全文获取类型
收费全文 | 1421篇 |
免费 | 97篇 |
国内免费 | 1篇 |
专业分类
1519篇 |
出版年
2022年 | 11篇 |
2021年 | 21篇 |
2020年 | 18篇 |
2019年 | 18篇 |
2018年 | 27篇 |
2017年 | 21篇 |
2016年 | 49篇 |
2015年 | 52篇 |
2014年 | 75篇 |
2013年 | 76篇 |
2012年 | 96篇 |
2011年 | 85篇 |
2010年 | 73篇 |
2009年 | 55篇 |
2008年 | 64篇 |
2007年 | 95篇 |
2006年 | 81篇 |
2005年 | 70篇 |
2004年 | 85篇 |
2003年 | 74篇 |
2002年 | 64篇 |
2001年 | 12篇 |
2000年 | 8篇 |
1999年 | 10篇 |
1998年 | 20篇 |
1997年 | 15篇 |
1996年 | 14篇 |
1995年 | 11篇 |
1994年 | 18篇 |
1993年 | 15篇 |
1992年 | 14篇 |
1991年 | 16篇 |
1990年 | 8篇 |
1989年 | 12篇 |
1988年 | 9篇 |
1987年 | 6篇 |
1986年 | 13篇 |
1985年 | 6篇 |
1984年 | 8篇 |
1983年 | 11篇 |
1982年 | 11篇 |
1981年 | 11篇 |
1980年 | 10篇 |
1979年 | 9篇 |
1978年 | 4篇 |
1977年 | 5篇 |
1974年 | 4篇 |
1972年 | 4篇 |
1971年 | 3篇 |
1959年 | 3篇 |
排序方式: 共有1519条查询结果,搜索用时 15 毫秒
1.
2.
Marcia Divoll David J. Greenblatt 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》1981,222(1):125-128
A simple and sensitive high-performance liquid chromatographic assay of methotrexate (MTX) and its two active metabolites, 7-hydroxymethotrexate (7-OH-MTX) and 2,4-di-amino-N10-methylpteroic acid (APA) in plasma, saliva and urine was developed. The method involved deproteinization with acetonitrile followed by addition of isoamyl alcohol and ethyl acetate. After extraction the sample was chromatographed on a cation-exchange column and monitored at 313 nm. The retention times were 5, 7 and 9 min and detection limits 20, 10 and 5 ng/ml for 7-OH-MTX, MTX and APA, respectively. For concentrations greater than 100 ng/ml one-step deproteinization of 0.1 ml sample with 0.25 ml acetonitrile was satisfactory for sample preparation. The method has been evaluated in samples from patients and rabbits receiving MTX. 相似文献
3.
Helge A. Thomsen Kurt R. Buck Susan L. Coale Dave L. Garrison Marcia M. Gowing 《Nordic Journal of Botany》1988,8(4):419-436
The examination of whole mounts prepared for transmission electron microscopy has resulted in the finding of thirteen taxa of nanoplanktonic coccolithophorids from the Weddell Sea, Antarctica. The material was collected as part of the AMERIEZ programme, March 1986. Cold-water adapted nanoplanktonic coccolithophorids have previously been shown to constitute a recurrent plankton element at subarctic and arctic localities. Three of the Weddell Sea species, Wigwamma annulifera, W. arctica , and Papposphaera sagittifera , are conspecific with northern hemisphere material, while two species, Calciarcus alaskensis and Turrisphaera arctica , are possibly identical with previously described arctidsubarctic material. Six taxa new to science have been described from the Weddell Sea, Wigwamma antarctica, W. triradiata, Trigonaspis melvillea, Pappomonas weddellensis, Papposphaera obpyramidalis , and P. simplicissima . The cooccurrence of identical forms at the two poles, and the fact that the species described are allocated to "arctic" genera, indicate a geologically relatively recent exchange of biological material between the poles. 相似文献
4.
Feeding ecology of five species of phaeodarians was analyzedusing samples collected in a timeseries of sediment trap deploymentsfrom October 1986-May 1988 from 1002000 m at an oligotrophicnorth Pacific site. Quantitative analysis using transmissionelectron microscopy of contents of feeding vacuoles showed thatheterotrophic flagellates and eukaryotic algae followed by bacteria,and then prokaryotic algae, constituted the largest volumesof recognizable organisms consumed throughout the depth rangesampled. Phaeodarians were predominantly generalists, appearedto feed on sinking organic aggregates, and may select eukaryotesover higher biomass bacteria. Other than a decrease in totalrecognizable cells with increasing depth, few seasonal or depthpatterns in consumed food occurred in this oligotrophic environment.Vacuole contents of co-occurring species showed no evidenceof food resource partitioning. The trophic role of phaeodariansis that of consumers of small eukaryotes and prokaryotes andrepackages of detritus throughout the water column.
