all the studied populations male plants predominate over the females, have a higher flower density and flower regularly every year while females are usually alternate-bearing. A strong negative association between fruit formation and vegetative growth has been found indicating that the presence of fruits on a branch reduces the growth of new shoots. This phenomenon, together with alternate-bearing, is evidence of a strong competition between vegetative growth and reproductive activity and indicates resource limitation. Flower morphology and quantity of pollen produced are typical of insect pollinated species, and flies of the Empididae and Muscidae were trapped bearing pollen at female plants but pollen is also dispersed by wind. Field experiments on pollination success showed that the exclusion of insects as pollen vectors produces a significant decrease in fruit development. Excess pollen provided by hand-pollination increases fruit set over open-pollination and reproductive success in the field is therefore pollen limited. The variability of sex expression, male fitness, fruit production, regrowth and pollination presented here suggests that these predispersal characteristics ofR. alaternus have been subject to evolutionary change since the tropical Tertiary climate was replaced by a Mediterranean one. This points to the possibility that reproductive, as well as morphological and physiological characteristics, may be equally selected for in extreme environments such as the Mediterranean. 相似文献
98.
Blackfoot disease is a peripheral vascular disease resulting in gangrene of the lower extremities. Although extensive epidemiological
study has implicated high arsenic content in artesian well water of the endemic area bears some important connection with
the disease, the etiology of the disease is still not clarified. In this study, attention is paid to chemical speciation of
arsenic in order to find out whether the concentrations of arsenic species in urine of Blackfoot disease patients are different
from those of controls. Experimental results indicate that the total arsenic, inorganic arsenic, monomethylarsonic acid, and
other forms of arsenic in the urine of patients are significantly higher than those of the contols. The possible connection
of those arsenic species with the etiology of the disease is discussed. 相似文献
99.
I. W. J. Debyser 《International journal of primatology》1995,16(6):889-907
I review literature on juvenile mortality of captive prosimians in order to evaluate the available information on captive
breeding. Juvenile mortality includes abortion, premature mortality, stillbirth, and death of the unweaned young. Prosimian
juvenile mortality ranges between 25 and 45% in captive populations. It is generally lower in the Lemuroidea, particularly
the Cheirogaleidae, than in the Lorisoidea. Mortality is particularly high in the Lorisinae. Most mortality, including a high
stillbirth rate, occurs on the first day and during the first 10 days thereafter. Stress, maternal neglect and traumatic insults,
not infrequently linked to each other, are the most frequently reported causes of death. The percentage of congenital malformations
tends to be high in some colonies. Sex of the infant and parity seem to be important risk factors for juvenile mortality,
whereas litter size does not appear to be important. Based on few data, wild- caught females appear to have higher breeding
success than those born in captivity. Synchronized births in lemuroids and isolated births in Galagoare more likely to result in successfully weaned infants. 相似文献
100.
L. P. Ruse 《Hydrobiologia》1995,315(2):135-142
Species abundances of Chironomidae (Insecta: Diptera) have often been excluded from studies of benthic river communities because of difficulties associated with sampling and identifying larvae. Chironomid pupal exuviae are easier to collect and identify and could be used to determine community structure if shown to be representative of local larval assemblages. Larvae were sampled along a 20 m chain secured over mid-channel gravels, upstream of two collection points for pupal exuviae. Proportional taxa abundances of pupal exuviae and larvae sampled from 130 m of stream were directly compared by a 2 test of independence and also separately fitted to four models of species abundance distribution. Observed proportions of taxa were not independent of the life stage sampled. The greatest discrepancies occurred with species of pupal exuviae that were absent as larvae from the gravel. The log series model provided the best fit with both pupal and larval data. Collections of pupal exuviae had greater species richness and evenness than samples of larvae. This was considered to be a consequence of sampling larvae from the gravel habitat alone. 相似文献