3Present address: Electron Microscope Laboratory, 26 GianniniHall, University of California, Berkeley, CA 94720, USA 相似文献
5.
6.
7.
A new species ofCeratozamia (Cycadaceae) from San Luis Potosí, Mexico is described. This species differs from all others in the genus by the clustering of leaflets along the rachis. 相似文献
8.
C. Rebekah Hand Marcia O. Burns Ellen Ireland 《Applied psychophysiology and biofeedback》1979,4(2):171-181
An EMG biofeedback program was developed for a 56-year-old Parkinsonism patient who exhibited pathological lip hypertonia and retraction. The program was designed to achieve the following goals: (1) to demonstrate a reduction in postural lip hypertonicity and (2) to demonstrate a reduction in lip hypertonicity during a series of increasingly complex speech activities. To achieve the first goal, contrastive tasks of full contraction and relaxation were utilized. Each posture was sustained while voltage measurements were made at specifici intervals. Procedures to modify lip retraction during speech included five tasks in which the patient was to monitor the audio feedback signal. The tasks involved: prolongation of a neutral vowel, consonant-vowel combinations, monosyllabic words, sentences, and a paragraph-reading task. Data collected over six biofeedback sessions are presented. Trend analyses showed consistent muscular reduction within each task. The following explanations for the decrease in the patient's hypertonicity were discussed: (1) reduction of anisometric contraction, (2) reduction of isometric contraction, (3) relearning of agonistic-antagonistic muscle balance.This research was supported in part by the University of Tennessee Neuropathology Services Program. 相似文献
9.
Long-term speeding in perceptual switches mediated by attention-dependent plasticity in cortical visual processing 总被引:3,自引:0,他引:3
Binocular rivalry has been extensively studied to understand the mechanisms that control switches in visual awareness and much has been revealed about the contributions of stimulus and cognitive factors. Because visual processes are fundamentally adaptive, however, it is also important to understand how experience alters the dynamics of perceptual switches. When observers viewed binocular rivalry repeatedly over many days, the rate of perceptual switches increased as much as 3-fold. This long-term rivalry speeding exhibited a pattern of image-feature specificity that ruled out primary contributions from strategic and nonsensory factors and implicated neural plasticity occurring in both low- and high-level visual processes in the ventral stream. Furthermore, the speeding occurred only when the rivaling patterns were voluntarily attended, suggesting that the underlying neural plasticity selectively engages when stimuli are behaviorally relevant. Long-term rivalry speeding may thus reflect broader mechanisms that facilitate quick assessments of signals that contain multiple behaviorally relevant interpretations. 相似文献
10.
Clarissa Neves Spitz Roberto Mogami Izabela Jardim Rodrigues Pitta Mariana Andrea Vilas Boas Hacker Anna Maria Sales Euzenir Nunes Sarno Marcia Rodrigues Jardim 《PLoS neglected tropical diseases》2022,16(4)
Leprosy is still a prevalent disease in Brazil, representing 93% of all occurrences in the Americas. Leprosy neuropathy is one of the most worrying manifestations of the disease. Acute neuropathy usually occurs during reaction episodes and is called neuritis. Twenty-two leprosy patients were included in this study. These patients had neural pain associated with ulnar sensory neuropathy, with or without adjunct motor involvement. The neurological picture began within thirty days of the clinical evaluation. The patients underwent a nerve conduction study and the demyelinating findings confirmed the diagnosis of neuritis. Ultrasonographic study (US) of the ulnar nerve was performed in all patients by a radiologist who was blinded to the clinical or neurophysiological results. Morphological characteristics of the ulnar nerve were analyzed, such as echogenicity, fascicular pattern, transverse cross-sectional area (CSA), aspect of the epineurium, as well as their anatomical relationships. The volume of selected muscles referring to the ulnar nerve, as well as their echogenicity, was also examined. Based on this analysis, patients with increased ulnar nerve CSA associated with loss of fascicular pattern, epineurium hyperechogenicity and presence of power Doppler flow were classified as neuritis. Therefore, patients initially classified by the clinical-electrophysiological criteria were reclassified by the imaging criteria pre-established in this study as with and without neuritis. Loss of fascicular pattern and flow detection on power Doppler showed to be significant morphological features in the detection of neuritis. In 38.5% of patients without clinical or neurophysiological findings of neuritis, US identified power Doppler flow and loss of fascicular pattern. The US is a method of high resolution and portability, and its low cost means that it could be used as an auxiliary tool in the diagnosis of neuritis and its treatment, especially in basic health units. 相似文献